GHC 7.6下工作的代碼的歧義檢查和寬泛的覆蓋條件失敗

Question

我有一個巧妙的類型級別的機制，可以在GHC 7.6上工作，但不是更高的版本。回首我不完全知道爲什麼它沒有計算過，但無論如何，我想找回這個功能莫名其妙：

{-# LANGUAGE 
    PolyKinds 
    , FunctionalDependencies , FlexibleInstances , FlexibleContexts 
    , OverlappingInstances 
    , ScopedTypeVariables 
    , TypeFamilies 
    , UndecidableInstances 
#-} 
module M where 

import Data.Proxy 

-- | A relation between a (maybe-partially-applied) type and that type fully 
-- applied. 
class Applied t (tab :: *) | t -> tab where 
    -- | Fully apply a type @t@ with polymorphic arguments, yielding @tab@. 
    applied :: Proxy t -> Proxy tab 

instance Applied (t a) tab=> Applied t tab where 
    applied _ = applied (Proxy :: Proxy (t a)) 

instance t ~ tab=> Applied t tab where -- always matches when `t` is kind `*` 
    applied _ = Proxy :: Proxy tab 

這依賴於tagged庫上GHC 7.6。我們可以用它喜歡：

$ ghci-7.6.3 
Prelude> :l M.hs 
[1 of 1] Compiling M    (M.hs, interpreted) 
Ok, modules loaded: M. 
*M> 
*M> :t applied (Proxy :: Proxy Either) 
applied (Proxy :: Proxy Either) :: Proxy (Either a a1) 
*M> (return $ Right 'a') == applied (Proxy :: Proxy Either) 
True 

然而，這並不編譯至少GHC 7.8.3或更高版本：

$ ghci-7.8.3 
GHCi, version 7.8.3: http://www.haskell.org/ghc/ :? for help 
Loading package ghc-prim ... linking ... done. 
Loading package integer-gmp ... linking ... done. 
Loading package base ... linking ... done. 
Prelude> :l M.hs 
[1 of 1] Compiling M    (M.hs, interpreted) 

M.hs:19:10: 
    Could not deduce (Applied (t a0) tab) 
     arising from the ambiguity check for an instance declaration 
    from the context (Applied (t a) tab) 
     bound by an instance declaration: 
       Applied (t a) tab => Applied t tab 
     at M.hs:19:10-42 
    The type variable ‘a0’ is ambiguous 
    In the ambiguity check for: 
     forall (k :: BOX) (k1 :: BOX) (t :: k1 -> k) tab (a :: k1). 
     Applied (t a) tab => 
     Applied t tab 
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes 
    In the instance declaration for ‘Applied t tab’ 

M.hs:19:10: 
    Illegal instance declaration for ‘Applied t tab’ 
     The liberal coverage condition fails in class ‘Applied’ 
     for functional dependency: ‘t -> tab’ 
     Reason: lhs type ‘t’ does not determine rhs type ‘tab’ 
    In the instance declaration for ‘Applied t tab’ 
Failed, modules load 

我覺得answer here是相關的，但我還不知道這個建議。

有可能我可以解決這個問題。我使用這個類的唯一地方是在形式的簽名：

instance (Foo tab, Applied t tab)=> Bar (Proxy t) where 

這可能表明我要讓Foo種多態，但這是一個複雜的大圖書館，我不知道這是否改變將是可能的。

Answer 1

如果我擺脫了FD，並啓用-XAllowAmbiguousTypes，您的ghci示例適用於ghc-7.8.3中的我。該擴展需要您在instance (Foo tab, Applied t tab)=> Bar (Proxy t)中使用applied函數的註釋（使用ScopedTypeVariables）。