Question

在学习Haskell时，我试图理解Haskell中clojure传感器的类型。

{-# LANGUAGE RankNTypes #-}

module Transducers where

-- r = reduced
type Reducer r a = r -> a -> r

type Transducer a b = forall r . Reducer r a -> Reducer r b

我无法理解如何输入以下功能：

-- type inference
transduce :: Foldable t => (t1 -> b -> a -> b) -> t1 -> b -> t a -> b
-- what I actually want
transduce :: forall t1 . Foldable t => Transducer a b -> t1 -> b -> t a -> b
transduce xform f init coll = foldl (xform f) init coll

这给我带来麻烦，它不会编译。我错过了语法方面的东西吗？或者这不可能吗？

Answer 1

在我看来，你可能意味着类似

transduce :: Foldable t => Transducer a b -> Reducer r a -> r -> t b -> r

作为user2407038 suggested，如果您想强制调用者提供Transducer，您只需要这样一种花哨的类型。否则，您可以将其简化为

transduce :: Foldable t => (x -> Reducer r b) -> x -> r -> t b -> r

Haskell排名n类型＆amp;类型语法

1 个答案: