为什么这个内射型家族实际上不是单射的？

Question

我试过这个：

{-# LANGUAGE TypeFamilyDependencies #-}
module Injective where

type family F (a :: *) = (fa :: *) | fa -> a

convert :: F a ~ F b => a -> b
convert x = x

GHC 8.6.4给了我这个错误

    • Could not deduce: a ~ b
      from the context: F a ~ F b
        bound by the type signature for:
                   convert :: forall a b. (F a ~ F b) => a -> b
        at Injective.hs:6:1-30

为什么？当然，注入的全部意义在于，人们可以从a ~ b中推断出F a ~ F b？

Answer 1

原来这是a known issue in GHC。显然这是因为它没有被证明是合理的。