从ADT创建无限列表 [英] Creating infinite list out of ADT

问题描述

在Haskell中，

 > a = [1,1..]

创建一个无限列表.现在，我有以下内容

creates an infinite list. Now I have the following

data Subunit = O | P deriving (Eq, Show)           

如果我愿意

b :: [Subunit]                                                                   
b = take 6 [P,P..]  

我得到以下信息:

 parse error on input ‘]’

为什么失败了?我需要添加什么才能创建无限列表?

Why this is failing? What I need to add to be able to create an infinite list?

推荐答案

不错！确实，它出错了...

Nice catch! Indeed it errors out ...

> take 10 [P, P..]

<interactive>:6:16: parse error on input ‘]’

...但这不是

> take 10 [P, P ..]   -- one more space
[P,P,P,P,P,P,P,P,P,P]

为什么空白很重要?因为否则语法会与模块前缀的名称重叠，该名称的格式为Module.name.例如，下面是访问Prelude中的运算符.的方法.

Why the whitespace is significant? Because otherwise the syntax overlaps with module-prefixed names, which have the form Module.name. Here's how the operator . from Prelude is accessed, for instance.

> :t (Prelude..)
(Prelude..) :: (b -> c) -> (a -> b) -> a -> c
> :t succ Prelude.. succ   -- infix use!
succ Prelude.. succ :: Enum c => c -> c

因此，P..是模块P中的.，而P ..在列表枚举中工作正常.

Hence, P.. is . from module P, while P .. works fine in a list enumeration.

(是的，这是一个不幸的语法问题……)

(Yes, this is an unfortunate quirk of the syntax ...)

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