mirror of
https://github.com/pavelchristof/syntax-example.git
synced 2024-09-11 09:55:56 +03:00
67 lines
1.9 KiB
Haskell
67 lines
1.9 KiB
Haskell
{-# LANGUAGE TemplateHaskell #-}
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{-# LANGUAGE FlexibleContexts #-}
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{-# LANGUAGE OverloadedStrings #-}
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import Control.Applicative
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import Control.Lens.TH
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import qualified Data.Attoparsec.Text as AP
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import Data.Char
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import Data.SemiIsoFunctor
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import Data.Syntax (Syntax)
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import qualified Data.Syntax as S
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import qualified Data.Syntax.Attoparsec.Text as S
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import qualified Data.Syntax.Char as S
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import qualified Data.Syntax.Pretty as S
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import Data.Text (Text)
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import qualified Data.Text.IO as T
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import qualified Text.PrettyPrint as P
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-- | A simple untyped lambda calculus.
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data AST = Var Text
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| App AST AST
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| Abs Text AST
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deriving (Show)
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$(makePrisms ''AST)
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-- | A variable name.
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name :: Syntax syn Text => syn Text
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name = S.takeWhile1 isAlphaNum
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-- | Encloses a symbol in parentheses.
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parens :: Syntax syn Text => syn a -> syn a
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parens m = S.char '(' */ S.spaces_ */ m /* S.spaces_ /* S.char ')'
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-- | An atom is a variable or an expression in parentheses.
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atom :: Syntax syn Text => syn AST
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atom = _Var /$/ name
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/|/ parens expr
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-- | An expression of our lambda calculus.
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expr :: Syntax syn Text => syn AST
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expr = _App /$/ atom /* S.spaces1 /*/ atom
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/|/ _Abs /$/ S.char '\\' /* S.spaces_
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*/ name /* S.spaces
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/* S.string "->" /* S.spaces
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/*/ expr
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/|/ atom
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main :: IO ()
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main = do
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-- Load the standard input.
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t <- T.getContents
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-- Try to parse it.
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case AP.parseOnly (S.getParser expr <* AP.skipSpace <* AP.endOfInput) t of
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Left err -> putStrLn err
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Right ast -> do
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-- If parsing succeeded print the AST.
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print ast
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-- Try to pretty print it.
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-- (Printing cannot really fail in this example)
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case S.runPrinter expr ast of
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Left err -> putStrLn err
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Right doc -> putStrLn (P.render doc)
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return ()
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