How can I create a delegate in F# of the type System.Buffers.SpanAction for use in String.Create?

Question

This question was also asked on F# Slack, but since that's not visible to the wider community, and since I don't have a solution yet, I figured it made sense to ask it here.

Basically, String.Create works with Span to create a new string and takes a delegate to fill the span. This is generally faster than changing a string the normal way: create a char array, call AsSpan , and calling the proper String constructor with the result.

This works fine:

let create value = 
    String.Create(12, value, fun buf v-> 
        for i = 0 to buf.Length do
            buf.[i] <- v)

But the minute I try to use a function that's curried, or try to use a delegate, I get a type error. According to F#, the delegate is of type SpanAction<'T, 'TArg> , where 'T translates to Span<'T> in the callback, and 'TArg is any argument you can use to prevent a closure.

I've tried several variants, but all come down to this basic idea:

type SpanDelegate<'T, 'TArg> = delegate of Span<'T> * 'TArg -> unit

let callback = 
    new SpanDelegate<char, char>(fun buf v -> 
      for i = 0 to buf.Length do buf.[i] <- v)

let create (value: char) = 
    String.Create(12, value, callback)   // type error on callback

Whether or not I use it with the original lambda, I can't seem to get it to work. Ideas are welcome:).

Answer 1

The reason why your code does not work is that you are creating a custom delegate SpanDelegate , but the code expects a pre-defined delegate SpanAction .

I'm not exactly sure I understand your issue correct (and so I may be missing something), but if you create the pre-defined SpanAction delegate, then everything seems to be working fine:

open System
open System.Buffers

let callback = 
    SpanAction<char, char>(fun buf v -> 
      for i = 0 to buf.Length do buf.[i] <- v)

let create (value: char) = 
    String.Create(12, value, callback)