Can't deduce f = f₁ from f x = f₁ y?
{-# LANGUAGE GADTs #-}
data Foo x y where
Composition :: Foo b c -> Foo a b -> Foo a c
FMap :: Functor f => (a->b) -> Foo (f a) (f b)
asFunction :: Foo a b -> a->b
asFunction (FMap m) = fmap m
-- asFunction (Composition (FMap m) (FMap n)) = fmap m . fmap n
asFunction (Composition m n) = asFunction m . asFunction n
This works as expected... until you uncomment the second clause of asFunction ! This is actually just an inlined version of a special case the two other patterns already match, so I'd expect it to be ok. But ( ghc-7.6.2 , or also ghc-7.4.1 )
Could not deduce (f ~ f1)
from the context (b1 ~ f a1, b ~ f b2, Functor f)
bound by a pattern with constructor
FMap :: forall (f :: * -> *) a b.
Functor f =>
(a -> b) -> Foo (f a) (f b),
in an equation for `asFunction'
...
Why does this happen, and why then not in the other clauses? What exactly should be done to prevent this trouble in more complicated applications?
这可能与为了支持更灵活（“不饱和”）类型的功能而暂时从GHC的类型推断系统中删除的特征的左/右分解特征相关，然后当它被发现具有这样的恼人效果时被重新引入。
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