在F＃中的列表理解 [英] list comprehension in F#

问题描述

我试图在F＃中做一些列表理解。我找到了这个。

  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）
  

第一个仍然正常，但第二是过时的。最新的（1.9.7.8）F＃编译器不支持这种风格。

经过一番搜索，我发现这个作品

  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] 
  

有人可以指出语法的改变吗？谢谢。

解决方案

• 嵌套for循环需要 do 。




• 您需要使用 seq {..} 。 {..} 没有 seq 的表单不再适用。




• 当在中保护循环模式时，也不再受支持。 p>




• print_any东西已弃用。使用 printf％A取而代之。




代码应该工作：

  let evens n = 
 seq {for x in 1 .. n do if x％2 = 0 then 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）
  

您仍然可以使用 $ b

 <$ c    - > 如果您只想返回一个值， $ c> let vec1 = [1; 2; 3] 
 let vec2 = [4; 5; 6] 
 let products = [for vec1 do for y in vec2  - > x * y] 
  

顺便说一下，我发现看看F＃随着时间的推移是如何演变的。太糟糕了，早期的采用者已经在书架上放了部分过时的书（不是我介意的）。

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)

The first still works ok, but the second is outdated.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

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]

Can someone point why the syntax changed? Thanks.

解决方案

• Nested for loops require a do.

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

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

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

This code should work:

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)

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]

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 Too bad the early adopters have partially outdated books on their shelves (not that I mind).

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