Support putting two format specifiers next to each other

[Happypig375]

Right now, trySscanf on e.g. "%c%c" won't work because all specifiers are translated to (.*?) under the hood.

Also added:

• format specifier consumes spaces to left
• - format specifier consumes spaces to right
• + format specifier consumes preceding plus for numbers
• (|Scan|_|) that turns trySscanf into an active pattern

[gusty]

Indeed

> trySscanf "%c%c" "ab" ;;
val it: (char * char) option = None

But I was expecting Some ('a','b').
Hopefully you can fix it.

[Happypig375]

Test failure seems... unrelated?

[gusty]

Not sure, at first sight it looks related, here's what I am seeing:

           Failed scanfParsing [62 ms]
           Error Message:
            System.IndexOutOfRangeException : Index was outside the bounds of the array.
           Stack Trace:
              at FSharpPlus.Tests.Parsing.scanfParsing() in C:\a\FSharpPlus\FSharpPlus\tests\FSharpPlus.Tests\Parsing.fs:line 87
            at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
            at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

Have you tried to repro locally ?

I normally paste code fragments in a script to work it out. It's way quicker than compiling the whole lib and trying stuff.

[Happypig375]

When I try to debug locally, Visual Studio just hangs on me...

[gusty]

That's why I always use (and suggest) a script.
Loading the whole solution is not a practical approach at all.

[Happypig375]

Ok, so the root cause is that sometimes we do want to consume spaces before and after as shown in one of the existing tests. I've added support via the and - modifiers - consumers spaces before and - consumes spaces after.

[Happypig375]

I think it's now fixed

[Happypig375]

I rewrote it to be more performant and also removed the spaces

[gusty]

Does it makes sense to re-use our existing Parsed active pattern ?

It think this one expand it, but it doesn't break it.

[Happypig375]

How can you fit a format string into (|Parsed|_|): string -> 'T option?

[gusty]

You can't, it is the other way around. Scan is an upgrade of Parsed.

[Happypig375]

(|Parsed|_|) uses tryParse inside and (|Scan|_|) uses TryParseArray inside which uses tryParse. The logic is already shared.

[Happypig375]

or you mean this active pattern should be named (|Parse|_|)?

[Happypig375]

Can I just call it (|Parse|_|)?

[gusty]

Good question. In general, active patterns should describe what something is, rather than what it does. That’s why we avoid naming them after actions like Parse.

We actually used to have an active pattern named Parse, but we deprecated it in favor of Parsed, to better reflect that it's identifying a parsed value, not performing the parsing itself.

So I’d recommend sticking with Parsed (or something similar that describes the result), to stay consistent with F# naming conventions and improve code readability.

[Happypig375]

Reusing the name Parsed + F# naming convention for formatted strings f -> Parsedf

But it doesn't read well. Something like ParsedFormat would be too long especially when used across multiple lines. Maybe an adjective like Like per fsharp/fslang-suggestions#1426 would read better?

[gusty]

I propose, let's move the Active Pattern to its own issue/PR.
Then let's merge this PR, which is a different thing and it is already approved.

[Happypig375]

Separated to #635

[gusty]

Nice job !