在下面的代码中,我试图匹配GADT构造函数Cons
以使编译器看到xs
非空:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
import Data.Typeable
data Foo (ts :: [*]) where
Nil :: Foo '[]
Cons :: (Typeable t) => Foo ts -> Foo ( t ': ts)
foo :: Foo xs -> IO ()
foo Nil = print "done"
foo (Cons rest :: Foo (y ': ys)) = do
print $ show $ typeRep (Proxy::Proxy y)
foo rest
不幸的是,这个简单的例子无法用GHC 8编译:
• Couldn't match type ‘xs’ with ‘y : ys’
‘xs’ is a rigid type variable bound by
the type signature for:
foo :: forall (xs :: [*]). Foo xs -> IO ()
Expected type: Foo (y : ys)
Actual type: Foo xs
• When checking that the pattern signature: Foo (y : ys)
fits the type of its context: Foo xs
In the pattern: Cons rest :: Foo (y : ys)
In an equation for ‘foo’:
foo (Cons rest :: Foo (y : ys))
= print $ (show $ typeRep (Proxy :: Proxy y))
我知道类型推断对于GADT来说可能很棘手(例如#9695,#10195,#10338),但这是所以简单...
我需要做些什么来说服GHC当我在Cons
上匹配时,GADT参数至少有一个元素?
答案 0 :(得分:3)
您需要的只是从Proxy t
Foo (t ': ts)
的功能
fooFstType :: Foo (t ': ts) -> Proxy t
fooFstType _ = Proxy
请注意,由于Foo
的类型参数为t ': ts
,而不仅仅是ts
,因此您可以引用表示类型签名中第一个元素的类型变量(而不是不知何故,身体与ScopedTypeVariables
)。
您的功能变为
foo :: Foo xs -> IO ()
foo Nil = print "done"
foo f@(Cons rest) = do
print $ show $ typeRep (fooFstType f)
foo rest
另一种可能性是将工作转移到类型级别:
type family First (xs :: [k]) :: k where
First (x ': xs) = x
foo :: forall xs . Foo xs -> IO ()
foo Nil = print "done"
foo (Cons rest) = do
print $ show $ typeRep (Proxy :: Proxy (First xs))
foo rest