Hi,
I've looked at:
http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7
but still have a problem with the attached program that fails with:
GHCi, version 7.1.20101010: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
[1 of 1] Compiling Main ( HasKey.hs, interpreted )
HasKey.hs:17:10:
Couldn't match type `key' with `key1'
because this skolem type variable would escape: `key1'
This skolem is bound by the instance declaration
In the instance declaration for `Ord (Keyed x)'
at the final instance for Ord.
The original code comes from
http://hackage.haskell.org/packages/archive/uni-util/2.2.0.0/doc/html/Util-VariableSet.html
Cheers Christian
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE UndecidableInstances #-}
class Ord key => HasKey x key | x -> key where
toKey :: x -> key
newtype Keyed x = Keyed { unKey :: x }
lift :: (HasKey x1 key1,HasKey x2 key2)
=> (key1 -> key2 -> a) -> (Keyed x1 -> Keyed x2 -> a)
lift f x1 x2 = f (toKey . unKey $ x1) (toKey . unKey $ x2)
instance HasKey x key => Eq (Keyed x) where
(==) = lift (==)
instance HasKey x key => Ord (Keyed x)
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