sexp-grammar

Invertible grammar combinators for S-expressions https://github.com/esmolanka/sexp-grammar

This package is not currently in any snapshots. If you're interested in using it, we recommend adding it to Stackage Nightly. Doing so will make builds more reliable, and allow stackage.org to host generated Haddocks.

BSD-3-Clause licensed by Yevhen Smolanka, Sergey Vinokurov
Maintained by Yevhen Smolanka

Build Status

sexp-grammar

Library of invertible parsing combinators for S-expressions. The combinators define primitive grammars and ways to compose them. A grammar constructed with these combinators can be run in two directions: parsing from S-expressions direction (forward) and serialising to S-expressions direction (backward).

The approach used in sexp-grammar is inspired by the paper Invertible syntax descriptions: Unifying parsing and pretty printing and a similar implementation of invertible grammar approach for JSON, library by Martijn van Steenbergen called JsonGrammar2.

Let’s have a look at sexp-grammar at work:

{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators     #-}

import GHC.Generics
import Data.Text (Text)
import Language.SexpGrammar
import Language.SexpGrammar.Generic

data Person = Person
  { pName    :: Text
  , pAddress :: Text
  , pAge     :: Maybe Int
  } deriving (Show, Generic)

instance SexpIso Person where
  sexpIso = with $ \person ->  -- Person is isomorphic to:
    list (                           -- a list with
      el (sym "person") >>>          -- a symbol "person",
      el string         >>>          -- a string, and
      props (                        -- a property-list with
        "address" .:  string >>>     -- a keyword :address and a string value, and
        "age"     .:? int))  >>>     -- an optional keyword :age with int value.
    person

We’ve just defined an isomorphism between S-expression representation and Haskell data record representation of the same information.

ghci> :set -XTypeApplications
ghci> import Language.SexpGrammar
ghci> import Data.ByteString.Lazy.Char8 (pack, unpack)
ghci> person <- either error return . decode @Person . pack =<< getLine
(person "John Doe" :address "42 Whatever str." :age 25)
ghci> person
Person {pName = "John Doe", pAddress = "42 Whatever str.", pAge = Just 25}
ghci> putStrLn (either id unpack (encode person))
(person "John Doe" :address "42 Whatever str." :age 25)

See more examples in the repository.