51 lines
1.2 KiB
Haskell
51 lines
1.2 KiB
Haskell
{-# LANGUAGE TemplateHaskell, TypeFamilies, DeriveFunctor, DeriveFoldable, DeriveTraversable #-}
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module UntypedLambdaCalculus.Parser ( Ast (AstVar, AstLam, AstApp)
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, AstF (AstVarF, AstLamF, AstAppF)
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, expr
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) where
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import Data.Functor.Foldable (cata)
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import Data.Functor.Foldable.TH (makeBaseFunctor,)
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import Data.List (foldl1')
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import Text.Parsec
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import Text.Parsec.String
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data Ast = AstVar String
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| AstLam String Ast
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| AstApp Ast Ast
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makeBaseFunctor ''Ast
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instance Show Ast where
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show = cata alg
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where alg (AstVarF v) = v
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alg (AstLamF v e) = "(\\" ++ v ++ ". " ++ e ++ ")"
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alg (AstAppF f x) = "(" ++ f ++ " " ++ x ++ ")"
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name :: Parser String
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name = do
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c <- letter
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cs <- many alphaNum
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return $ c : cs
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var :: Parser Ast
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var = AstVar <$> name
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parens :: Parser a -> Parser a
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parens = between (char '(') (char ')')
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lam :: Parser Ast
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lam = do
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char '\\'
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vars <- name `sepBy1` spaces
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char '.'
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spaces
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body <- expr
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return $ foldr AstLam body vars
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safeExpr :: Parser Ast
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safeExpr = var <|> parens (lam <|> expr)
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expr :: Parser Ast
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expr = foldl1' AstApp <$> sepBy1 safeExpr spaces
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