Hello
the following program fails to load in Hugs2005 with all Haskell
extensions enabled:
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-}
class (Monad m) => Stream m h where
class (Stream m h) => CharStream m h where
vGetChar :: h -> m Char
vGetContents :: h -> m String
vGetContents h = func
where func = do vGetChar h
return ""
class (Stream m h) => ByteStream m h where
vGetByte :: h -> m ()
instance (CharStream m h) => ByteStream m h where
vGetByte h = undefined
instance (ByteStream m h) => CharStream m h where
vGetChar = undefined
message printed is:
ERROR file:.\SystemStreamClasses.hs -
*** The type checker has reached the cutoff limit while trying to
*** determine whether:
*** CharStream a b
*** can be deduced from:
*** ()
*** This may indicate that the problem is undecidable. However,
*** you may still try to increase the cutoff limit using the -c
*** option and then try again. (The current setting is -c40)
this program compiles ok in GHC 6.4.1. if call to `func` is replaced
with its body, then program loads ok:
vGetContents h = do vGetChar h
return ""
can problem with loading my code in Hugs be resolved by giving to
`func` some type signmature? i can't replace call with its body
because code in real program is much more complicated:
vGetContents h =
let loop =
let func = do c <- vGetChar h
next <- loop
c `seq` return (c : next)
handler e = if isEOFError e then return []
else mError e
in mCatch func handler
in
do firstchar <- vGetChar h
rest <- loop
return (firstchar : rest)
class (Monad m) => MonadHelper m where
mError :: IOError -> m a
mCatch :: m a -> (IOError -> m a) -> m a
class (Show h, MonadHelper m) => Stream m h | h -> m where
....
--
Best regards,
Bulat mailto:bulatz@HotPOP.com
