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我在让 GHC 在一个应该很明显的地方推断出一个类型时遇到了问题。以下是演示问题的完整片段。

{-# LANGUAGE DataKinds, ScopedTypeVariables, KindSignatures, TypeOperators, GADTs #-}

import Data.Reflection
import Data.Proxy
import Data.Tagged

-- heterogeneous list, wrapping kind [*] as *
data HList :: [*] -> * where
              HNil :: HList '[]
              HCons :: a -> HList as -> HList (a ': as)

main = test2

test1 = do
    let x = HCons 3 HNil :: HList '[Int]
        c = case x of (HCons w HNil) -> w
    print c

test2 = reify True (\(_::Proxy a) -> do

    let x = HCons (Tagged 3) HNil :: HList '[Tagged a Int]
        c = case x of (HCons w HNil) -> w
    print $ untag (c :: Tagged a Int))

在中，正如我所期望的那样，我可以在不给出明确类型的情况下进行test1打印。的类型由 : 上的显式签名推断，即has type中的第一个元素。cccxHListInt

然而test2，在 中，c需要显式签名。如果我只是print $ untag c在test2，我得到

Test.hs:22:32:
    Couldn't match type `s0' with `s'
      `s0' is untouchable
           inside the constraints (as ~ '[] *)
           bound at a pattern with constructor
                      HNil :: HList ('[] *),
                    in a case alternative
      `s' is a rigid type variable bound by
          a type expected by the context:
            Reifies * s Bool => Proxy * s -> IO ()
          at Test.hs:19:9
    Expected type: Tagged * s0 Int
      Actual type: a
    In the pattern: HNil
    In the pattern: HCons w HNil
    In a case alternative: (HCons w HNil) -> w

为什么 GHC 不能从给定的显式类型推断出c类型？xtest1

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1 回答 1

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I've found these errors to be related to let-bindings... though I don't know the precise cause or if it's actually bug in GHC. The workaround is to use a case statement instead:

test4 = reify True $ \ (_::Proxy a) -> do
  let x = HCons (Tagged 4) HNil :: HList '[Tagged a Int]
      c = case x of (HCons w HNil) -> w
  print $ untag (c :: Tagged a Int)

test5 = reify True $ \ (_::Proxy a) -> do
  case HCons (Tagged 5) HNil :: HList '[Tagged a Int] of
    HCons w HNil -> print $ untag w
于 2013-09-12T18:23:30.253 回答