Sexps.hs

{-# language PatternSynonyms #-}

import Text.ParserCombinators.Parsec hiding (token)

data SExpr info atom
    = Atom  info atom
    | SExpr info [SExpr info atom]

sexpr_elim onAtom onSexpr = recure
  where
    recure sexpr = case sexpr of
        Atom  info atom     -> onAtom info atom
        SExpr info children -> onSexpr info (map recure children)

instance (Show info, Show atom) => Show (SExpr info atom) where
    show = sexpr_elim (\i -> show) (\_ list -> "(" ++ unwords list ++ ")")

sexpr :: Parser info -> Parser atom -> Parser (SExpr info atom)
sexpr info atom = recure
  where
    recure
        =   info >>= \info ->   
        (   pure (SExpr info)
                <*  token "("
                <*> many recure
                <*  token ")"
        <|> pure (Atom info)
                <*> atom
        )

token s = try (string s) <* spaces