Haskell/GHC:用相同的模式匹配多个一元构造函数 [英] Haskell/GHC: Matching multiple unary constructors with the same pattern

问题描述

因此，我一直在尝试定义TrieSet数据类型(即使我知道我不需要):

So I was playing around with defining a TrieSet datatype (even though I know I don't need to):

module Temp where

import Data.Map

data TrieSet a = Nonterminal (Data.Map a (TrieSet a)) | Terminal (Data.Map a (TrieSet a))

insert :: Ord a => [a] -> TrieSet a -> TrieSet a
insert [] (_ m) = Terminal m
insert (a:as) (c m) = c $ insertWith (insert as . flip const) a (insert as $ Nonterminal empty) m

当我遇到错误时，我从未见过:

When I got an error I've never seen before:

% ghc -c Temp.hs
Temp.hs:8:11: Parse error in pattern

因此，GHC似乎不喜欢以相同的模式匹配多个一元构造函数.我进行了另一项测试，以确保这是问题所在:

So it seemed like GHC doesn't like matching multiple unary constructors with the same pattern. I did another test to make sure that was the problem:

module Temp2 where

extract :: Either String String -> String
extract (_ s) = s

似乎证实了我的怀疑:

% ghc -c Temp2.hs
Temp2.hs:4:9: Parse error in pattern

所以我的问题是(分为多个部分):

So my question is (in multiple parts):

1. 我对GHC为什么不喜欢这些功能吗?
2. 为什么这不是Haskell标准的一部分?毕竟，我们可以用相同的模式匹配多个null构造函数.
3. 我可以给GHC使其接受这些语言吗?

推荐答案

1. 是的.从来没有支持过这种通配符.
2. 在我看来，如果您不知道匹配的数据构造函数，则很难推断出函数的类型.只需考虑一个函数 f(_ n)= n .它的类型应该是什么?Haskell的类型系统无法描述类型构造函数的多样性，因此不存在像 f 这样的函数.
3. 我不这样认为.

1. Yes. That kind of wildcards was never supported.
2. In my opinion, it would be much more difficult to infer a function's type if you don't know the data-constructor that was matched on. Just think about a function f (_ n) = n. What should be its type? The type-system of Haskell has no way to describe the arity of a type's constructors, so a function like f could not exist.
3. I don't think so.

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