haskell - 为什么这个单射类型族实际上不是单射的？

Question

我尝试过这个:

{-# 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，单射性的全部意义在于 可以从 F a ~ F b推导出 a ~ b？

Answer 1

原来这是a known issue in GHC 。显然这是因为它尚未被证明是正确的。