G'day all.
Quoting Hugo Macedo <[EMAIL PROTECTED]>:
After checking the Haskell98 grammar I found out that this is allowed
syntactically and probably semantically too. Is there any reason to do that?
One thing that hasn't come up yet is that empty instance declarations are
the only decent option (that I know of) that we have in the absence of
real class aliases. Here's an example from Dfa.lhs, which could probably
be written slightly more simply now:
{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-undecidable-instances #-}
import Control.Monad.Identity
import Control.Monad.Reader
import Control.Monad.State
data (Ord t) => ReRead t
= {- detail unimportant -}
data (Ord t) => ReState t
= {- detail unimportant -}
type ReM m t a = StateT (ReState t) (ReaderT (ReRead t) m) a
class (Monad m, Ord t) => ReVars m t where { }
instance (Monad m, Ord t) => ReVars m t where { }
remNullSet :: (ReVars m t) => ReM m t (SimplRe t)
{- etc -}
class (ReVars m t, MonadIO m, Show t) => ReVarsIO m t where { }
instance (ReVars m t, MonadIO m, Show t) => ReVarsIO m t where { }
remDump :: (ReVarsIO m t) => ReM m t ()
{- etc -}
Cheers,
Andrew Bromage
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