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我对以下代码中类型检查器的行为感到困惑：

{-# LANGUAGE ScopedTypeVariables 
 , TypeFamilies , MultiParamTypeClasses, FlexibleContexts #-}

class (BarClass (Foo a) a)=> FooClass a where
    type Foo a
    foo :: Foo a -> a

    -- THIS WORKS:
    theBarFooOf :: a -> Bar (Foo a) a
    foo = bar $ theBarFooOf (undefined :: a)

    -- THIS DOES NOT:
 -- theBarFoo :: Bar (Foo a) a
 -- foo = bar (theBarFoo :: Bar (Foo a) a)

class BarClass t r where
    type Bar t r
    bar :: (Bar t r) -> t -> r

第二个版本（foo通过稍微简单的方法实现theBarFoo）生成以下错误：

Could not deduce (Bar (Foo a) a ~ Bar (Foo a0) a0)
from the context (FooClass a)
  bound by the class declaration for `FooClass'
  at demo.hs:(5,1)-(15,42)
NB: `Bar' is a type function, and may not be injective
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Expected type: Bar (Foo a) a
  Actual type: Bar (Foo a) a
In the first argument of `bar', namely
  `(theBarFoo :: Bar (Foo a) a)'
In the expression: bar (theBarFoo :: Bar (Foo a) a)
In an equation for `foo': foo = bar (theBarFoo :: Bar (Foo a) a)

当实例头的范围内时，为什么值应该Bar (Foo a) a是模棱两可a的，但这工作正常：a -> Bar (Foo a) a？这是一个错误吗？

我正在使用 GHC 7.6.3。

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