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.
personWe'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.