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[英]Liberal coverage condition introduced in GHC 7.7 breaks code valid in GHC 7.6
[英]ambiguity check and liberal coverage condition failures for code working under GHC 7.6
我有一個聰明的類型級機械,適用於GHC 7.6但不是更高版本。 回想起來,我不完全確定它為什么會工作,但無論如何我想以某種方式取回這個功能:
{-# 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
這取決於GHC 7.6上的tagged
庫。 我們可以使用它:
$ 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
但是,這至少不能在GHC 7.8.3或更高版本上編譯:
$ 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
我可能能夠解決這個問題。 我使用這個類的唯一地方是形式的簽名:
instance (Foo tab, Applied t tab)=> Bar (Proxy t) where
這可能表明我想讓Foo
變為多態,但這是一個大型復雜的庫,我不知道這種改變是否可行。
如果我擺脫FD,你的ghci示例在ghc-7.8.3中適用於我,並啟用-XAllowAmbiguousTypes
。 該擴展將要求您在instance (Foo tab, Applied t tab)=> Bar (Proxy t)
使用該applied
函數的類型時使用ScopedTypeVariables
進行注釋( instance (Foo tab, Applied t tab)=> Bar (Proxy t)
。
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