Hello.
Today I wrote a small program to experiment with the Applicative
class. The program is supposed to use an "applicative reader", similar
to a "monad reader", to evaluate arithmetic expressions.
But when compiling the program with ghc-7.6.1, I get the following message:
$ ghc --make applicative-eval
[1 of 1] Compiling Main ( applicative-eval.hs, applicative-eval.o
)
ghc: panic! (the 'impossible' happened)
(GHC version 7.6.1 for x86_64-unknown-linux):
expectJust cpeBody:collect_args
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
If the line
eval (Let s a b) = \m -> eval b ((s,eval a m):m)
is commented out, the program compiles without problems.
Is this a known issue with the compiler?
The source code is attached.
Romildo
module Main where
import Control.Applicative (pure,(<*>),(<$>),(<$),(<*),(*>))
import Text.Parsec
import System.IO (stdout,hSetBuffering,BufferMode(NoBuffering))
data Exp = Cte Integer
| Var String
| Sum Exp Exp
| Sub Exp Exp
| Mul Exp Exp
| Div Exp Exp
| Let String Exp Exp
deriving (Show)
type Memory = [(String,Integer)]
eval :: Exp -> (->) Memory Integer
eval (Cte i) = pure i
eval (Var s) = \m -> case lookup s m of
Just v -> v
Nothing -> 0
eval (Sum a b) = (+) <$> eval a <*> eval b
eval (Sub a b) = (-) <$> eval a <*> eval b
eval (Mul a b) = (*) <$> eval a <*> eval b
eval (Div a b) = (div) <$> eval a <*> eval b
eval (Let s a b) = \m -> eval b ((s,eval a m):m)
pExp, pTerm, pFactor :: Parsec String () Exp
pExp = chainl1 pTerm (lexeme (Sum <$ char '+' <|> Sub <$ char '-'))
pTerm = chainl1 pFactor (lexeme (Mul <$ char '*' <|> Div <$ char '/'))
pFactor = Cte <$> lexeme pInteger <|>
Var <$> lexeme pVariable <|>
lexeme (char '(') *> pExp <* lexeme (char ')')
pInteger = read <$> many1 digit
pVariable = (:) <$> letter <*> many (alphaNum <|> char '-')
lexeme p = p <* spaces
main = do hSetBuffering stdout NoBuffering
calc 1
calc n = do putStr ("[" ++ show n ++ "] ")
input <- getLine
case parse pExp "-" input of
Left err -> putStrLn (show err)
Right exp -> do putStrLn (show exp)
putStrLn (show (eval exp []))
calc (n+1)
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