如何在Haskell实例中使用类型变量

Question

我尝试在类型实例的where-body中使用type变量.但是GHC不对类型实例中的函数使用类型变量.

我尝试实现类型类Bits的[a].

instance forall a. Bits a => Bits [a] where
    xor = zipWith xor
    rotateL list dis = keeped .|. overlap
    where
        overlap = tail moved ++ [head moved] 
        (keeped, moved) = unzip $ map (\n -> let rot = rotate n dis in (rot.&.mask, rot.&.filter)) list
        mask = (complement 0) `shiftL` dis -- this line
        filter = complement mask

GHC说:

Could not deduce (Num a) arising from the literal ‘0’

预期:

这0应该是类型a,它是在中定义的类型变量instance forall a. Bits a => Bits [a]

Answer 1

在不同的上下文中有不同的方法来写"零".

你只有一个约束Bits a,那么写一个"零"的方法就是zeroBits.

0是具有Num a实例的类型的"零" .