可比性在榆树中意味着什么？ [英] What does comparable mean in Elm?

问题描述

我很难理解榆树中可比的到底是什么。榆树似乎和我一样困惑。

I'm having trouble understanding what exactly a comparable is in Elm. Elm seems as confused as I am.

在REPL上：

> f1 = (<)
<function> : comparable -> comparable -> Bool

所以 f1 接受可比对象。

> "a"
"a" : String
> f1 "a" "b"
True : Bool

所以看起来 String 是可比较的。

So it seems String is comparable.

> f2 = (<) 1
<function> : comparable -> Bool

所以 f2 接受可比较的内容。 / p>

So f2 accepts a comparable.

> f2 "a"
As I infer the type of values flowing through your program, I see a conflict
between these two types:

    comparable

    String

所以字符串 是和不具有可比性吗？

为什么 f2 的类型不是 number- >布尔？ f2 还能接受什么其他可比对象？

So String is and is not comparable?
Why is the type of f2 not number -> Bool? What other comparables can f2 accept?

推荐答案

通常，当您看到Elm中某个类型中的type变量，此变量不受限制。然后，当您提供某种特定类型的东西时，该变量将被该特定类型替换：

Normally when you see a type variable in a type in Elm, this variable is unconstrained. When you then supply something of a specific type, the variable gets replaced by that specific type:

-- says you have a function:
foo : a -> a -> a -> Int
-- then once you give an value with an actual type to foo, all occurences of `a` are replaced by that type:
value : Float
foo value : Float -> Float -> Int

可比是类型变量，具有内置特殊含义。意思是说，它将仅与可比较类型匹配，例如 Int ， String 和其他一些类型。但除此之外，它的行为应相同。因此，我认为类型系统中存在一个小错误，因为您得到了：

comparable is a type variable with a built-in special meaning. That meaning is that it will only match against "comparable" types, like Int, String and a few others. But otherwise it should behave the same. So I think there is a little bug in the type system, given that you get:

> f2 "a"
As I infer the type of values flowing through your program, I see a conflict
between these two types:

    comparable

    String

如果错误不存在，您将得到：

If the bug weren't there, you would get:

> f2 "a"
As I infer the type of values flowing through your program, I see a conflict
between these two types:

    Int

    String

编辑：我打开了问题针对此错误

I opened an issue for this bug

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