Perhaps you should give us the error the compiler give you.
Plus:
data LegGram nt t s = Ord nt => LegGram (M.Map nt [RRule nt t s])
will become invalid. Currently, such class constraints are ignored.
You should remove the 'Ord nt' constraint and add it to you legSome
function. (Maybe that's a track to solve your problem...)
You have also another solution: make your LegGram type available *for
all*Ord nt (with GADTs or ExistentialQuantification), thus making you
unable to
know which type 'nt' exactly is:
data LegGram t s = forall nt. Ord nt => LegGram (M.Map nt [RRule nt t s])
or
data LegGram t s where
LegGram :: Ord nt => M.Map nt [RRule nt t s] -> LegGram t s
should be both valid. I tend to prefer the latter (the use of a GADT), as
it makes you declare and handle your type constructor just like any
function.
But I don't know if it fits you requirements.
2012/1/3 AUGER Cédric
Hi all, I am an Haskell newbie; can someone explain me why there is no reported error in @legSome@ but there is one in @legSomeb@
(I used leksah as an IDE, and my compiler is: $ ghc -v Glasgow Haskell Compiler, Version 7.2.1, stage 2 booted by GHC version 6.12.3 )
What I do not understand is that the only difference was a typing anotation to help the type inference, but I believed that this annotation was already given by the signature I give, so I am quite lost.
Thanks in advance!
====================================================================== {-# OPTIONS_GHC -XScopedTypeVariables #-} -- why isn't this option always enabled...
{-# OPTIONS_GHC -XGADTs #-}
import Data.Word import qualified Data.Map as M import qualified Data.Set as S
data Symbols nt t = NT nt -- ^ non terminal | T t -- ^ terminal deriving (Eq, Ord)
type Sem s = [s]->s
data Rule nt t s = Rule { refined :: nt , expression :: [Symbols nt t] , emit :: Sem s }
type RRule nt t s = ([Symbols nt t], Sem s) data LegGram nt t s = Ord nt => LegGram (M.Map nt [RRule nt t s])
legSome :: LegGram nt t s -> nt -> Either String ([t], s) -- ^^^^^^^^^^^^^^ -- isn't this redundant? -- vvvvvvvvvvvvvv legSome ((LegGram g)::LegGram nt t s) ntV = case M.lookup ntV g of Nothing -> Left "No word accepted!" Just l -> let sg = legSome (LegGram (M.delete ntV g)) subsome :: [RRule nt t s] -> Either String ([t], s) subsome [] = Left "No word accepted!" subsome ((r,sem):l) = let makeWord [] = Right ([],[]) makeWord ((NT nnt):ll) = do (m, ss) <- sg nnt (mm, sss) <- makeWord ll return (m++mm, ss:sss) makeWord ((T tt):ll) = do (mm, sss) <- makeWord ll return (tt:mm, sss) in case makeWord r of Right (ll, mm) -> Right (ll, sem mm) Left err -> subsome l in subsome l
legSomeb :: LegGram nt t s -> nt -> Either String ([t], s) -- but without it I have an error reported legSomeb (LegGram g) ntV = case M.lookup ntV g of Nothing -> Left "No word accepted!" Just l -> let sg = legSomeb (LegGram (M.delete ntV g)) subsome :: [RRule nt t s] -> Either String ([t], s) subsome [] = Left "No word accepted!" subsome ((r,sem):l) = let makeWord [] = Right ([],[]) makeWord ((NT nnt):ll) = do (m, ss) <- sg nnt (mm, sss) <- makeWord ll return (m++mm, ss:sss) makeWord ((T tt):ll) = do (mm, sss) <- makeWord ll return (tt:mm, sss) in case makeWord r of Right (ll, mm) -> Right (ll, sem mm) Left err -> subsome l in subsome l
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe