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Programming language: Haskell
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A simple and comprehensive Haskell parsing library

Differences and similarities from Parsec and Attoparsec

Similarities to Parsec
  • Allows for parsing arbitrary Streams
  • Makes extensive use of combinators
Similarities to Attoparsec
  • Allows for return partial results
  • Is not a monad transformer
Differences from both
  • Returns a list of all possible parses
  • Allows for conversion of a regular expression to a parser

Non-greedy parsing

The last item in that list is the most important. In both Parsec and Attoparsec, parsers such as "many" are greedy. That is, they will consume as much input as possible. This is makes writing a parser equivalent to the regular expression a[ab]*a a bit tricky. We would be tempted to write:

p = do
    a <- char 'a'
    b <- many (oneOf "ab")
    c <- char 'a'
    return (a,b,c)

The problem is that the many (oneOf "ab") parser is greedy, and will consume the final char 'a' term that we try to bind to c, resulting in a failed parse. We could write this using a combination of try, notFollowedBy, and lookAhead parsers, but it doesn't capture the same elegance of "parse an 'a', then some 'a's or 'b's, then an 'a'".

JustParse removes this problem with its ability to match all possible parses. That same parser in JustParse (with many changed to many_), applied to the input abaaba would return:

('a', "b", 'a')
('a', "ba", 'a')
('a', "baab", 'a')

The Partial result represents the branch of the parse tree in which the "many_" term consumes all available input. Supplying it with something like a would yield an additional result of ('a', "baaba", 'a') (and another Partial), since it will resume parsing.

For compatability reasons, the parsers many, sepBy, etc. operate as they do in Parsec and Attoparsec. To use the ones that return all possible parses, merely append an underscore, such as many_ and sepBy_. For general purpose parse branching, one may use the branch function, or its infix name of <||>.

Regex convenience

JustParse provides the regex parser. This parser is of the type Stream s Char => Parser s Match. A Match object contains all of the text matched within it, and a list of Match objects which represent any subgroups (which may themselves contain subgroups, etc). These regular expressions are truly regular in that they do not have backreferences (for now). If one only wants the entirety of the matched text, the regex' parser will do that. Example:

p = regex' "ab+cd?"

is equivalent to the standard parser:

p = do
    a <- char 'a'
    b <- many1 (char 'b')
    c <- char 'c'
    d <- option "" (string "d")
    return (a:b++c:d)

So for small String parsers, or for use in larger parsers, the regex or regex' parsers prove very convenient.