F#中的列表理解 [英] list comprehension in F#

问题描述

我正在尝试在 F# 中进行一些列表理解.我发现了这个.

I am trying to do some list comprehension in F#. And I found this.

let evens n =
    { for x in 1 .. n when x % 2 = 0 -> x }
print_any (evens 10)

let squarePoints n =
    { for x in 1 .. n
      for y in 1 .. n  -> x,y }
print_any (squarePoints 3)

第一个仍然可以正常工作，但第二个已经过时了.最新的 (1.9.7.8) F# 编译器不支持这种风格.

The first still works ok, but the second is outdated. The latest (1.9.7.8) F# compiler does not support this style.

经过一番搜索，我发现这个作品

After some search I found this works

let vec1 = [1;2;3]
let vec2 = [4;5;6]
let products = [for x in vec1 do for y in vec2 do yield x*y]

有人能指出为什么语法改变了吗?谢谢.

Can someone point why the syntax changed? Thanks.

推荐答案

• 嵌套 for 循环需要 do.

  您需要使用 seq {..}.没有 seq 的形式 {..} 不再起作用.

  You need to use seq {..}. The form {..} without seq doesn't work anymore.

  也不再支持 for 循环模式中的 when 保护.

  A when guard in a for loop pattern is also not supported anymore.

  print_any something 已弃用.使用 printf "%A" something 代替.

  print_any something is deprecated. Use printf "%A" something instead.

  此代码应该可以工作:

  let evens n =
      seq { for x in 1 .. n do if x%2=0 then yield x }
  printf "%A" (evens 10)
  
  let squarePoints n =
      seq { for x in 1 .. n do
              for y in 1 .. n  -> x,y }
  printf "%A" (squarePoints 3)
  

  如果您只想返回单个值，您仍然可以使用 ->:

  You can still use the -> if all you want to do is return a single value:

  let vec1 = [1;2;3]
  let vec2 = [4;5;6]
  let products = [for x in vec1 do for y in vec2 -> x*y]
  

  顺便说一下，我觉得看看 F# 如何随着时间的推移而发展很有趣.太糟糕了，早期采用者的书架上有部分过时的书籍(我并不介意).

  By the way, I find it interesting to see how F# evolved over time. Too bad the early adopters have partially outdated books on their shelves (not that I mind).

  这篇关于F#中的列表理解的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！