Parsec (parser)


Parsec is a library for writing parsers in Haskell. It is based on higher-order parser combinators, so a complicated parser can be made out of many smaller ones. It has been reimplemented in many other languages, including
Erlang,,
Elixir,,
OCaml, and
F#,
as well as imperative languages such as
C#, and
Java.
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.
Other Haskell parser combinator libraries that have been derived from Parsec include Megaparsec and Attoparsec.
Parsec is free software released under the BSD-3-Clause license.

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.
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 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
-- 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"