Haskell中的参数数目和无点数 [英] Number of arguments and point-free in Haskell

问题描述

使用多个模式匹配，即使没有点，也无法使用不同数量的参数！

With multiple pattern-matching, different numbers of arguments are impossible, even with point-free!

foo True b = b + 2
foo _ = id

例如，

无效.但是

doesn't work for example. But

foo True = (+2)
foo _ = id

确实.有时我们只能在函数的一部分中使用无点，所以...

does. Sometimes we can use point-free only in one part of the function, so...

为什么?对GHC来说太难了吗? :'(

Why? Is it too hard for GHC? :'(

推荐答案

为什么?对GHC来说太难了吗?

Why? Is it too hard for GHC?

否.对于GHC来说一点都不难.实际上，这是Haskell报告的错.

No. It is not at all too hard for GHC. Actually, this is the fault of the Haskell Report.

请参阅: Haskell报告2010>声明和绑定>函数绑定

函数绑定将变量绑定到函数值.变量x的函数绑定的一般形式为:

A function binding binds a variable to a function value. The general form of a function binding for variable x is:

x p1 ₁ …p1 _k match1
…
x pn ₁ …pn _k 匹配

x p1₁ … p1_k match1
…
x pn₁ … pn_k matchn

[...等等等等...

[...blah blah...]

翻译:函数的一般绑定形式在语义上等价于等式(即简单模式绑定):

Translation: The general binding form for functions is semantically equivalent to the equation (i.e. simple pattern binding):

x = \ x ₁ …x _k -> case(x ₁ ，…，x _k 的

x = \ x₁ … x_k -> case (x₁, …, x_k) of

(p11，…，p1k)match1
…
(pn1，…，pnk)matchn
其中x _i 是新标识符.

(p11, …, p1k) match1
…
(pn1, …, pnk) matchn
where the x_i are new identifiers.

(重点是我的)

虽然函数定义与lambda&案例表达，不一定像Mihai所建议的那样编译.

While function definitions are semantically equivalent to a lambda & case expression, they are not necessarily compiled that way, as Mihai suggests.

问题是，Haskell报告定义函数声明，以使它们必须在等式的左侧具有相同数量的输入.这可以通过以下事实清楚地表明:k在第1和第n个函数声明行上都保持相同(并且暗示之间的所有行). 这是限制的原因； 与GHC的实施细节无关.

The thing is, the Haskell report defines function declarations such that they must have the same number of inputs on the left-hand side of the equation. This is made clear by the fact that k remains the same on both the 1st and the nth function declaration lines (and by implication, all lines in-between). This is the reason for the restriction; it has nothing to do with GHC's implementation details.

tl; dr

不允许这样做的选择只是风格问题. –八月

The choice not to allow it is just a matter of style. – augustss

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