parsec.el

A parser combinator library for Emacs Lisp similar to Haskell’s Parsec library.

Overview

This work is based on John Wiegley’s emacs-pl. The original emacs-pl is awesome, but I found following problems when I tried to use it:

• It only contains a very limited set of combinators
• Some of its functions (combinators) have different behaviors than their Haskell counterparts
• It can’t show error messages when parsing fails

So I decided to make a new library on top of it. This library, however, contains most of the parser combinators in Text.Parsec.Combinator, which should be enough in most use cases. Of course more combinators can be added if necessary! Most of the parser combinators have the same behavior as their Haskell counterparts. parsec.el also comes with a simple error handling mechanism so that it can display an error message showing how the parser fails.

So we can

• use these parser combinators to write parsers easily from scratch in Emacs Lisp like what we can do in Haskell
• port existing Haskell program using Parsec to its equivalent Emacs Lisp program easily

Parsing Functions & Parser Combinators

We compare the functions and macros defined in this library with their Haskell counterparts, assuming you’re already familiar with Haskell’s Parsec. If you don’t have any experience with parser combinators, look at the docstrings of these functions and macros and try them to see the results! They are really easy to learn and use!

The Usage column for each function/combinator in the following tables is much simplified. Check the docstring of the function/combinator to see the full description.

Basic Parsing Functions

These parsing functions are used as the basic building block for a parser. By default, their return value is a string.

parsec.el	Haskell’s Parsec	Usage
parsec-ch	char	parse a character
parsec-any-ch	anyChar	parse an arbitrary character
parsec-satisfy	satisfy	parse a character satisfying a predicate
parsec-newline	newline	parse ‘\n’
parsec-crlf	crlf	parse ‘\r\n’
parsec-eol	eol	parse newline or CRLF
parsec-eof, parsec-eob	eof	parse end of file
parsec-eol-or-eof	N/A	parse EOL or EOF
parsec-re	N/A	parse using a regular expression
parsec-one-of	oneOf	parse one of the characters
parsec-none-of	noneOf	parse any character other than the supplied ones
parsec-str	N/A	parse a string but consume input only when successful
parsec-string	string	parse a string and consume input for partial matches
parsec-num	N/A	parse a number
parsec-letter	letter	parse a letter
parsec-digit	digit	parse a digit

Note:

• parsec-str and parsec-string are different. parsec-string behaves the same as string in Haskell, and parsec-str is more like combining string and try in Haskell. Personally I found parsec-str easier to use because parsec-str is “atomic”, which is similar to parsec-ch.
• Use the power of regular expressions provided by parsec-re and simplify the parser!

Parser Combinators

These combinators can be used to combine different parsers.

parsec.el	Haskell’s Parsec	Usage
parsec-or	choice	try the parsers until one succeeds
parsec-try	try	try parser and consume no input when an error occurs
parsec-lookahead	lookahead	try parser and consume no input when successful
parsec-peek	try && lookahead	try parser without comsuming any input
parsec-peek-p	try && lookahead	same as parsec-peek except the return value for failure
parsec-with-error-message	<?> (similar)	use the new error message when an error occurs
parsec-many	many	apply the parser zero or more times
parsec-many1	many1	apply the parser one or more times
parsec-many-till	manyTill	apply parser zero or more times until end succeeds
parsec-until	N/A	parse until end succeeds
parsec-not-followed-by	notFollowedBy	succeed when the parser fails
parsec-endby	endby	apply parser zero or more times, separated and ended by end
parsec-sepby	sepby	apply parser zero or more times, separated by sep
parsec-between	between	apply parser between open and close
parsec-count	count	apply parser n times
parsec-option	option	apply parser, if it fails, return opt
parsec-optional	N/A	apply parser zero or one time and return the result
parsec-optional*	optional	apply parser zero or one time and discard the result
parsec-optional-maybe	optionMaybe	apply parser zero or one time and return the result in Maybe

Note:

• parsec-or can also be used to replace <|>.
• parsec-with-error-message is slightly different from <?>. It will replace the error message even when the input is consumed.
• By default, parsec-many-till behaves as Haskell’s manyTill. However, parsec-many-till and parsec-until can accept an optional argument to specify which part(s) to be returned. You can use :both or :end as the optional argument to change the default behavior. See the docstrings for more information.

Parser Utilities

These utilities can be used together with parser combinators to build a parser and ease the translation process if you’re trying to port an existing Haskell program.

parsec.el	Haskell’s Parsec	Usage
parsec-and	do block	try all parsers and return the last result
parsec-return	do block	try all parsers and return the first result
parsec-ensure	N/A	quit the parsing when an error occurs
parsec-ensure-with-error-message	N/A	quit the parsing when an error occurs with new message
parsec-collect	sequence	try all parsers and collect the results into a list
parsec-collect*	N/A	try all parsers and collect non-nil results into a list
parsec-start	parse	entry point
parsec-parse	parse	entry point (same as parsec-start)
parsec-with-input	parse	perform parsers on input
parsec-from-maybe	fromMaybe	retrieve value from Maybe
parsec-maybe-p	N/A	is a Maybe value or not
parsec-query	N/A	change the parser’s return value

Variants that Return a String

By default, the macros/functions that return multiple values will put the values into a list. These macros/functions are:

• parsec-many
• parsec-many1
• parsec-many-till
• parsec-until
• parsec-count
• parsec-collect and parsec-collect*

They all have a variant that returns a string by concatenating the results in the list:

• parsec-many-as-string or parsec-many-s
• parsec-many1-as-string or parsec-many1-s
• parsec-many-till-as-string or parsec-many-till-s
• parsec-until-as-string or parsec-until-s
• parsec-collect-as-string or parsec-collect-s
• parsec-count-as-string or parsec-count-s

The *-s and *-as-string variants are the same, except the *-s variants have a shorter name. Using these *-s functions are recommended if you’re dealing with strings very frequently in your code. These variants accept the same arguments and have the same behavior as their original counterpart that returns a list. The only difference is the return value.

Code Examples

Some very simple examples are given here. You can see many code examples in the test files in this GitHub repo.

The following code extract the “hello” from the comment:

(parsec-with-input "/* hello */"
  (parsec-string "/*")
  (parsec-many-till-as-string (parsec-any-ch)
                              (parsec-try
                               (parsec-string "*/"))))

The following Haskell program does a similar thing:

import           Text.Parsec

main :: IO ()
main = print $ parse p "" "/* hello */"
  where
    p = do string "/*"
           manyTill anyChar (try (string "*/"))

The following code returns the “aeiou” before “end”:

(parsec-with-input "if aeiou end"
  (parsec-str "if ")
  (parsec-return
      (parsec-many-as-string (parsec-one-of ?a ?e ?i ?o ?u))
    (parsec-str " end")))

Write a Parser: a Simple CSV Parser

You can find the code in examples/simple-csv-parser.el. The code is based on the Haskell code in Using Parsec.

An end-of-line should be a string \n. We use (parsec-str "\n") to parse it (Note that since \n is also one character, (parsec-ch ?\n) also works). Some files may not contain a newline at the end, but we can view end-of-file as the end-of-line for the last line, and use parsec-eof (or parsec-eob) to parse the end-of-file. We use parsec-or to combine these two combinators:

(defun s-csv-eol ()
  (parsec-or (parsec-str "\n")
             (parsec-eof)))

A CSV file contains many lines and ends with an end-of-file. Use parsec-return to return the result of the first parser as the result.

(defun s-csv-file ()
  (parsec-return (parsec-many (s-csv-line))
    (parsec-eof)))

A CSV line contains many CSV cells and ends with an end-of-line, and we should return the cells as the results:

(defun s-csv-line ()
  (parsec-return (s-csv-cells)
    (s-csv-eol)))

CSV cells is a list, containing the first cell and the remaining cells:

(defun s-csv-cells ()
  (cons (s-csv-cell-content) (s-csv-remaining-cells)))

A CSV cell consists any character that is not , or \n, and we use the parsec-many-as-string variant to return the whole content as a string instead of a list of single-character strings:

(defun s-csv-cell-content ()
  (parsec-many-as-string (parsec-none-of ?, ?\n)))

For the remaining cells: if followed by a comma ,, we try to parse more csv cells. Otherwise, we should return the nil:

(defun s-csv-remaining-cells ()
  (parsec-or (parsec-and (parsec-ch ?,) (s-csv-cells)) nil))

OK. Our parser is almost done. To begin parsing the content in buffer foo, you need to wrap the parser inside parsec-start (or parsec-parse):

(with-current-buffer "foo"
  (goto-char (point-min))
  (parsec-parse
   (s-csv-file)))

If you want to parse a string instead, we provide a simple wrapper macro parsec-with-input, and you feed a string as the input and put arbitraty parsers inside the macro body. parsec-start or parsec-parse is not needed.

(parsec-with-input "a1,b1,c1\na2,b2,c2"
  (s-csv-file))

The above code returns:

(("a1" "b1" "c1") ("a2" "b2" "c2"))

Note that if we replace parsec-many-as-string with parsec-many in s-csv-cell-content:

(defun s-csv-cell-content ()
  (parsec-many (parsec-none-of ?, ?\n)))

The result would be:

((("a" "1") ("b" "1") ("c" "1")) (("a" "2") ("b" "2") ("c" "2")))

More Parser Examples

I translate some Haskell Parsec examples into Emacs Lisp using parsec.el. You can see from these examples that it is very easy to write parsers using parsec.el, and if you know haskell, you can see that basically I just translate the Haskell into Emacs Lisp one by one because most of them are just the same!

You can find five examples under the examples/ directory.

Three of the examples are taken from the chapter Using Parsec in the book of Real World Haskell:

• simple-csv-parser.el: a simple csv parser with no support for quoted cells, as explained in previous section.
• full-csv-parser.el: a full csv parser
• url-str-parser.el: parser parameters in URL

pjson.el is a translation of Haskell’s json library using Parsec.

scheme.el is a much simplified Scheme parser based on Write Yourself a Scheme in 48 Hours.

They’re really simple but you can see how this library works!

Change the Return Values using parsec-query

Parsing has side-effects such as forwarding the current point. In the original emacs-pl, you can specify some optional arguments to some parsing functions (pl-ch, pl-re etc.) to change the return values. In parsec.el, these functions don’t have such a behavior. Instead, we provide a unified interface parsec-query, which accepts any parser, and changes the return value of the parser.

You can speicify following arguments:

:beg      --> return the point before applying the PARSER
:end      --> return the point after applying the PARSER
:nil      --> return nil
:groups N --> return Nth group for `parsec-re'."

So instead of returning “b” as the result, the following code returns 2:

(parsec-with-input "ab"
  (parsec-ch ?a)
  (parsec-query (parsec-ch ?b) :beg))

Returning a point means that you can also incorporate parsec.el with Emacs Lisp functions that can operate on points/regions, such as goto-char and kill-region.

:group can be specified when using parsec-re:

(parsec-with-input "ab"
  (parsec-query (parsec-re "\\(a\\)\\(b\\)") :group 2))

The above code will return “b” instead of “ab”.

Error Messages

parsec.el implements a simple error handling mechanism. When an error happens, it will show how the parser fails.

For example, the following code fails:

(parsec-with-input "aac"
  (parsec-count 2 (parsec-ch ?a))
  (parsec-ch ?b))

The return value is:

(parsec-error . "Found \"c\" -> Expected \"b\"")

This also works when parser combinators fail:

(parsec-with-input "a"
  (parsec-or (parsec-ch ?b)
             (parsec-ch ?c)))

The return value is:

 (parsec-error . "None of the parsers succeeds:
	Found \"a\" -> Expected \"c\"
	Found \"a\" -> Expected \"b\"")

If an error occurs, the return value is a cons cell that contains the error message in its cdr. Compared to Haskell’s Parsec, it’s really simple, but at least the error message could tell us some information. Yeah, not perfect but usable.

To test whether a parser returns an error, use parsec-error-p. If it returns an error, you can use parsec-error-str to retrieve the error message as a string.

You can decide what to do based on the return value of a parser:

(let ((res (parsec-with-input "hello"
             (parsec-str "world"))))
  (if (parsec-error-p res)
      (message "Parser failed:\n%s" (parsec-error-str res))
    (message "Parser succeeded by returning %s" res)))

Acknowledgement

• Daan Leijen for Haskell’s Parsec
• John Wiegley for emacs-pl