在SML中将字符串与文件夹连接 [英] Concatenating strings with foldr in SML

问题描述

我正在尝试声明一个函数，字符串列表->字符串，例如使用
[ Chicago， city， USA] 应该返回美国芝加哥城市 。到目前为止，我所做的是：

I'm trying to declare a function, string list -> string, that with the input for instance ["Chicago","city","USA"] should return "Chicago city USA". What I did so far was this:

fun gather ts = foldr op ^ "" ts;

这似乎有些相似，但是问题是，我想包括空格在单词之间，因为此函数将返回 ChigagocityUSA 。

This seems to be somewhat along the lines, however the problem is, I would like to include the spaces between the words, as this function would return "ChigagocityUSA".

推荐答案

是的，问题在于 ^ 是一个函数，它为两个字符串 foo和 bar返回 foobar，尽管您希望使用 foo bar。

Yes, the problem is that ^ is a function that for two strings "foo" and "bar" returns "foobar", although you want "foo bar".

因此，您需要定义一个函数，该函数接受两个字符串参数（作为一个元组），并返回两个字符串，且两个字符串之间有一个空格（因此 string1 ^ ^ string2 ）。

So what you need to do is to define a function that takes two string arguments (as a tuple) and returns the two strings with a space in between them (so string1 ^ " " ^ string2).

然后您可以将该函数作为 foldr的参数并获得所需的结果。

You can then give that function as an argument to foldr and get the result you want.

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