Parsec (parser)
Parsec is a library for writing parsers in Haskell.[1] It is based on higher-order parser combinators, so a complicated parser can be made out of many smaller ones.[2] It has been reimplemented in many other languages, including Erlang,[3] OCaml,[4] and F#,[5][6] as well as imperative languages such as C#,[7] and Java.[8]
Because a parser combinator-based program is generally slower than a parser generator-based program, Parsec is normally used for small domain-specific languages, while Happy is used for compilers such as GHC.[9]
Other Haskell parser combinator libraries that have been derived from Parsec include Megaparsec[10] and Attoparsec.[11]
Parsec is free software released under the BSD-3-Clause license.[12]
Example
Parsers written in Parsec start with simpler parsers, such as ones that recognize certain strings, and combine them to build a parser with more complicated behavior. For example, digit
parses a digit, and string
parses a specific string (like "hello"
).
Parser combinator libraries like Parsec provide utility functions to run the parsers on real values. A parser to recognize a single digit from a string can be split into two functions: one to create the parser, and a main
function that calls one of these utility functions (parse
in this case) to run the parser:
import Text.Parsec -- has general parsing utility functions
import Text.Parsec.Char -- contains specific basic combinators
type Parser = Stream s m Char => ParsecT s u m String
parser :: Parser
parser = string "hello"
main :: IO ()
main = print (parse parser "<test>" "hello world")
-- prints 'Right "hello"'
We define a Parser
type to make the type signature of parser
easier to read. If we wanted to alter this program, say to read either the string "hello"
or the string "goodbye"
, we could use the operator <|>
, provided by the Alternative
typeclass, to combine two parsers into a single parser that tries either:
parser = string "hello" <|> string "goodbye"
References
- ↑ "Parsec on Haskell wiki". Retrieved 29 May 2017.
- ↑ "Parsec: Direct Style Monadic Parser Combinators For The Real World" (PDF). Retrieved 22 November 2014.
- ↑ "Parsec Erlang". Retrieved 23 November 2014.
- ↑ "Parsec OCaml" (PDF). Retrieved 23 November 2014.
- ↑ "XParsec by corsis". Retrieved 29 May 2017.
- ↑ "FParsec". Retrieved 29 May 2017.
- ↑ "CSharp monad". Retrieved 10 December 2014.
- ↑ "JParsec". Retrieved 14 October 2016.
- ↑ "The Glasgow Haskell Compiler (AOSA Vol. 2)". Retrieved 23 November 2014.
- ↑ "megaparsec: Monadic parser combinators". Hackage. Retrieved 2018-09-10.
- ↑ "attoparsec: Fast combinator parsing for bytestrings and text". Hackage. Retrieved 2018-09-10.
- ↑ https://github.com/haskell/parsec/blob/master/LICENSE