SML 列表相等奇怪

发布于 2024-10-08 03:54:34 字数 385 浏览 12 评论 0原文

我有这样一段代码：

fun foldr2(f, x::xs) =
    if xs = [] then
      x
    else
      f(x, foldr2(f, xs))

使用类型签名

(''a * ''a -> ''a) * ''a list -> ''a

看起来非常简单，它需要一个作用于相等类型的函数和一个相等类型列表作为参数，因为 xs = [] 比较。但是，由于某种原因，当在 SML/NJ 中实数不是相等类型时，它适用于诸如 (op +, [2.3, 2.7, 4.0]) 之类的输入。谁能帮我解释一下为什么会发生这种魔法？

原文

I have this bit of code:

fun foldr2(f, x::xs) =
    if xs = [] then
      x
    else
      f(x, foldr2(f, xs))

With the type signature

(''a * ''a -> ''a) * ''a list -> ''a

Looks pretty straight-forward, it takes a function that works over equality types and a list of equality type as arguments, because of the xs = [] comparison. However, for some reason it works on input such as (op +, [2.3, 2.7, 4.0]), when in SML/NJ reals are not an equality type. Can anyone help me shed some light on why this magic occurs?

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握住我的手 2024-10-15 03:54:34

我相信这与 + 重载实数的神奇方式有关。对我来说，这几乎接近于编译器错误，尽管我必须查看 SML97 定义才能准确了解正确的行为是什么。恕我直言，+ 上的重载是 SML 中令人讨厌的黑暗角落。

例如，如果您定义一个类型为 real * real -> 的函数real 并将其作为参数传递给 foldr2 你会得到你所期望的类型错误：

fun f (x : real * real) = 134.5
foldr2 (f, [1.4, 2.25, 7.0])
  stdIn:8.1-8.29 Error: operator and operand don't agree [equality type required]

如果你只是向 op +< 添加类型注释，你甚至可以引发类型错误/code>，这基本上让我得出这样的结论：正是 + 的重载导致了神秘的效果。

I believe it's to do with the magical way in which + is overloaded for reals. To me, this almost verges on being a compiler bug, although I would have to look at the SML97 definition to see exactly what the correct behaviour is meant to be. The overloading over + is something of a nasty dark corner in SML, IMHO.

For example, if you define a function that is of type real * real -> real and pass that as an argument to foldr2 you get the type error you were expecting:

fun f (x : real * real) = 134.5
foldr2 (f, [1.4, 2.25, 7.0])
  stdIn:8.1-8.29 Error: operator and operand don't agree [equality type required]

You can even induce the type error if you just add a type annotation to op +, which basically led me to the conclusion that it is the overloading of + that is causing the mysterious effect.

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