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
<sedrikov@gmail.com>
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