propane/doc/user_guide.md

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#> Overview

Propane is a LALR Parser Generator (LPG) which:

• accepts LR(0), SLR, and LALR grammars
• generates a built-in lexer to tokenize input
• supports UTF-8 lexer inputs
• generates a table-driven shift/reduce parser to parse input in linear time
• targets C, C++, D, or Rust language outputs
• optionally supports automatic full parse tree generation
• supports starting parsing from multiple start rules
• tracks input text start and end positions for all matched tokens/rules
• is MIT-licensed
• is distributable as a standalone Ruby script

#> Installation

Propane is designed to be distributed as a stand-alone single file script that can be copied into and versioned in a project's source tree. The only requirement to run Propane is that the system has a Ruby interpreter installed. The latest release can be downloaded from https://github.com/holtrop/propane/releases. Simply copy the propane executable script into the desired location within the project to be built (typically the root of the repository) and mark it executable.

#> Command Line Usage

Propane is typically invoked from the command-line as ./propane.

Usage: ./propane [options] <input-file> <output-file>
Options:
  -h, --help  Show this usage and exit.
  --log LOG   Write log file. This will show all parser states and their
              associated shifts and reduces. It can be helpful when
              debugging a grammar.
  --version   Show program version and exit.
  -w          Treat warnings as errors. This option will treat shift/reduce
              conflicts as fatal errors and will print them to stderr in
              addition to the log file.

The user must specify the path to a Propane input grammar file and a path to an output file. The generated source code will be written to the output file. If a log file path is specified, Propane will write a log file containing detailed information about the parser states and transitions.

##> Target language selection

Propane determines the target output language from the extension of the output file name given on the command line:

• .c selects the C target. A header file with a .h extension is generated alongside the implementation file.
• .cc, .cpp, or .cxx selects the C++ target. A header file with a .h extension is generated alongside the implementation file.
• .d selects the D target.
• .rs selects the Rust target.

Propane reports an error if the target language cannot be determined from the output file name.

#> Propane Grammar File

A Propane grammar file provides Propane with the patterns, tokens, grammar rules, and user code blocks from which to build the generated lexer and parser.

Example grammar file:

<<
import std.math;
>>

# Parser values are unsigned integers.
ptype ulong;

# A few basic arithmetic operators.
token plus /\+/;
token times /\*/;
token power /\*\*/;
token integer /\d+/ <<
  ulong v;
  foreach (c; match_text)
  {
    v *= 10;
    v += (c - '0');
  }
  $$ = v;
>>
token lparen /\(/;
token rparen /\)/;
# Drop whitespace.
drop /\s+/;

Start -> E1 << $$ = $1; >>
E1 -> E2 << $$ = $1; >>
E1 -> E1 plus E2 << $$ = $1 + $3; >>
E2 -> E3 << $$ = $1; >>
E2 -> E2 times E3 << $$ = $1 * $3; >>
E3 -> E4 << $$ = $1; >>
E3 -> E3 power E4 << $$ = pow($1, $3); >>
E4 -> integer << $$ = $1; >>
E4 -> lparen E1 rparen << $$ = $2; >>

Grammar files can contain comment lines beginning with # which are ignored. White space in the grammar file is also ignored.

It is convention to use the extension .propane for the Propane grammar file, however any file name is accepted by Propane.

This user guide follows the convention of beginning a token name with a lowercase character and beginning a rule name with an uppercase character.

##> User Code Blocks

User code blocks begin following a "<<" token and end with a ">>" token found at the end of a line. All text lines in the code block are copied verbatim into the output file.

Standalone Code Blocks

C example:

<<
#include &lt;stdio.h>
>>

D example:

<<
import std.stdio;
>>

Rust example:

<<
use std::collections::HashMap;
>>

Standalone code blocks are emitted early in the output file as top-level code outside the context of any function. Standalone code blocks are a good place to include/import any other necessary supporting code modules. They can also define helper functions that can be reused by lexer or parser user code blocks. They are emitted in the order they are defined in the grammar file.

For a C target, the word "header" may immediately follow the "<<" token to cause Propane to emit the code block in the generated header file rather than the generated implementation file. This allows including another header that may be necessary to define any types needed by a ptype directive, for example:

<&lt;header
#include "mytypes.h"
>>

Lexer pattern code blocks

Lexer code blocks appear between << and >> markers following a drop, token, or pattern expression. User code in a lexer code block will be executed when the lexer matches the given pattern. Assignment to the $$ symbol will associate a parser value with the lexed token. This parser value can then be used later in a parser rule.

The input text positions of the matched token can also be accessed from within a lexer code block. Each of these positions is an instance of the p_position_t structure (see ${#p_position_t}), which contains 1-based row and col fields. The start position of the matched token is accessed with ${position}, and the end position of the matched token is accessed with ${end_position}.

Example:

token integer /\d+/ <<
  printf("integer token on row %d, col %d\n",
      ${position}.row, ${position}.col);
  $$ = parse_integer(match_text, match_length);
>>

C/C++ lexer code block arguments

The lexer code block is passed the following arguments:

• match_text (uint8_t const *) - points to the text matched by the lexer pattern.
• match_length (size_t) - length of the matched text.

Example:

ptype long;

token integer /\d+/ <<
  long v = 0;
  for (size_t i = 0u; i < match_length; i++)
  {
    v *= 10;
    v += (match_text[i] - '0');
  }
  $$ = v;
>>

D lexer code block arguments

The lexer code block is passed the following arguments:

• match_text (string) - a slice containing the text matched by the lexer pattern.

ptype ulong;

token integer /\d+/ <<
  ulong v;
  foreach (c; match_text)
  {
    v *= 10;
    v += (c - '0');
  }
  $$ = v;
>>

Rust lexer code block arguments

The lexer code block is passed the following arguments:

• match_text (&[u8]) - a slice containing the text matched by the lexer pattern.

The matched text is a byte slice rather than a string; use std::str::from_utf8() or String::from_utf8_lossy() to view it as a string.

ptype i64;

token integer /\d+/ <<
  let mut v: i64 = 0;
  for c in match_text
  {
      v *= 10;
      v += (c - b'0') as i64;
  }
  $$ = v;
>>

Parser rule code blocks

Example:

E1 -> E1 plus E2 << $$ = $1 + $3; >>

Parser rule code blocks appear following a rule expression. User code in a parser rule code block will be executed when the parser reduces the given rule. Assignment to the $$ symbol will associate a parser value with the reduced rule. Parser values for the rules or tokens in the rule pattern can be accessed positionally with tokens $1, $2, $3, etc...

The input text positions for the reduced rule and for the individual rule components can also be accessed from within a parser rule code block. Each of these positions is an instance of the p_position_t structure (see ${#p_position_t}), which contains 1-based row and col fields.

The start position of the overall reduced rule is accessed with ${$.position}, and the end position of the overall reduced rule is accessed with ${$.end_position}.

The start and end positions of an individual rule component are accessed positionally with ${N.position} and ${N.end_position}, where N is the 1-based index of the component (${1.position} for the first component, ${2.position} for the second, and so on).

Example:

Assignment -> ident equals Expr <<
    printf("assignment on row %d, col %d\n",
        ${$.position}.row, ${$.position}.col);
    printf("target identifier ends on row %d, col %d\n",
        ${1.end_position}.row, ${1.end_position}.col);
    printf("expression starts on row %d, col %d\n",
        ${3.position}.row, ${3.position}.col);
>>

A rule or rule component that allows for an empty match may not have valid positions. In this case the position should be checked for validity before its row and col fields are used (see ${#p_position_valid}). For C targets this can be accomplished with if (p_position_valid(${$.position})), for D targets with if (${$.position}.valid), and for Rust targets with if ${$.position}.valid().

In tree generation mode, a full parse tree is automatically constructed in memory for user code to traverse after parsing is complete. Parser rule code blocks are still supported in tree generation mode, but they behave differently than when tree generation mode is not active. The code block for a rule is executed after the rule has been matched and its tree node has been fully formed. Within the code block, $$ refers to the tree node handle for the reduced rule, and the rule components are accessed positionally with $1, $2, $3, etc..., each a tree node handle for that component (a rule node or a Token node). Field aliases (see the "Specifying parser rules" section) may also be used to reference a component tree node by name; a field alias behaves identically to the positional reference for that component.

Tree nodes are stored in a compact arena owned by the parser context and are referenced by lightweight handles rather than pointers. The whole tree is freed together with the context by p_context_delete(); there is no separate tree delete function, and tree node handles are only valid while the context is alive.

Child fields, positions, and token payloads are accessed through per-language accessors on a node handle:

• C: field accessor functions p_TYPE_field(node) and tree walk macros p_tree_walk_TYPE(node, field1, field2, ...); generic accessors p_node_valid(node), p_node_position(node), p_node_end_position(node), p_node_n_fields(node), p_node_data(node) (a pointer to the node record, for token payload and user fields), and p_node_id(node) (for identity comparison).
• C++: handle methods called with (), e.g. node.field(), node.valid(), node.position(), node.token(), node.pvalue(), node.data(), and node.node_id() (for identity comparison). The C-style functions and macros above are also available.
• D: @property accessors, e.g. node.field, node.valid, node.position, node.token, node.pvalue, and node.node_id (for identity comparison).
• Rust: handle methods called with (), e.g. node.field(), node.valid(), node.position(), node.end_position(), node.n_fields(), node.token(), node.pvalue(), node.data() (a reference to the node record, for token user fields), and node.node_id() (for identity comparison).

The positional position expansions (${$.position}, ${N.position}, etc...) are not available in tree generation mode; use the position accessors above instead.

C example:

tree;

Assignment -> ident equals Expr <<
    /* $$ is the Assignment tree node, $1 is the ident Token node, and $3 is
     * the Expr rule node. */
    printf("assignment on row %d, col %d\n",
        p_node_position($$).row, p_node_position($$).col);
    printf("target identifier ends on row %d, col %d\n",
        p_node_end_position($1).row, p_node_end_position($1).col);
>>

Rust example:

tree;

Assignment -> ident equals Expr <<
    /* $$ is the Assignment tree node, $1 is the ident Token node, and $3 is
     * the Expr rule node. */
    println!("assignment on row {}, col {}",
        $$.position().row, $$.position().col);
    println!("target identifier ends on row {}, col {}",
        $1.end_position().row, $1.end_position().col);
>>

##> context_user_fields statement - adding custom fields to the context

Propane uses a context structure for lexer and parser operations. Custom fields may be added to the context structure by using the grammar context_user_fields statement. This allows lexer pattern or parser rule code blocks to access user-defined fields within the context structure.

Example:

context_user_fields <<
    int mycontextval;
>>

Lexer user code blocks or parser user code blocks can access user-defined context fields by using the ${context.<field>} syntax.

C++ example:

context_user_fields <<
    std::string comments;
>>
drop /#(.*)\n/ <<
    /* Accumulate comments before the next parser tree node. */
    ${context.comments} += std::string((const char *)match_text, match_length);
>>

Rust example:

context_user_fields <<
    pub comments: String,
>>
drop /#(.*)\n/ <<
    /* Accumulate comments before the next parser tree node. */
    ${context.comments} += std::str::from_utf8(match_text).unwrap();
>>

For the Rust target, the code block contents are inserted directly into the generated p_context_t struct definition. Each field must therefore be written as a Rust struct field, terminated with a comma. The generated p_context_t derives Default, so each user context field type must implement Default. Mark a field pub if code outside of the generated module needs to access it; grammar user code blocks are emitted into the generated module itself and can access a field regardless.

If a pointer to any allocated memory is stored in a user-defined context field, it is up to the user to free any memory when the program is finished using the context structure.

##> drop statement - ignoring input patterns

A drop statement can be used to specify a lexer pattern that when matched should result in the matched input being dropped and lexing continuing after the matched input.

A common use for a drop statement would be to ignore whitespace sequences in the user input.

Example:

drop /\s+/;

See also ${#Regular expression syntax}.

free_token_node statement - freeing user-allocated memory in token node fields

If user lexer code block allocates memory to store in a token node's pvalue or any custom token user fields store pointers to allocated memory, the free_token_node grammar statement can be used to provide a code block which can be used to free memory properly.

Example freeing pvalue (C):

tree;
free_token_node <<
    free(${token.pvalue});
>>
ptype int *;
token a <<
  $$ = (int *)malloc(sizeof(int));
  *$$ = 1;
>>
token b <<
  $$ = (int *)malloc(sizeof(int));
  *$$ = 2;
>>
Start -> a:a b:b;

Example freeing custom token user fields (C):

token_user_fields <<
    char * comments;
>>
on_token_node <<
    ${token.comments} = (char *)malloc(some_len);
>>
free_token_node <<
    free(${token.comments});
>>

Example freeing pvalue (Rust):

tree;
free_token_node <<
    if !${token.pvalue}.is_null()
    {
        unsafe { drop(Box::from_raw(${token.pvalue})); }
    }
>>
ptype *mut i32;
token a <<
  $$ = Box::into_raw(Box::new(1));
>>
token b <<
  $$ = Box::into_raw(Box::new(2));
>>
Start -> a:a b:b;

The free_token_node statement user code block is not emitted for D language since D has a garbage collector.

The code block is emitted for the Rust target, where it is run from a Drop implementation generated for p_context_t. It therefore runs exactly once however the context is disposed of, whether that is by calling p_context_delete() or by simply letting the context go out of scope. A ptype or token user field which owns its memory (a String, a Vec, a Box, and so on) is released when the context is dropped and does not need a free_token_node code block. The statement is only needed for memory which Rust does not track, such as a raw pointer obtained from Box::into_raw().

##> lex_fn statement - specifying a custom lexer function

Propane generates both a lexer and a parser. By default, the parser uses the generated p_lex() function directly to return information for the next lexed token from the input stream.

However, the user can specify a custom lex function. This function may or may not use the Propane generated p_lex() function under the hood. For example, a token sequence could be injected or repeated from a previously saved macro definition.

Example (C/C++):

<<
static size_t mylexfn(p_context_t * context, p_token_info_t * out_token_info)
{
    static size_t count;
    size_t result = P_SUCCESS;
    if (count > 0)
    {
        out_token_info->token = TOKEN_a;
        out_token_info->pvalue = p_value(count);
        count--;
    }
    else
    {
        result = p_lex(context, out_token_info);
        if (out_token_info->token == TOKEN_c)
        {
            count = 3;
        }
    }
    return result;
}
>>

lex_fn mylexfn;

Example (Rust):

<<
fn mylexfn(context: &mut p_context_t, out_token_info: &mut p_token_info_t) -> usize
{
    let mut result = P_SUCCESS;
    if context.count > 0
    {
        out_token_info.token = TOKEN_a;
        out_token_info.pvalue = p_value(context.count);
        context.count -= 1;
    }
    else
    {
        result = p_lex(context, out_token_info);
        if out_token_info.token == TOKEN_c
        {
            context.count = 3;
        }
    }
    result
}
>>

context_user_fields <<
    count: usize,
>>

lex_fn mylexfn;

The lex_fn statement takes one argument specifying the name of the custom lexer function. The user must supply a value for the token field of the p_token_info_t output structure so that the parser knows what token was lexed.

Additionally, if the parser user code makes use of the token's pvalue, then the lexer function must supply a value for the pvalue field of the p_token_info_t structure. The p_value() generated API function could be useful for specifying parser values to associate with the lexed token when tree generation is not enabled. When tree generation is enabled, the pvalue field can be set to an instance of whatever type the user has defined as the ptype type.

The custom lex function returns a result code to the parser. Returning P_SUCCESS indicates that the function has produced a token in the token output field and the parse should proceed. Returning any other result code stops the parse immediately; the parser propagates that code out of p_parse() (and the p_parse_XXX() and p_parse_inner_XXX() functions) unchanged. This allows a custom lex function to surface an error condition, for example by propagating a P_UNEXPECTED_TOKEN, P_UNEXPECTED_INPUT, or P_DECODE_ERROR result from a nested p_parse_inner_XXX() or p_lex() call.

Observe the following contract when returning a result code other than P_SUCCESS:

• Do not return P_DROP. This code is an internal lexer signal and is never returned to the parser by the generated p_lex() function. Returning it from a custom lex function would be reported as a spurious parse failure. If drop functionality is required, the custom lex function should loop and return the next non-drop token.
• Do not return P_EOF to signal the end of the input. The end of input is communicated to the parser by returning P_SUCCESS with the token field set to the end-of-input token (TOKEN___EOF), which is what the generated p_lex() function does. Returning P_EOF aborts the parse rather than allowing it to complete.
• The p_token() and p_position() accessors are populated by the parser only when the parser itself detects an unexpected token. When a custom lex function returns an error code, p_token() is not updated and may not reflect a meaningful token, and p_position() reflects the lexer's current text position rather than a parser-identified error location. A custom lex function that wants a specific reported position can set it with p_set_position() before returning.
• Returning P_USER_TERMINATED does not populate the user terminate code returned by p_user_terminate_code(). This user terminate code is normally populated by the $terminate() function in the user code block.

##> module statement - specifying the generated parser module name

The module statement can be used to specify the module name for a generated D module.

module proj.parser;

If a module statement is not present, then the generated D module will not contain a module statement and the default module name will be used.

##> noline statement - disabling #line directives

By default, Propane emits #line directives into the generated output around user code blocks. These directives instruct the compiler to report any warnings or errors in the user code using the file name and line number of the original grammar file, rather than the generated output file. This makes it easier to locate the source of a compiler diagnostic in the grammar file.

The noline statement disables the emission of these #line directives.

noline;

When the noline statement is present, user code blocks are copied to the generated output without any surrounding #line directives. This can be useful when debugging the generated parser itself, or when the #line directives interfere with other tooling.

Rust has no #line directive equivalent. For a Rust target, Propane instead emits a comment before and after each section of user code naming the grammar file and the line number the code was taken from:

/* Begin user code from myparser.propane line 42. */
  let mut v: i64 = 0;
/* End user code from myparser.propane line 42. */

A compiler diagnostic that points into the generated Rust module can be traced back to the grammar by reading up to the nearest such comment. The noline statement suppresses these comments for a Rust target, in the same way that it suppresses #line directives for the other targets.

##> on_tree_node statement - custom initialization of a token tree node

The on_token_node statement can be used to provide code that initializes any token user fields when a token tree node instance is created.

For example (C++):

context_user_fields <<
    std::string comments;
>>
token_user_fields <<
    std::string comments;
>>
on_token_node <<
    ${token.comments} = ${context.comments};
    ${context.comments} = "";
>>
drop /#(.*)\n/ <<
    /* Accumulate comments before the next parser tree node. */
    ${context.comments} += std::string((const char *)match_text, match_length);
>>

For example (Rust):

context_user_fields <<
    pub comments: String,
>>
token_user_fields <<
    pub comments: String,
>>
on_token_node <<
    ${token.comments} = std::mem::take(&mut ${context.comments});
>>
drop /#(.*)\n/ <<
    /* Accumulate comments before the next parser tree node. */
    ${context.comments} += std::str::from_utf8(match_text).unwrap();
>>

##> prefix statement - specifying the generated API prefix

By default the public API (types, constants, and functions) of the generated lexer and parser uses a prefix of p_.

This prefix can be changed with the prefix statement.

Example:

prefix myparser_;

With a parser generated with this prefix statement, instead of calling p_context_new() you would call myparser_context_new().

The prefix statement can be optionally used if you would like to change the prefix used by your generated lexer and parser to something other than the default.

It can also be used when generating multiple lexers/parsers to be used in the same program to avoid symbol collisions.

##> ptype statement - specifying parser value types

The ptype statement is used to define parser value type(s). Example:

ptype void *;

This defines the default parser value type to be void * (this is, in fact, the default parser value type if the grammar file does not specify otherwise).

Each defined lexer token type and parser rule has an associated parser value type. When the lexer runs, each lexed token has a parser value associated with it. When the parser runs, each instance of a reduced rule has a parser value associated with it. Propane supports using different parser value types for different rules and token types. The example ptype statement above defines the default parser value type. A parser value type name can optionally be specified following the ptype keyword. For example:

ptype Value;
ptype array = Value[];
ptype dict = Value[string];

Object -> lbrace rbrace << $$ = new Value(); >>

Values (array) -> Value << $$ = [$1]; >>
Values -> Values comma Value << $$ = $1 ~ [$3]; >>

KeyValue (dict) -> string colon Value << $$ = [$1: $3]; >>

In this example, the default parser value type is Value. A parser value type named array is defined to mean Value[]. A parser value type named dict is defined to mean Value[string]. Any defined tokens or rules that do not specify a parser value type will have the default parser value type associated with them. To associate a different parser value type with a token or rule, write the parser value type name in parentheses following the name of the token or rule. In this example:

• a reduced Object's parser value has a type of Value.
• a reduced Values's parser value has a type of Value[].
• a reduced KeyValue's parser value has a type of Value[string].

When tree generation mode is active, the ptype functionality works differently. In this mode, only one ptype is used by the parser. Lexer user code blocks may assign a parse value to the generated Token node by assigning to $$ within a lexer code block. The type of the parse value $$ is given by the global ptype type.

Rust ptype requirements

For the Rust target, every ptype type must implement both Clone and Default. When tree generation mode is not active, the generated p_value_t is an enum with one variant per defined ptype. The generated accessor functions clone the held value, and return Default::default() when the variant currently held is not the one requested.

A user-defined ptype type therefore normally needs a #[derive(Clone, Default)]:

<<
#[derive(Clone, Default)]
pub struct Value
{
    pub n: i64,
}
>>

ptype Value;

##> start statement - specifying the parser start rule name

The start rule can be changed from the default of Start by using the start statement. Example:

start MyStartRule;

Multiple start rules can be specified, either with multiple start statements or one start statement listing multiple start rules. Example:

start Module ModuleItem Statement Expression;

When multiple start rules are specified, multiple p_parse_*(), p_parse_inner_*(), and p_result_*() functions are generated. A default p_parse() and p_result() are generated corresponding to the first start rule. Additionally, each start rule causes the generation of another version of each of these functions, for example p_parse_Statement(), p_parse_inner_Statement(), and p_result_Statement().

##> token statement - specifying tokens

The token statement allows defining a lexer token and a pattern to match that token. The name of the token must be specified immediately following the token keyword. A regular expression pattern may optionally follow the token name. If a regular expression pattern is not specified, the name of the token is taken to be the pattern. See also: ${#Regular expression syntax}.

Example:

token for;

In this example, the token name is for and the pattern to match it is /for/.

Example:

token lbrace /\{/;

In this example, the token name is lbrace and a single left curly brace will match it.

The token statement can also include a user code block. The user code block will be executed whenever the token is matched by the lexer.

Example:

token if << writeln("'if' keyword lexed"); >>

The token statement is actually a shortcut statement for a combination of a tokenid statement and a pattern statement. To define a lexer token without an associated pattern to match it, use a tokenid statement. To define a lexer pattern that may or may not result in a matched token, use a pattern statement.

##> tokenid statement - defining tokens without a matching pattern

The tokenid statement can be used to define a token without associating it with a lexer pattern that matches it.

Example:

tokenid string;

The tokenid statement can be useful when defining a token that may optionally be returned by user code associated with a pattern.

It is also useful when lexer modes and multiple lexer patterns are required to build up a full token. A common example is parsing a string. See the ${#Lexer modes} chapter for more information.

##> token_user_fields statement - adding custom token fields

When tree generation mode is active, Propane generates a tree node structure and a token node structure for each matching rule and token instance in the input string. The user may add custom fields to token tree nodes using the token_user_fields statement. The code block supplied to the token_user_fields is inserted in the struct generated by the parser to hold a token tree node.

Example (D/C++):

token_user_fields <<
    string mytokenval;
>>

Example (Rust):

token_user_fields <<
    pub mytokenval: String,
>>

For the Rust target, the code block contents are inserted directly into the generated token tree node struct, so each field must be written as a Rust struct field terminated with a comma. Each user token field type must implement Clone and Default.

The on_token_node statement can be used to provide code that initializes any token user fields when a token tree node instance is created.

For example (C++):

context_user_fields <<
    std::string comments;
>>
token_user_fields <<
    std::string comments;
>>
on_token_node <<
    ${token.comments} = ${context.comments};
    ${context.comments} = "";
>>
drop /#(.*)\n/ <<
    /* Accumulate comments before the next parser tree node. */
    ${context.comments} += std::string((const char *)match_text, match_length);
>>

If a pointer to any allocated memory is stored in a user-defined context field, the free_token_node statement can be used to supply a code block which will be executed immediately before the token node is freed. For C++, the delete statement is used to free the token tree node, so the destructor for any custom token user fields will be called. For Rust, a token user field which owns its memory is dropped along with the context, so a free_token_node code block is only needed for memory which Rust does not track.

##> tree statement - tree generation mode

To activate tree generation mode, place the tree statement in your grammar file:

tree;

It is recommended to place this statement early in the grammar.

In tree generation mode various aspects of propane's behavior are changed:

• Only one ptype is allowed.
• Parser user code blocks execute after the rule's tree node has been formed and access the tree nodes via $$, $1, $2, etc... (see the "Parser rule code blocks" section).
• Structure types are generated to represent the parsed tokens and rules as defined in the grammar.
• The parse result from p_result() is a Start tree node handle referring to the root of the parse tree for the input. If the user has changed the start rule with the start grammar statement, the name of the start structure will be given by the user-specified start rule instead of Start.

Example tree generation grammar:

tree;

ptype int;

token a << $$ = 11; >>
token b << $$ = 22; >>
token one /1/;
token two /2/;
token comma /,/ <<
  $$ = 42;
>>
token lparen /\(/;
token rparen /\)/;
drop /\s+/;

Start -> Items;

Items -> Item:item ItemsMore;
Items -> ;

ItemsMore -> comma Item:item ItemsMore;
ItemsMore -> ;

Item -> a;
Item -> b;
Item -> lparen Item:item rparen;
Item -> Dual;

Dual -> One Two;
Dual -> Two One;
One -> one;
Two -> two;

The following unit test describes the fields that will be present for an example parse:

string input = "a, ((b)), b";
p_context_t * context = p_context_new(input);
assert_eq(P_SUCCESS, p_parse(context));
Start start = p_result(context);
assert(start.pItems1.valid);
assert(start.pItems.valid);
Items items = start.pItems;
assert(items.item.valid);
assert(items.item.pToken1.valid);
assert_eq(TOKEN_a, items.item.pToken1.token);
assert_eq(11, items.item.pToken1.pvalue);
assert(items.pItemsMore.valid);
ItemsMore itemsmore = items.pItemsMore;
assert(itemsmore.item.valid);
assert(itemsmore.item.item.valid);
assert(itemsmore.item.item.item.valid);
assert(itemsmore.item.item.item.pToken1.valid);
assert_eq(TOKEN_b, itemsmore.item.item.item.pToken1.token);
assert_eq(22, itemsmore.item.item.item.pToken1.pvalue);
assert(itemsmore.pItemsMore.valid);
itemsmore = itemsmore.pItemsMore;
assert(itemsmore.item.valid);
assert(itemsmore.item.pToken1.valid);
assert_eq(TOKEN_b, itemsmore.item.pToken1.token);
assert_eq(22, itemsmore.item.pToken1.pvalue);
assert(!itemsmore.pItemsMore.valid);
p_context_delete(context);

The equivalent traversal for a Rust target, where tree node fields are accessor methods and an absent child is a handle whose valid() method returns false:

let mut context = p_context_new(b"a, ((b)), b");
assert_eq!(P_SUCCESS, p_parse(&mut context));
let start = p_result(&context);
let items = start.pItems();
assert!(items.valid());
assert!(items.item().valid());
assert_eq!(TOKEN_a, items.item().pToken1().token());
assert_eq!(11, items.item().pToken1().pvalue());
let mut itemsmore = items.pItemsMore();
assert!(itemsmore.valid());
assert_eq!(TOKEN_b, itemsmore.item().item().item().pToken1().token());
assert_eq!(22, itemsmore.item().item().item().pToken1().pvalue());
itemsmore = itemsmore.pItemsMore();
assert_eq!(TOKEN_b, itemsmore.item().pToken1().token());
assert!(!itemsmore.pItemsMore().valid());
p_context_delete(context);

tree_prefix and tree_suffix statements

In tree generation mode, structure types are defined and named based on the rules in the grammar. Additionally, a structure type called Token is generated to hold parsed token information.

These structure names can be modified by using the tree_prefix or tree_suffix statements in the grammar file. The field names that refer to instances of the structures are not affected by the tree_prefix or tree_suffix values.

For example, if the following two lines were added to the example above:

tree_prefix ABC;
tree_suffix XYZ;

Then the types would be used as such instead:

string input = "a, ((b)), b";
p_context_t * context = p_context_new(input);
assert_eq(P_SUCCESS, p_parse(context));
ABCStartXYZ start = p_result(context);
assert(start.pItems1.valid);
assert(start.pItems.valid);
ABCItemsXYZ items = start.pItems;
assert(items.pItem.valid);
assert(items.pItem.pToken1.valid);
assert_eq(TOKEN_a, items.pItem.pToken1.token);
assert_eq(11, items.pItem.pToken1.pvalue);
assert(items.pItemsMore.valid);
ABCItemsMoreXYZ itemsmore = items.pItemsMore;
assert(itemsmore.pItem.valid);
assert(itemsmore.pItem.pItem.valid);
assert(itemsmore.pItem.pItem.pItem.valid);
assert(itemsmore.pItem.pItem.pItem.pToken1.valid);

##> Specifying a lexer pattern

A pattern statement is used to define a lexer pattern that can execute user code but may not result in a matched token.

Example:

/foo+/ << writeln("saw a foo pattern"); >>

This can be especially useful with ${#Lexer modes}.

See also ${#Regular expression syntax}.

##> Regular expression syntax

A regular expression ("regex") is used to define lexer patterns in token, pattern, and drop statements. A regular expression begins and ends with a / character.

Example:

/#.*/

Regular expressions can include many special characters/sequences:

• The . character matches any input character other than a newline.
• The * character matches any number of the previous regex element.
• The + character matches one or more of the previous regex element.
• The ? character matches 0 or 1 of the previous regex element.
• The [ character begins a character class.
• The ( character begins a matching group.
• The { character begins a count qualifier.
• The \ character escapes the following character and changes its meaning:
  • The \a sequence matches an ASCII bell character (0x07).
  • The \b sequence matches an ASCII backspace character (0x08).
  • The \d sequence is shorthand for the [0-9] character class.
  • The \D sequence matches every code point not matched by \d.
  • The \f sequence matches an ASCII form feed character (0x0C).
  • The \n sequence matches an ASCII new line character (0x0A).
  • The \r sequence matches an ASCII carriage return character (0x0D).
  • The \s sequence is shorthand for the [ \t\r\n\f\v] character class.
  • The \S sequence matches every code point not matched by \s.
  • The \t sequence matches an ASCII tab character (0x09).
  • The \v sequence matches an ASCII vertical tab character (0x0B).
  • The \w sequence is shorthand for the [a-zA-Z0-9_] character class.
  • The \W sequence matches every code point not matched by \w.
  • Any other character matches itself.
• The | character creates an alternate match.

Any other character just matches itself in the input stream.

A character class consists of a list of character alternates or character ranges that can be matched by the character class. For example [a-zA-Z_] matches any lowercase character between a and z or any uppercase character between A and Z or the underscore _ character. Character classes can also be negative character classes if the first character after the [ is a ^ character. In this case, the set of characters matched by the character class is the inverse of what it otherwise would have been. For example, [^0-9] matches any character other than 0 through 9.

A matching group can be used to override the pattern sequence that multiplicity specifiers apply to. For example, the pattern /foo+/ matches "foo" or "foooo", while the pattern /(foo)+/ matches "foo" or "foofoofoo", but not "foooo".

A count qualifier in curly braces can be used to restrict the number of matches of the preceding atom to an explicit minimum and maximum range. For example, the pattern \d{3} matches exactly 3 digits 0-9. Both a minimum and maximum multiplicity count can be specified and separated by a comma. For example, /a{1,5}/ matches between 1 and 5 a characters. Either the minimum or maximum count can be omitted to omit the corresponding restriction in the number of matches allowed.

An alternate match is created with the | character. For example, the pattern /foo|bar/ matches either the sequence "foo" or the sequence "bar".

##> Lexer modes

Lexer modes can be used to change the set of patterns that are matched by the lexer. A common use for lexer modes is to match strings.

Example:

<<
string mystringvalue;
>>

tokenid str;

# String processing
/"/ <<
  mystringvalue = "";
  $mode(string);
>>
string: /[^"]+/ << mystringvalue ~= match_text; >>
string: /"/ <<
  $mode(default);
  return $token(str);
>>

A lexer mode is defined by placing the name before a colon (:) character that precedes a token or pattern statement. The token or pattern statement is restricted to only applying if the named mode is active.

By default, the active lexer mode is named default. A $mode() call within a lexer code block can be used to change lexer modes.

In the above example, when the lexer in the default mode sees a doublequote (") character, the lexer code block will clear the mystringvalue variable and will set the lexer mode to string. When the lexer begins looking for patterns to match against the input, it will now look only for patterns tagged for the string lexer mode. Any non-" character will be appended to the mystringvalue string. A " character will end the string lexer mode and return to the default lexer mode. It also returns the str token now that the token is complete.

Note that the token name str above could have been string instead - the namespace for token names is distinct from the namespace for lexer modes.

Multiple modes can be specified for a token or pattern or drop statement. For example, if the grammar wanted to only recognize an identifier following a . token and not other keywords, it could switch to an identonly mode when matching a . The ident token pattern will be matched in either the default or identonly mode.

ptype char;
token abc;
token def;
default, identonly: token ident /[a-z]+/ <<
  $$ = match_text[0];
  $mode(default);
  return $token(ident);
>>
token dot /\./ <<
  $mode(identonly);
>>
default, identonly: drop /\s+/;

##> Specifying parser rules

Rule statements create parser rules which define the grammar that will be parsed by the generated parser.

Multiple rules with the same name can be specified. Rules with the same name define a rule set for that name and act as alternatives that the parser can accept when attempting to match a reference to that rule.

The default start rule name is Start. This can be changed with the start statement. The grammar file must define a rule with the name of the start rule name which will be used as the top-level starting rule that the parser attempts to reduce.

Rule statements are composed of the name of the rule, a -> token, the fields defining the rule pattern that must be matched, and a terminating semicolon or user code block.

Example:

ptype ulong;
start Top;
token word /[a-z]+/ << $$ = match_text.length; >>
Top -> word << $$ = $1; >>

In the above example the Top rule is defined to match a single word token.

Another example:

Start -> E1 << $$ = $1; >>
E1 -> E2 << $$ = $1; >>
E1 -> E1 plus E2 << $$ = $1 + $3; >>
E2 -> E3 << $$ = $1; >>
E2 -> E2 times E3 << $$ = $1 * $3; >>
E3 -> E4 << $$ = $1; >>
E3 -> E3 power E4 << $$ = pow($1, $3); >>
E4 -> integer << $$ = $1; >>
E4 -> lparen E1 rparen << $$ = $2; >>

This example uses the default start rule name of Start.

A parser rule has zero or more fields on the right side of its definition. Each of these fields is either a token name or a rule name. A field can be immediately followed by a ? character to signify that it is optional. A field can optionally be followed by a : and then a field alias name. If present, the field alias name is used to refer to the field value in user code blocks, or if tree generation mode is active, the field alias name is used as the field name in the generated tree node structure. An optional and named field must use the format field?:name. Example:

token public;
token private;
token int;
token ident /[a-zA-Z_][a-zA-Z_0-9]*/;
token semicolon /;/;
IntegerDeclaration -> Visibility?:visibility int ident:name semicolon;
Visibility -> public;
Visibility -> private;

In a parser rule code block, parser values for the right side fields are accessible as $1 for the first field's parser value, $2 for the second field's parser value, etc... For the IntegerDeclaration rule, the first field value can also be referred to as ${visibility} and the third field value can also be referred to as ${name}. The $$ symbol accesses the output parser value for this rule. The above examples demonstrate how the parser values for the rule components can be used to produce the parser value for the accepted rule.

In tree generation mode, parser rule code blocks access the reduced rule tree node and its component tree nodes via $$, $1, $2, etc... (see the "Parser rule code blocks" section). Field aliases may still be used in tree generation mode to reference a component tree node by name, behaving identically to the corresponding positional reference.

##> User termination of the lexer or parser

Propane supports allowing lexer or parser user code blocks to terminate execution of the parser. Some example uses of this functionality could be to:

• Detect integer overflow when lexing an integer literal constant.
• Detect and report an error as soon as possible during parsing before continuing to parse any more of the input.
• Determine whether parsing should stop and instead be retried using a different parser version.

To terminate parsing from a lexer or parser user code block, use the $terminate(code) function, passing an integer expression argument. For example:

NewExpression -> new Expression << $terminate(42); >>

The value passed to the $terminate() function is known as the "user terminate code". If the parser returns a P_USER_TERMINATED result code, then the user terminate code can be accessed using the p_user_terminate_code() API function.

#> Propane generated API

By default, Propane uses a prefix of p_ when generating a lexer/parser. This prefix is used for all publicly declared types and functions. The uppercase version of the prefix is used for all constant values.

This section documents the generated API using the default p_ or P_ names.

##> Constants

Propane generates the following result code constants:

• P_SUCCESS: A successful decode/lex/parse operation has taken place.
• P_DECODE_ERROR: An error occurred when decoding UTF-8 input.
• P_UNEXPECTED_INPUT: Input was received by the lexer that does not match any lexer pattern.
• P_UNEXPECTED_TOKEN: A token was seen in a location that does not match any parser rule.
• P_DROP: The lexer matched a drop pattern.
• P_EOF: The lexer reached the end of the input string.
• P_USER_TERMINATED: A parser user code block has requested to terminate the parser.

Result codes are returned by the API functions p_decode_code_point(), p_lex(), and p_parse().

##> Types

p_code_point_t

The p_code_point_t type is aliased to a 32-bit unsigned integer. It is used to store decoded code points from the input text and perform lexing based on the grammar's lexer patterns.

p_context_t

Propane defines a p_context_t structure type. The structure is intended to be used opaquely and stores information related to the state of the lexer and parser. A p_context_t instance is allocated and initialied with the p_context_new() function.

p_position_t

The p_position_t structure contains two fields: row and col. These fields contain the 1-based row and column describing a parser position.

For D targets, the p_position_t structure can be checked for validity by querying the valid property.

For C targets, the p_position_t structure can be checked for validity by calling p_position_valid(pos) where pos is a p_position_t structure instance.

For Rust targets, the p_position_t structure can be checked for validity by calling its valid() method (e.g. if pos.valid()).

p_value_t

If tree generation mode is enabled, the p_value_t type is defined to be the type given to the ptype statement in the grammar file.

If tree generation mode is not enabled, there could be more than one ptype given, so the p_value_t type is a union of all possible ptype types. In this case, the API functions p_value() and p_value_XXX() for each given ptype name XXX are generated to return p_value_t instances holding the corresponding ptype.

For Rust targets, p_value_t is an enum rather than a union, and every ptype type must implement Clone and Default (see ${#Rust ptype requirements}). Reading a p_value_t with an accessor for a ptype other than the one it currently holds returns Default::default() rather than reinterpreting the stored bytes.

p_token_info_t

The p_token_info_t structure contains the following fields:

• position (p_position_t) holds the text position of the first code point in the token.
• end_position (p_position_t) holds the text position of the last code point in the token.
• length (size_t) holds the number of input bytes used by the token.
• token (p_token_t) holds the token ID of the lexed token
• pvalue (p_value_t) holds the parser value associated with the token. The actual user value can be extracted with p_value_get(&token_info.pvalue) for the default value or p_value_get_XXX(&token_info.pvalue) for named ptype values.

For Rust targets, p_token_info_t implements Default, so a token info structure to pass to p_lex() can be created with p_token_info_t::default().

Tree Node Types

If tree generation mode is enabled, a structure type for each rule will be generated. The name of the structure type is given by the name of the rule. Additionally a structure type called Token is generated to represent a tree node which refers to a raw parser token rather than a composite rule.

Tree Node Fields

All tree nodes have a position field specifying the text position of the beginning of the matched token or rule, and an end_position field specifying the text position of the end of the matched token or rule. Each of these fields are instances of the p_position_t structure.

A Token node will always have a valid position and end_position. A rule node may not have valid positions if the rule allows for an empty match. In this case the position structure should be checked for validity before using it. For C targets this can be accomplished with if (p_position_valid(node->position)), for D targets with if (node.position.valid), and for Rust targets with if node.position().valid().

A Token node has the following additional fields:

• token which specifies which token was parsed (one of TOKEN_*)
• pvalue which specifies the parser value for the token. If a lexer user code block assigned to $$, the assigned value will be stored here.

Tree node structures for rules contain generated fields based on the right hand side components specified for all rules of a given name.

In this example:

Start -> Items;

Items -> Item ItemsMore;
Items -> ;

The Start structure will have a field called pItems and another field of the same name but with a positional suffix (pItems1) which both refer to the parsed Items node. Both will be invalid node handles if the parsed Items rule was empty.

Tree node fields are not data members; they are read through the per-language accessors described in the "Parser rule code blocks" section, so this field is read as p_Start_pItems(node) for C, node.pItems() for C++ and Rust, and node.pItems for D.

The Items structure will have fields:

• pItem and pItem1 which refer to the parsed Item node.
• pItemsMore and pItemsMore2 which refer to the parsed ItemsMore node.

If a rule can be empty (for example in the second Items rule above), then the field referring to that rule's generated tree node will be an invalid node handle if the parser matches the empty rule pattern.

The non-positional tree node field will not be generated if there are multiple positions in which an instance of the node it refers to could be present. For example, in the below rules:

Dual -> One Two;
Dual -> Two One;

The generated Dual structure will contain pOne1, pTwo2, pTwo1, and pOne2 fields. However, a pOne field and pTwo field will not be generated since it would be ambiguous which one was matched.

If the first rule is matched, then pOne1 and pTwo2 will be valid node handles while pTwo1 and pOne2 will be invalid. If the second rule is matched instead, then the opposite would be the case.

Reading a field of an invalid node handle produces another invalid node handle rather than failing. An invalid result therefore propagates along a chain of field accesses, so a walk which descends several levels only has to be checked once, at the end, instead of at every step. Using the tree generation grammar from the ${#tree statement - tree generation mode} section:

let token = start.pItems().item().pDual().pOne1().pToken1();
if token.valid()
{
    println!("{}", p_token_names[token.token() as usize]);
}

If the parsed input did not contain a Dual, then pDual() returns an invalid handle and each remaining call in the chain passes that invalid result along. The walk does not fail and does not read invalid memory.

Reading a field which the matched rule did not fill behaves the same way. Every node of a rule set reserves a slot for each field that the rule set can have, so reading pTwo1 from a Dual node when Dual -> One Two was matched returns an invalid handle rather than reading past the end of the node's fields.

This behavior is the same for every target language:

• C: p_One_pToken1(p_Dual_pOne1(dual)), or p_tree_walk_Dual(dual, pOne1, pToken1)
• C++ and Rust: dual.pOne1().pToken1()
• D: dual.pOne1.pToken1

The result at the end of the chain must still be checked before it is used. An invalid handle reports an invalid position and end_position and an n_fields of 0, but its token is token ID 0 and its pvalue is a default-constructed parser value. Neither of those can be distinguished from a node which genuinely holds those values.

If a field alias is present in a rule definition, an additional field will be generated in the tree node with the field alias name. For example:

Exp -> Exp:left plus ExpB:right;

In the generated Exp structure, the fields pExp, pExp1, and left will all refer to the same child node (an instance of the Exp structure), and the fields pExpB, pExpB3, and right will all refer to the same child node (an instance of the ExpB structure).

##> Functions

p_context_new

The p_context_new() function must be called to allocate and initialize the context structure. The input to be used for lexing/parsing is passed in when initializing the context structure.

C example:

p_context_t * context = p_context_new(input, input_length);

D example:

p_context_t * context = p_context_new(input);

Rust example:

let mut context = p_context_new(b"a = 1");

For Rust targets, p_context_new() accepts the input as a &[u8] byte slice and copies it into the returned context.

p_context_delete

The p_context_delete() function must be called to deinitialize and deallocate a context structure allocated by p_context_new().

In tree generation mode, the whole parse tree is owned by the context and is freed by p_context_delete(). Tree node handles are only valid while the context is alive. If a lexer user code block allocates memory to store in a token node's pvalue or in a custom token user field, the free_token_node statement can be used to provide a code block which frees that memory; if specified, the free_token_node code block is executed from p_context_delete().

For Rust targets, p_context_delete() takes the context by value and consumes it. Everything the context owns is released when it is dropped, including running any free_token_node code block, so calling this function is optional for a Rust target; letting the context go out of scope has the same effect.

Rust example:

p_context_delete(context);

p_lex

The p_lex() function is the main entry point to the lexer. It is normally called automatically by the generated parser to retrieve the next input token for the parser and does not need to be called by the user. However, the user may initialize a context and call p_lex() to use the generated lexer in a standalone mode.

Example:

p_context_t * context = p_context_new(input, input_length);
p_token_info_t token_info;
size_t result = p_lex(context, &token_info);
switch (result)
{
case P_DECODE_ERROR:
    /* UTF-8 decode error */
    break;
case P_UNEXPECTED_INPUT:
    /* Input text does not match any lexer pattern. */
    break;
case P_USER_TERMINATED:
    /* Lexer user code block requested to terminate the lexer. */
    break;
case P_SUCCESS:
    /*
     * token_info.position holds the text position of the first code point in the token.
     * token_info.end_position holds the text position of the last code point in the token.
     * token_info.length holds the number of input bytes used by the token.
     * token_info.token holds the token ID of the lexed token
     * token_info.pvalue holds the parser value associated with the token.
     */
    break;
}

Rust example:

let mut context = p_context_new(input);
let mut token_info = p_token_info_t::default();
loop
{
    match p_lex(&mut context, &mut token_info)
    {
        P_SUCCESS =>
        {
            if token_info.token == TOKEN___EOF
            {
                break;
            }
            println!("{} ({} bytes)", p_token_names[token_info.token as usize],
                token_info.length);
        }
        /* P_DECODE_ERROR, P_UNEXPECTED_INPUT, or P_USER_TERMINATED. */
        _ => break,
    }
}

p_parse

The p_parse() function is the main entry point to the parser. It must be passed a pointer to an initialized context structure.

Example:

p_context_t * context = p_context_new(input, input_length);
size_t result = p_parse(context);

Rust example:

let mut context = p_context_new(input);
let result = p_parse(&mut context);

When multiple start rules are specified, a separate parse function is generated for each which starts parsing at the given rule. For example, if Statement is specified as a start rule:

size_t result = p_parse_Statement(context);

In this case, the parser will start parsing with the Statement rule.

p_parse_inner_XXX

For each start rule, a p_parse_inner_XXX() function is also generated. This variant of the parser entry point accepts a caller-provided array of "follow tokens" -- tokens the caller allows to appear immediately after the start rule in some outer grammar context. It is useful when embedding a Propane-generated sub-parser within an outer parser and the outer parser knows which tokens naturally terminate the sub-parse.

For C targets, the signature is (example for a rule named Statement):

size_t p_parse_inner_Statement(p_context_t * context,
    p_token_t const * follow_tokens, size_t n_follow_tokens);

Passing a NULL pointer (or a count of zero) makes the function behave identically to p_parse_Statement().

For D targets, the signature accepts a slice:

size_t p_parse_inner_Statement(p_context_t * context,
    const(p_token_t)[] follow_tokens);

Passing null for the slice makes the function behave identically to p_parse_Statement().

For Rust targets, the signature accepts a slice:

pub fn p_parse_inner_Statement(context: &mut p_context_t,
    follow_tokens: &[p_token_t]) -> usize;

Passing an empty slice (&[]) makes the function behave identically to p_parse_Statement():

let result = p_parse_inner_Statement(&mut context, &[TOKEN_rbrace]);

When the parse is completed via a non-EOF follow token, that follow token is not consumed from the input stream. The parser rewinds the input index and text position to the start of the follow token so that a subsequent call to p_lex() or another parse function sees the same token.

p_position_valid

The p_position_valid() function is only generated for C targets. it is used to determine whether or not a p_position_t structure is valid.

Example:

if (p_position_valid(node->position))
{
    ....
}

For D targets, rather than using p_position_valid(), the valid property function of the p_position_t structure can be queried (e.g. if (node.position.valid)).

For Rust targets, rather than using p_position_valid(), the valid() method of the p_position_t structure can be called (e.g. if node.position().valid()).

p_result

The p_result() function can be used to retrieve the final parse value after p_parse() returns a P_SUCCESS value.

Example:

p_context_t * context = p_context_new(input, input_length);
size_t result = p_parse(context);
if (p_parse(context) == P_SUCCESS)
{
    result = p_result(context);
}

If tree generation mode is active, then the p_result() function returns a Start tree node handle referring to the root of the parse tree.

When multiple start rules are specified, a separate result function is generated for each which returns the parse result for the corresponding rule. For example, if Statement is specified as a start rule:

p_context_t * context = p_context_new(input, input_length);
size_t result = p_parse(context);
if (p_parse_Statement(context) == P_SUCCESS)
{
    result = p_result_Statement(context);
}

In this case, the parser will start parsing with the Statement rule and the parse result from the Statement rule will be returned.

Rust example:

let mut context = p_context_new(input);
if p_parse(&mut context) == P_SUCCESS
{
    let result = p_result(&context);
}

For Rust targets in tree generation mode, p_result() returns a Start tree node handle which borrows the context, so the handle must be dropped before the context is passed to p_context_delete().

p_position

The p_position() function can be used to retrieve the parser position where an error occurred.

Example:

p_context_t * context = p_context_new(input, input_length);
size_t result = p_parse(context);
if (p_parse(context) == P_UNEXPECTED_TOKEN)
{
    p_position_t error_position = p_position(context);
    fprintf(stderr, "Error: unexpected token at row %u column %u\n",
        error_position.row, error_position.col);
}

Rust example:

let mut context = p_context_new(input);
if p_parse(&mut context) == P_UNEXPECTED_TOKEN
{
    let error_position = p_position(&context);
    eprintln!("Error: unexpected token at row {} column {}",
        error_position.row, error_position.col);
}

p_set_position

The p_set_position() function sets the current text position stored in the context. This is useful when performing a nested parse operation over a slice of a larger enclosing document: by setting the initial text position to something other than the default (1, 1), any token positions or error positions subsequently reported by the lexer/parser are relative to the enclosing document rather than the slice.

Example:

p_context_t * context = p_context_new(input, input_length);
p_position_t start = { .row = 5, .col = 20 };
p_set_position(context, start);
p_parse(context);

Rust example:

let mut context = p_context_new(input);
p_set_position(&mut context, p_position_t { row: 5, col: 20 });
p_parse(&mut context);

p_input_index

The p_input_index() function returns the current input text byte offset, measured from the start of the input text passed to p_context_new(). This is useful for slicing out the remaining input after a partial parse, or for locating tokens in the original input buffer.

Example:

p_context_t * context = p_context_new(input, input_length);
p_parse_inner_Statement(context, follow_tokens, n_follow_tokens);
size_t offset = p_input_index(context);
/* Remaining input starts at `input + offset`. */

Rust example:

let mut context = p_context_new(input);
p_parse_inner_Statement(&mut context, follow_tokens);
let offset = p_input_index(&context);
/* Remaining input starts at `&input[offset..]`. */

p_set_input_index

The p_set_input_index() function sets the current input text byte offset, measured from the start of the input text passed to p_context_new(). This moves the lexer's read cursor, which can be used together with p_set_position() to rewind the input part-way through a parse in order to re-read an earlier section of the input. The byte offset is not validated; the caller is responsible for providing an offset within the bounds of the input text. A value previously returned by p_input_index() is a suitable argument.

Example:

/* Save the cursor and text position at the start of a section. */
size_t saved_index = p_input_index(context);
p_position_t saved_position = p_position(context);
/* ... later, rewind to re-read that section. */
p_set_input_index(context, saved_index);
p_set_position(context, saved_position);

Rust example:

/* Save the cursor and text position at the start of a section. */
let saved_index = p_input_index(&context);
let saved_position = p_position(&context);
/* ... later, rewind to re-read that section. */
p_set_input_index(&mut context, saved_index);
p_set_position(&mut context, saved_position);

p_user_terminate_code

The p_user_terminate_code() function can be used to retrieve the user terminate code after p_parse() returns a P_USER_TERMINATED value. User terminate codes are arbitrary values that can be defined by the user to be returned when the user requests to terminate parsing. They have no particular meaning to Propane.

Example:

if (p_parse(context) == P_USER_TERMINATED)
{
    size_t user_terminate_code = p_user_terminate_code(context);
}

Rust example:

if p_parse(&mut context) == P_USER_TERMINATED
{
    let user_terminate_code = p_user_terminate_code(&context);
}

p_token

The p_token() function can be used to retrieve the current parse token. This is useful after p_parse() returns a P_UNEXPECTED_TOKEN value. terminate code after p_parse() returns a P_USER_TERMINATED value to indicate what token the parser was not expecting.

Example:

if (p_parse(context) == P_UNEXPECTED_TOKEN)
{
    p_token_t unexpected_token = p_token(context);
}

Rust example:

if p_parse(&mut context) == P_UNEXPECTED_TOKEN
{
    let unexpected_token = p_token(&context);
}

p_decode_code_point

The p_decode_code_point() function can be used to decode code points from a UTF-8 string. It does not require a lexer/parser context structure and can be used as a standalone UTF-8 decoder or from within a lexer or parser user code block.

D Example:

size_t result;
p_code_point_t code_point;
ubyte code_point_length;

result = p_decode_code_point("\xf0\x9f\xa7\xa1", &code_point, &code_point_length);
assert(result == P_SUCCESS);
assert(code_point == 0x1F9E1u);
assert(code_point_length == 4u);

Rust Example:

let mut code_point: p_code_point_t = 0;
let mut code_point_length: u8 = 0;

let result = p_decode_code_point(b"\xf0\x9f\xa7\xa1",
    &mut code_point, &mut code_point_length);
assert_eq!(P_SUCCESS, result);
assert_eq!(0x1F9E1, code_point);
assert_eq!(4, code_point_length);

p_value

The p_value(v) function builds an instance of the p_value_t with the default member set to the value of v.

A p_value_XXX(v) function is defined for each user-defined ptype name with the user-given name in place of the XXX.

These functions are useful for custom lexer functions which need to return a parser value corresponding to a lexed token.

They are especially useful when tree generation mode is not active. In that case, the p_value_t union can hold one of several different possible value types.

Rust example:

out_token_info.pvalue = p_value(count);

p_value_get

The p_value_get() accessor functions can be used to extract a user value from a p_value_t union.

The p_value_get(pvalue) function accepts a pointer to a p_value_t and returns the value of its default member.

A p_value_get_XXX(pvalue) function is generated for each user-defined ptype name with the user-given name in place of the XXX, returning the value of the corresponding member.

These functions are the counterpart to p_value() constructor functions and are useful for reading the parser value associated with a lexed token, for example p_value_get(&token_info.pvalue).

Rust example:

let value = p_value_get(&token_info.pvalue);

For Rust targets these accessors return a clone of the held value. If the p_value_t holds a different ptype than the one requested, the accessor returns Default::default().

##> Data

p_token_names

The p_token_names array contains the grammar-specified token names. It is indexed by the token ID.

C example:

p_context_t * context = p_context_new(input, input_length);
size_t result = p_parse(context);
if (p_parse(context) == P_UNEXPECTED_TOKEN)
{
    p_position_t error_position = p_position(context);
    fprintf(stderr, "Error: unexpected token `%s' at row %u column %u\n",
        p_token_names[context->token],
        error_position.row, error_position.col);
}

Rust example:

let mut context = p_context_new(input);
if p_parse(&mut context) == P_UNEXPECTED_TOKEN
{
    let error_position = p_position(&context);
    eprintln!("Error: unexpected token `{}' at row {} column {}",
        p_token_names[p_token(&context) as usize],
        error_position.row, error_position.col);
}

#> License

Propane is licensed under the terms of the MIT License:

${include LICENSE.txt}

#> Contributing

Propane is developed on github.

Issues may be submitted to https://github.com/holtrop/propane/issues.

Pull requests may be submitted as well:

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request

#> Change Log

${changelog}