Parsec is a library for writing parsers written in the programming language 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, Racket, F#, and the imperative programming languages 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 the Glasgow Haskell Compiler (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, codice_1 parses a digit, and codice_2 parses a specific string (like codice_3).
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 codice_4 function that calls one of these utility functions (codice_5 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 codice_6 type to make the type signature of codice_7 easier to read. If we wanted to alter this program, say to read either the string codice_3 or the string codice_9, we could use the operator codice_10, provided by the codice_11 typeclass, to combine two parsers into a single parser that tries either:
parser = string "hello" ;| ; string "goodbye"
References.
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