Documentation contains invalid type extension for seq<'T> which is a type abbreviation

[abelbraaksma]

The documentation about type extensions contains the following example code:

type seq<’T> with
    /// Repeat each element of the sequence n times
    member xs.RepeatElements(n: int) =
        seq { for x in xs do for i in 1 .. n do yield x }

This example code won't compile, it will throw:

error FS0964: Type abbreviations cannot have augmentations

and

error FS0895: Type abbreviations cannot have members

The proper code would be (assuming System.Collections.Generic is opened)

type IEnumerable<'T> with
    /// Repeat each element of the sequence n times
    member xs.RepeatElements(n: int) =
        seq { for x in xs do for i in 1 .. n do yield x }

Note that the text itself mentions that type abbreviations cannot be extended.

[dsyme]

@abelbraaksma Please fix by making a PR to the docs directly?

[dsyme]

See dotnet/docs#1178

[abelbraaksma]

@dsyme, yes, I'll look into that (never worked with github sources before). I'm wondering though if my suggested change is a good one. Having type IEnumerable<'T> with... does not append the new method to Seq, because Seq is a module and modules cannot be extended (unfortunately so, but that's a different discussion).

Creating a type Seq = ... would do what you'd expect, but that is not a type extension.

What the code does is (of course) creating an instance member extension for sequences, but that's hardly idiomatic. Perhaps the whole suggestion should go and a different extension that's more fsharpish should be used instead? Any ideas?