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 {- CIS 194 HW 11
    due Monday, 8 April
 -}
 
 module Week11.SExpr where
 
 import Week11.AParser
 import Control.Applicative
 
 import Data.Char(isSpace, isAlpha, isAlphaNum)
 
 ------------------------------------------------------------
 --  1. Parsing repetitions
 ------------------------------------------------------------
 
 zeroOrMore :: Parser a -> Parser [a]
 zeroOrMore p = many_v
     where many_v = some_v <|> pure []
           some_v = (:) <$> p <*> many_v
 
 oneOrMore :: Parser a -> Parser [a]
 oneOrMore p = some_v
     where many_v = some_v <|> pure []
           some_v = (:) <$> p <*> many_v
 
 ------------------------------------------------------------
 --  2. Utilities
 ------------------------------------------------------------
 
 spaces :: Parser String
 spaces = zeroOrMore (satisfy isSpace)
 
 ident :: Parser String
 ident = (:) <$> satisfy isAlpha <*> zeroOrMore (satisfy isAlphaNum)
 
 ------------------------------------------------------------
 --  3. Parsing S-expressions
 ------------------------------------------------------------
 
 -- An "identifier" is represented as just a String; however, only
 -- those Strings consisting of a letter followed by any number of
 -- letters and digits are valid identifiers.
 type Ident = String
 
 -- An "atom" is either an integer value or an identifier.
 data Atom = N Integer | I Ident
   deriving Show
 
 -- An S-expression is either an atom, or a list of S-expressions.
 data SExpr = A Atom
            | Comb [SExpr]
   deriving Show
 
 parseAtom :: Parser Atom
 parseAtom = ( N <$> (spaces *> posInt)) <|> ( I <$> (spaces *> ident))
 
 
 parseSExpr :: Parser SExpr
 parseSExpr = (A <$> parseAtom) <|> Comb <$> (spaces *> char '(' *> oneOrMore (parseSExpr) <* spaces <* char ')')