未解析的 flex 记录(需要知道此上下文中所有字段的名称) [英] unresolved flex record (need to know the names of ALL the fields in this context)

问题描述

我一直在尝试使用元组列表作为参数创建一个函数，但我不断收到错误消息:未解析的弹性记录(需要知道此上下文中所有字段的名称)" 我的代码是:

I've been trying to create a function with a tuple-list as an argument but I keep getting the error: "unresolved flex record (need to know the names of ALL the fields in this context)" My code is:

fun convert d = ( (map (#1) d) , (map (#2) d) );

这基本上是试图将一对列表转换为一对列表.我还尝试将 d 的类型声明为 :('a * 'b) list 但这导致了更多错误.我认为这与元组的未知大小有关，可以使用一些帮助来了解它.

This is basicaly trying to convert a list of pairs into a pair of lists.I've also tried to declare the type of d as :('a * 'b) list but that resulted in even more errors. I assume that this has something to do with the unknown size of the tupple and could use some help on how to make it known.

推荐答案

我怀疑您在注释 d 类型时遇到的问题实际上只是您没有用括号，这对我有用:

I suspect that the issue you ran into with annotating the type of d was actually just that you weren't surrounding the annotation with parens, this works for me:

fun convert (d : ('a * 'b) list) = ( (map (#1) d) , (map (#2) d) )

然而，这不是很好的 SML 风格.不鼓励使用 #1、#2、#n 等，因为它会导致这样的问题，你已经失去了一些类型推断为您提供的通常简洁性.

However, this isn't great SML style. Using #1, #2, #n and so on is somewhat discouraged since it leads to issues like this, where you've lost some of the usual succintness that type inference gives you.

相反，您可以为对定义一些显式选择函数:

Instead, you can define some explicit selection functions for pairs:

fun first  (a, _) = a
fun second (_, b) = b

fun convert d = (map first d, map second d)

(请注意，我还从 convert 的主体中删除了一些多余的括号，因为函数应用程序的优先级高于元组构造，而且我还删除了分号这仅在对命令式代码进行排序或在 REPL 中键入代码时才是真正需要的)

(Note that I've also dropped some superfluous parenthesis from the body of convert, since function application has higher precedence then tuple construction, and I've also dropped the semi-colon which is only really necessary when sequencing imperative code or when typing code into the REPL)

这是一个非常好的标准风格指南ML，来自哈佛(或塔夫茨大学)的课程.该文档的旧版本在要避免的常见错误"下特别提到了这一点.

This is a pretty good style guide for Standard ML, from a course at Harvard (or maybe Tufts). An older version of that doc mentioned this specifically under "Common mistakes to avoid".

一些初学者认为获得一对 p 的第二个元素的好方法是编写 #2 p.

Avoid #1 and #2

Some beginners pick up the idea that a good way to get the second element of a pair p is to write #2 p.

这种风格不符合习惯或可读性，它可能会混淆类型检查器.处理对的正确方法是通过模式匹配，所以

This style is not idiomatic or readable, and it can confuse the type checker. The proper way to handle pairs is by pattern matching, so

fun first (x, _) = x
fun second (_, y) = y

是首选，而不是

fun bogus_first p = #1 p (* WRONG *)
fun bogus_second p = #2 p

(由于我不想讨论的原因，这些版本甚至不进行类型检查.)

(For reasons I don’t want to discuss, these versions don’t even type-check.)

如果您的对或元组不是函数的参数，请使用 val 进行模式匹配:

If your pair or tuple is not an argument to a function, use val to do the pattern matching:

val (x, y) = lookup_pair mumble

但通常你可以在普通有趣的匹配中包含匹配.

But usually you can include matching in ordinary fun matching.

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