In this example I create a parser for a basic maths language. The function `parse_maths` takes as input some mathematical expression and outputs an instance of `mpc_ast_t`.
Parser Combinators are structures that encode how to parse a particular language. They can be combined using intuitive operators to create new parsers of increasing complexity. Using these operators detailed grammars and languages can be parsed and processed in a quick, efficient, and easy way.
The trick behind Parser Combinators is the observation that by structuring the library in a particular way, one can make building parser combinators look like writing a grammar itself. Therefore instead of describing _how to parse a language_, a user must only specify _the language itself_, and the computer will work out how to parse it ... as if by magic!
As is shown in the above example _mpc_ takes this one step further, and actually allows you to specify the grammar directly, or to built up parsers using library functions.
The Parser Combinator type in _mpc_ is `mpc_parser_t`. This encodes a function that attempts to parse some string and, if successful, returns a pointer to some data. Otherwise it returns some error. A parser can be run using `mpc_parse`.
This function returns `1` on success and `0` on failure. It takes as input some parser `p`, some input `string`, and some `filename`. It outputs into `r` the result of the parse - which is either a pointer to some data object, or an error. The type `mpc_result_t` is a union type defined as follows.
where `mpc_val_t` is synonymous with `void *` and simply represents some pointer to data - the exact type of which is dependant on the parser. Some variations on the above also exist. For almost all of the built-in and basic parsers the return type for a successful parser will be `char *`.
Parses the contents of `file` with parser `p` and returns the result in `r`. Returns `1` on success and `0` on failure. This is also the correct method to parse input from pipes or streams.
All the following functions return basic parsers. All of those parsers return a newly allocated `char*` with the character(s) they manage to match or an error on failure. They have the following functionality.
Combinators are functions that take one or more parsers and return a new parser of some given functionality. These combinators work independent of exactly what data type those input parsers return on success. In languages such as Haskell ensuring you don't input one type of data into a parser requiring a different type of data is done by the compiler. But in C we don't have that luxury. So it is at the discretion of the programmer to ensure that he or she deals correctly with the outputs of different parser types.
A second annoyance in C is that of manual memory management. Some parsers might get half-way and then fail. This means they need to clean up any partial data that has been collected in the parse. In Haskell this is handled by the Garbage Collector, but in C these combinators will need to take _destructor_ functions as input, which say how clean up any partial data that has been collected.
Returns a parser with the following behaviour. If parser `a` succeeds, then it fails and consumes no input. If parser `a` fails, then it succeeds, consumes no input and returns `NULL` (or the result of lift function `lf`). Destructor `da` is used to destroy the result of `a` on success.
Returns a parser that runs `a`. If `a` is successful then it returns the result of `a`. If `a` is unsuccessful then it succeeds, but returns `NULL` (or the result of `lf`).
Attempts to run `a` exactly `n` times. If this fails, any partial results are destructed with `da`. If successful results of `a` are combined using fold function `f`.
Attempts to run `n` parsers in sequence, returning the fold of the results using fold function `f`. First parsers must be specified, followed by destructors for each parser, excluding the final parser. These are used in case of partial success. For example: `mpc_and(3, mpcf_strfold, mpc_char('a'), mpc_char('b'), mpc_char('c'), free, free);` would attempt to match `'a'` followed by `'b'` followed by `'c'`, and if successful would concatenate them using `mpcf_strfold`. Otherwise would use `free` on the partial results.
Returns a parser that runs `a` with backtracking disabled. This means if `a` consumes any input, it will not be reverted, even on failure. Turning backtracking off has good performance benefits for grammars which are `LL(1)`. These are grammars where the first character completely determines the parse result - such as the decision of parsing either a C identifier, number, or string literal. This option should not be used for non `LL(1)` grammars or it will produce incorrect results or crash the parser.
Another way to think of `mpc_predictive` is that it can be applied to a parser (for a performance improvement) if either successfully parsing the first character will result in a completely successful parse, or all of the referenced sub-parsers are also `LL(1)`.
The combinator functions take a number of special function types as function pointers. Here is a short explanation of those types are how they are expected to behave. It is important that these behave correctly otherwise it is exceedingly easy to introduce memory leaks or crashes into the system.
Returns some data value when called. It can be used to create _empty_ versions of data types when certain combinators have no known default value to return. For example it may be used to return a newly allocated empty string.
This takes in some pointer to data and outputs some new or modified pointer to data, ensuring to free and old data no longer used. The `apply_to` variation takes in an extra pointer to some data such as state of the system.
This takes a list of pointers to data values and must return some combined or folded version of these data values. It must ensure to free and old data that is no longer used once after combination has taken place. This will ensure no memory is leaked.
First we build a fold function that will concatenate two strings together - freeing any data we no longer needed. For this sake of this tutorial we will write it by hand, but this (as well as many other useful fold functions) is actually included in _mpc_ as `mpcf_strfold`.
Note that only `ident` must be deleted. When we input a parser into a combinator we should consider it to be part of that combinator now. This means we shouldn't create a parser and input it into multiple places of it will be doubly feed.
Building parsers in the above way can have issues with self-reference or cyclic-reference. To overcome this we can separate the construction of parsers into two different steps. Construction and Definition.
This will construct a parser called `name` which can then be used by others, including itself, without ownership being transfered. Any parser created using `mpc_new` is said to be _retained_. This means it will behave differently to a normal parser when referenced. When deleting a parser that includes a _retained_ parser, the _retained_ parser it will not be deleted along with it. To delete a retained parser `mpc_delete` must be used on it directly.
This assigns the contents of parser `a` to `p`, and deletes `a`. With this technique parsers can now reference each other, as well as themselves, without trouble.
A final step is required. Parsers that reference each other must all be undefined before they are deleted. It is important to do any undefining before deletion. The reason for this is that to delete a parser it must look at each sub-parser that is used by it. If any of these have already been deleted a segfault is unavoidable - even if they were retained beforehand.
To ease the task of undefining and then deleting parsers `mpc_cleanup` can be used. It takes `n` parsers as input, and undefines them all, before deleting them all.
Note: _mpc_ may have separate stages for construction and definition, but it does not detect [left-recursive grammars](http://en.wikipedia.org/wiki/Left_recursion). These will go into an infinite loop when they attempt to parse input, and so should specified instead in right-recursive form instead.
*`mpc_start(mpc_parser_t *a);` Matches the start of input followed by `a`
*`mpc_end(mpc_parser_t *a, mpc_dtor_t da);` Matches `a` followed by the end of input
*`mpc_enclose(mpc_parser_t *a, mpc_dtor_t da);` Matches the start of input, `a`, and the end of input
*`mpc_strip(mpc_parser_t *a);` Matches `a` striping any surrounding whitespace
*`mpc_tok(mpc_parser_t *a);` Matches `a` and strips any trailing whitespace
*`mpc_sym(const char* s);` Matches string `s` and strips any trailing whitespace
*`mpc_total(mpc_parser_t *a, mpc_dtor_t da);` Matches the whitespace stripped `a`, enclosed in the start and end of input
*`mpc_between(mpc_parser_t *a, mpc_dtor_t ad, const char* o, const char* c);` Matches `a` between strings `o` and `c`
*`mpc_parens(mpc_parser_t *a, mpc_dtor_t ad);` Matches `a` between `"("` and `")"`
*`mpc_braces(mpc_parser_t *a, mpc_dtor_t ad);` Matches `a` between `"<"` and `">"`
*`mpc_brackets(mpc_parser_t *a, mpc_dtor_t ad);` Matches `a` between `"{"` and `"}"`
*`mpc_squares(mpc_parser_t *a, mpc_dtor_t ad);` Matches `a` between `"["` and `"]"`
*`mpc_tok_between(mpc_parser_t *a, mpc_dtor_t ad, const char* o, const char* c);` Matches `a` between `o` and `c`, where `o` and `c` have their trailing whitespace striped.
*`mpc_tok_parens(mpc_parser_t *a, mpc_dtor_t ad);` Matches `a` between trailing whitespace stripped `"("` and `")"`
*`mpc_tok_braces(mpc_parser_t *a, mpc_dtor_t ad);` Matches `a` between trailing whitespace stripped `"<"` and `">"`
*`mpc_tok_brackets(mpc_parser_t *a, mpc_dtor_t ad);` Matches `a` between trailing whitespace stripped `"{"` and `"}"`
*`mpc_tok_squares(mpc_parser_t *a, mpc_dtor_t ad);` Matches `a` between trailing whitespace stripped `"["` and `"]"`
*`mpc_val_t* mpcf_maths(int n, mpc_val_t** xs);` Examines second argument as string to see which operator it is, then operators on first and third argument as if they are `int*`.
Passing around all these function pointers might seem clumsy, but having parsers be type-generic is important as it lets users define their own syntax tree types, as well as allows them perform specific house-keeping or data processing in the parsing phase. For example we can specify a simple maths grammar that computes the result of the expression as it goes along.
Even with all that has been shown above, specifying parts of text can be a tedious task requiring many lines of code. So _mpc_ provides a simple regular expression matcher.
This returns a parser that will attempt to match the given regular expression pattern, and return the matched string on success. It does not have support for groups and match objects, but should be sufficient for simple tasks.
A cute thing about this is that it uses previous parts of the library to parse the user input string - and because _mpc_ is type generic, the parser spits out a new `mpc_parser_t` that matches the regular expression directly! It even uses many of the combinator functions indirectly as fold functions! This is a great case study in learning how to use _mpc_, so those curious are encouraged to find it in the source code.
One can avoid passing in and around all those clumbsy function pointer if they don't care what type is output by _mpc_. For this a generic Abstract Syntax Tree type `mpc_ast_t` is included in _mpc_. The combinator functions which act on this don't need information on how to destruct or fold instances of the result as they know it will be a `mpc_ast_t`. So there are a number of combinator functions which work specifically (and only) on parsers that return this type. They reside under `mpca_*`.
Doing things via this method means that all the data processing must take place after the parsing. In many instances this is no problem or even preferable.
It also allows for one more trick. As all the fold and destructor functions are implicit, the user can simply specify the grammar of the language in some nice way and the system can try to build a parser for the AST type from this alone. For this there are two functions supplied which take in a string and output a parser. The format for these grammars is simple and familar to those who have used parser generators before. It looks something like this.
String literals are surrounded in double quotes `"`. Character literals in single quotes `'` and regex literals in slashes `/`. References to other parsers are surrounded in braces `<>` and referred to by name.
Parts specified one after another are parsed in order (like `mpc_and`), while parts separated by a pipe `|` are alternatives (like `mpc_or`). Parenthesis `()` are used to specify precidence. `*` can be used to mean zero or more of. `+` for one or more of. `?` for zero or one of. `!` for negation. And a number inside braces `{5}` to mean N counts of.
Like with the regular expressions, this user input is parsed by existing parts of the _mpc_ library. It provides one of the more powerful features of the library.
This takes in some single right hand side of a rule, as well as a list of any of the parsers it refers to, and outputs a parser that does exactly what is specified by the rule.
* * *
```c
mpc_err_t* mpca_lang(const char* lang, ...);
```
This takes in a full language (one or more rules) as well as any parsers referred to by either the right or left hand sides. Any parsers specified on the left hand side of any rule will be assigned a parser equivalent to what is specified on the right. On valid user input this returns `NULL`, while if there are any errors in the user input it will return an instance of `mpc_err_t` describing the issues.
_mpc_ provides some automatic generation of error messages. These can be enhanced by the user, with use of `mpc_expect`, but many of the defaults should provide both useful and readable. An example of an error message might look something like this:
