Parser Combinators are structures that encode how to parse particular languages. 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 library will work out how to parse it … as if by magic!

mpc can be used in this mode, or, as shown in the above example, you can specify the grammar directly as a string or in a file.

Basic Parsers

String Parsers

All the following functions construct new basic parsers of the type mpc_parser_t *. All of those parsers return a newly allocated char * with the character(s) they manage to match. If unsuccessful they will return an error. They have the following functionality.

mpc_parser_t *mpc_any(void);

Matches any individual character

mpc_parser_t *mpc_char(char c);

Matches a single given character c

mpc_parser_t *mpc_range(char s, char e);

Matches any single given character in the range s to e (inclusive)

mpc_parser_t *mpc_oneof(const char *s);

Matches any single given character in the string s

mpc_parser_t *mpc_noneof(const char *s);

Matches any single given character not in the string s

mpc_parser_t *mpc_satisfy(int(*f)(char));

Matches any single given character satisfying function f

mpc_parser_t *mpc_string(const char *s);

Matches exactly the string s

Other Parsers

Several other functions exist that construct parsers with some other special functionality.

mpc_parser_t *mpc_pass(void);

Consumes no input, always successful, returns NULL

mpc_parser_t *mpc_fail(const char *m);
mpc_parser_t *mpc_failf(const char *fmt, ...);

Consumes no input, always fails with message m or formatted string fmt.

mpc_parser_t *mpc_lift(mpc_ctor_t f);

Consumes no input, always successful, returns the result of function f

mpc_parser_t *mpc_lift_val(mpc_val_t *x);

Consumes no input, always successful, returns x

mpc_parser_t *mpc_state(void);

Consumes no input, always successful, returns a copy of the parser state as a mpc_state_t *. This state is newly allocated and so needs to be released with free when finished with.

mpc_parser_t *mpc_anchor(int(*f)(char,char));

Consumes no input. Successful when function f returns true. Always returns NULL.

Function f is a anchor function. It takes as input the last character parsed, and the next character in the input, and returns success or failure. This function can be set by the user to ensure some condition is met. For example to test that the input is at a boundary between words and non-words.

At the start of the input the first argument is set to ''. At the end of the input the second argument is set to ''.

Parsing

Once you’ve build a parser, you can run it on some input using one of the following functions. These functions return 1 on success and 0 on failure. They output either the result, or an error to a mpc_result_t variable. This type is defined as follows.

typedef union {
  mpc_err_t *error;
  mpc_val_t *output;
} mpc_result_t;

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.

int mpc_parse(const char *filename, const char *string, mpc_parser_t *p, mpc_result_t *r);

Run a parser on some string.

int mpc_parse_file(const char *filename, FILE *file, mpc_parser_t *p, mpc_result_t *r);

Run a parser on some file.

int mpc_parse_pipe(const char *filename, FILE *pipe, mpc_parser_t *p, mpc_result_t *r);

Run a parser on some pipe (such as stdin).

int mpc_parse_contents(const char *filename, mpc_parser_t *p, mpc_result_t *r);

Run a parser on the contents of some file.

Combinators

Combinators are functions that take one or more parsers and return a new parser of some given functionality.

These combinators work independently of exactly what data type the parser(s) supplied as input return. In languages such as Haskell ensuring you don’t input one type of data into a parser requiring a different type 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 result 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.

Here are the main combinators and how to use then.

mpc_parser_t *mpc_expect(mpc_parser_t *a, const char *e);
mpc_parser_t *mpc_expectf(mpc_parser_t *a, const char *fmt, ...);

Returns a parser that runs a, and on success returns the result of a, while on failure reports that e was expected.

mpc_parser_t *mpc_apply(mpc_parser_t *a, mpc_apply_t f);
mpc_parser_t *mpc_apply_to(mpc_parser_t *a, mpc_apply_to_t f, void *x);

Returns a parser that applies function f (optionality taking extra input x) to the result of parser a.

mpc_parser_t *mpc_check(mpc_parser_t *a, mpc_dtor_t da, mpc_check_t f, const char *e);
mpc_parser_t *mpc_check_with(mpc_parser_t *a, mpc_dtor_t da, mpc_check_with_t f, void *x, const char *e);
mpc_parser_t *mpc_checkf(mpc_parser_t *a, mpc_dtor_t da, mpc_check_t f, const char *fmt, ...);
mpc_parser_t *mpc_check_withf(mpc_parser_t *a, mpc_dtor_t da, mpc_check_with_t f, void *x, const char *fmt, ...);

Returns a parser that applies function f (optionally taking extra input x) to the result of parser a. If f returns non-zero, then the parser succeeds and returns the value of a (possibly modified by f). If f returns zero, then the parser fails with message e, and the result of a is destroyed with the destructor da.

mpc_parser_t *mpc_not(mpc_parser_t *a, mpc_dtor_t da);
mpc_parser_t *mpc_not_lift(mpc_parser_t *a, mpc_dtor_t da, mpc_ctor_t lf);

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.

mpc_parser_t *mpc_maybe(mpc_parser_t *a);
mpc_parser_t *mpc_maybe_lift(mpc_parser_t *a, mpc_ctor_t lf);

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).

mpc_parser_t *mpc_many(mpc_fold_t f, mpc_parser_t *a);

Runs a zero or more times until it fails. Results are combined using fold function f. See the Function Types section for more details.

mpc_parser_t *mpc_many1(mpc_fold_t f, mpc_parser_t *a);

Runs a one or more times until it fails. Results are combined with fold function f.

mpc_parser_t *mpc_count(int n, mpc_fold_t f, mpc_parser_t *a, mpc_dtor_t da);

Runs 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.

mpc_parser_t *mpc_or(int n, ...);

Attempts to run n parsers in sequence, returning the first one that succeeds. If all fail, returns an error.

mpc_parser_t *mpc_and(int n, mpc_fold_t 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.

mpc_parser_t *mpc_predictive(mpc_parser_t *a);

Returns a parser that runs a with backtracking disabled. This means if a consumes more than one character, 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).

Function Types

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 easy to introduce memory leaks or crashes into the system.

typedef void(*mpc_dtor_t)(mpc_val_t*);

Given some pointer to a data value it will ensure the memory it points to is freed correctly.

typedef mpc_val_t*(*mpc_ctor_t)(void);

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.

typedef mpc_val_t*(*mpc_apply_t)(mpc_val_t*);
typedef mpc_val_t*(*mpc_apply_to_t)(mpc_val_t*,void*);

This takes in some pointer to data and outputs some new or modified pointer to data, ensuring to free the input data if it is no longer used. The apply_to variation takes in an extra pointer to some data such as global state.

typedef int(*mpc_check_t)(mpc_val_t**);
typedef int(*mpc_check_with_t)(mpc_val_t**,void*);

This takes in some pointer to data and outputs 0 if parsing should stop with an error. Additionally, this may change or free the input data. The check_with variation takes in an extra pointer to some data such as global state.

typedef mpc_val_t*(*mpc_fold_t)(int,mpc_val_t**);

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 any input data that is no longer used once the combination has taken place.

Combinator Method

Using the above combinators we can create a parser that matches a C identifier.

When using the combinators we need to supply a function that says how to combine two char *.

For this we build a fold function that will concatenate zero or more strings together. For this sake of this tutorial we will write it by hand, but this (as well as many other useful fold functions), are actually included in mpc under the mpcf_* namespace, such as mpcf_strfold.

mpc_val_t *strfold(int n, mpc_val_t **xs) {
  char *x = calloc(1, 1);
  int i;
  for (i = 0; i < n; i++) {
    x = realloc(x, strlen(x) + strlen(xs[i]) + 1);
    strcat(x, xs[i]);
    free(xs[i]);
  }
  return x;
}

We can use this to specify a C identifier, making use of some combinators to say how the basic parsers are combined.

mpc_parser_t *alpha = mpc_or(2, mpc_range('a', 'z'), mpc_range('A', 'Z'));
mpc_parser_t *digit = mpc_range('0', '9');
mpc_parser_t *underscore = mpc_char('_');

mpc_parser_t *ident = mpc_and(2, strfold,
  mpc_or(2, alpha, underscore),
  mpc_many(strfold, mpc_or(3, alpha, digit, underscore)),
  free);

/* Do Some Parsing... */

mpc_delete(ident);

Notice that previous parsers are used as input to new parsers we construct from the combinators. Note that only the final parser ident must be deleted. When we input a parser into a combinator we should consider it to be part of the output of that combinator.

Because of this we shouldn’t create a parser and input it into multiple places, or it will be doubly freed.

Regex Method

There is an easier way to do this than the above method. mpc comes with a handy regex function for constructing parsers using regex syntax. We can specify an identifier using a regex pattern as shown below.

mpc_parser_t *ident = mpc_re("[a-zA-Z_][a-zA-Z_0-9]*");

/* Do Some Parsing... */

mpc_delete(ident);

Library Method

Although if we really wanted to create a parser for C identifiers, a function for creating this parser comes included in mpc along with many other common parsers.

mpc_parser_t *ident = mpc_ident();

/* Do Some Parsing... */

mpc_delete(ident);

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.

mpc_parser_t *mpc_new(const char *name);

This will construct a parser called name which can then be used as input to others, including itself, without fear of being deleted. 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 will not be deleted along with it. To delete a retained parser mpc_delete must be used on it directly.

A retained parser can then be defined using…

mpc_parser_t *mpc_define(mpc_parser_t *p, mpc_parser_t *a);

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.

mpc_parser_t *mpc_undefine(mpc_parser_t *p);

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.

void mpc_cleanup(int n, ...);

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.

mpc_parser_t *mpc_copy(mpc_parser_t *a);

This function makes a copy of a parser a. This can be useful when you want to
use a parser as input for some other parsers multiple times without retaining
it.

mpc_parser_t *mpc_re(const char *re);
mpc_parser_t *mpc_re_mode(const char *re, int mode);

This function takes as input the regular expression re and builds a parser
for it. With the mpc_re_mode function optional mode flags can also be given.
Available flags are MPC_RE_MULTILINE / MPC_RE_M where the start of input
character ^ also matches the beginning of new lines and the end of input $
character also matches new lines, and MPC_RE_DOTALL / MPC_RE_S where the
any character token . also matches newlines (by default it doesn’t).

Common Parsers

Useful Parsers