XIncoherentInstances不起作用
我一直在使用语法库（github上的2.0版本），我有一个小例子，它导致GHC（i）7.6.2抱怨IncoherentInstances 。
{-# LANGUAGE FlexibleInstances,
TypeFamilies,
TypeOperators,
OverlappingInstances,
UndecidableInstances,
GADTs,
FlexibleContexts,
IncoherentInstances #-}
import Data.Syntactic
import Data.Syntactic.Functional
type family FunToAST (dom :: * -> *) f
type instance FunToAST dom (Full a) = ASTF dom a
type instance FunToAST dom (a :-> fi) = ASTF dom a -> FunToAST dom fi
data Let a where
Let :: Let (a :-> (a -> b) :-> Full b)
share :: (Let :<: dom,
ApplySym sig fi dom,
SyntacticN (Denotation sig) fi,
fi ~ FunToAST dom sig)
=> Denotation sig
share = sugarSym Let
精细。 我不问为什么我需要IncoherentInstances 。 问题是，当我包含该扩展时，我得到完全相同的错误消息：
Overlapping instances for SyntacticN
(Denotation sig)
(ASTF sup0 a0 -> ASTF sup0 (a0 -> b0) ->
ASTF sup0 b0)
arising from a use of `sugarSym'
Matching instances:
instance [overlap ok] (Syntactic f, Domain f ~ sym,
fi ~ AST sym (Full (Internal f))) =>
SyntacticN f fi
-- Defined in `Data.Syntactic.Sugar'
instance [overlap ok] (Syntactic a, Domain a ~ sym,
ia ~ Internal a, SyntacticN f fi) =>
SyntacticN (a -> f) (AST sym (Full ia) -> fi)
-- Defined in `Data.Syntactic.Sugar'
(The choice depends on the instantiation of `sig, sup0, a0, b0'
To pick the first instance above, use -XIncoherentInstances
when compiling the other instance declarations)
In the expression: sugarSym Let
In an equation for `share2': share2 = sugarSym Let
我怀疑这个问题实际上与图书馆有关，但我对IncoherentInstances知识不够，无法写出我自己的例子。