list comprehension in F#

Question

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The first still works ok, but the second is outdated.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.

Answer 1

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 Too bad the early adopters have partially outdated books on their shelves (not that I mind).