Improve replacements for StringBuilder.

[Alxandr]

I currently have a shim StringBuilder in my code so as to be able to use APIs that needs stringbuilders. This should probably be part of Fabe.Core (with replacements in place). In JS, it can be implemented as a simple ref<string> or similar.

[ncave]

@Alxandr It's already part of Fable.Core, just use it.

[Alxandr]

@ncave That's weird, cause I've been getting exceptions that no replacement was found for StringBuilder.Append (etc.)

[ncave]

It was added recently, try it.

[Alxandr]

You're right. I still get errors though 😛:

[!] Error: 'StringBuilder$$Append$$244C7CD6' is not exported by .fable\fable-core.2.0.0-alpha-031\System.Text.js

[ncave]

@Alxandr The current SB implementation is a bit simplistic, probably one of the overloads is missing, as the overload mangling is now done based on argument types. Post a bit of F# code so we can see which one.

[Alxandr]

I currently use the following implementation as a shim (be adviced that Remove and ToString with indexes has not been tested):

  type StringBuilder (s: string) =
    let mutable s = s

    new () = StringBuilder ("")

    member __.Length = s.Length

    member sb.Append (s': string) =
      s <- s + s'
      sb

    member inline sb.Append (c: char) = sb.Append (string c)

    member inline sb.Append (num: ^n) = sb.Append (sprintf "%d" num)

    member inline sb.Append (o: obj) = sb.Append (string o)

    member inline sb.AppendLine () = sb.Append System.Environment.NewLine

    member inline sb.AppendLine (s: string) =
      (sb.Append (s)).AppendLine ()

    member sb.Remove (startIndex: int, length: int) =
      if startIndex + length >= s.Length
      then s <- s.Substring (0, startIndex)
      else s <- s.Substring (0, startIndex) + s.Substring (startIndex + length)
      sb

    member inline __.ToString (startIndex: int, length: int) =
      s.Substring (startIndex, length)

    override __.ToString() = s

Not entirely sure which are missing, but that's at least the minimal implementation for my sake (wrt which methods are available).

[ncave]

@Alxandr Eventually all StringBuilder.Append overloads will have to be added. If you need to get rid of your shim sooner, you can submit a PR, at least for the trivial ones, it should work now that @alfonsogarciacaro fixed the overload mangling.

[Alxandr]

Yeah, no, the shim works, so this isn't a rush.

[Alxandr]

I can work on this issue now that the test issues fixes has been merged 🙂

[alfonsogarciacaro]

It'd be nice to have a proper implementation that actually prevents many string allocations, using a typed char array (another reason to merge #1473.

BTW, now that we can use F# for fable-core, it'd be also nice to translate the System.Decimal source to F# so we can actually keep accuracy. Just saying ;)

[Alxandr]

I think things like System.Decimal is probably a better fit for webassembly (like long.js did). As for string builder, as far as I've seen from tests, JS engines are really well optimized at appending strings together (probably because that's the only way you've been able to build strings), that I don't think the added complexity is worth it. At least not unless someone does a propper meassure to see that it is.

[Alxandr]

Speaking of long.js, should it maybe be updated from upstream? Wouldn't it be better if it could be imported as a file untouched, and simply write the fable shims needed on top?

[alfonsogarciacaro]

That's what we did for Fable 1, but long.js attaches everything to the prototype so any import would bring the whole file. That's why I changed the functions to get better advantage of tree shaking. I know it makes updating the file more complicated but...

[Alxandr]

Ah. You're right. I didn't notice.

[alfonsogarciacaro]

Let's close the issue as we already have PR #1505 to track this.