[API Proposal]: trySscanf active pattern

[Happypig375]

Background and motivation

There is (|Parsed|_|) wrapping tryParse. There should be a similar need for matching a string with a printf format instead of directly parsing.

Separated from #620 at the request of @gusty

API Proposal

The only unresolved question about this API is the name.

let (|Parsedf|_|) pf = trySscanf pf // doesn't read well but is in line with names like trySscanf
let (|Like|_|) pf = trySscanf pf // The "Like" operator of SQL and Visual Basic which also does simple string pattern matching
let (|Scanned|_|) pf = trySscanf pf // Scanned is to trySscanf like Parsed is to tryParse

API Usage

match "123" with
| Parsedf "%c%c%c" ('1', '2', '3') -> printf "yes"
| _ -> failwith "impossible"

Alternative Designs

No response

Risks

It's an API with heavy STRP type inference like most of F#+. Error messages are an abomination.

Are you willing to help with a proof-of-concept (as PR in that or a separate repo) first and as pull-request later on?

Yes, please assign this issue to me.

[gusty]

I think the other alternative name is the existing Parsed.
This would mean, replace (upgrade) the existing Parsed with this new implementation, unless this is a breaking change, but I think it is not.

[Happypig375]

@gusty I tried doing overloading on (|Parsed|_|) but it doesn't work.

• static members that support overloading cannot be active patterns
• attempting to use SRTP for return type overloading based on the first parameter fails because the compiler checks for the final return value being an option.

[gusty]

Yes, anyway I see now, both functions address different things, you can't exchange Parsed for the new one.

From the list you proposed, I like Scanned.

[wallymathieu]

I kind of like ’Scanned’ as well. ’Like’ implies a certain style of pattern matching more in line with SQL.

[Happypig375]

I kind of like ’Scanned’ as well. ’Like’ implies a certain style of pattern matching more in line with SQL.

There are direct equivalents - namely %c for _ and %s for %.

[gusty]

I think I like Like as well

[gusty]

But analysing the whole thing, Like sounds good at first glance, especially considering its connection to SQL and VB. However, as an active pattern, it doesn’t read quite right in real usage.

For example:

match input with
| Like "%i" 5 -> ...

This reads more like an instruction ("match something that is like...") rather than a pattern you're matching against. Active patterns are meant to convey the result of a computation, not the action to be taken. That's why names like Parsed or Scanned feel more natural in this context.

Here’s a quick breakdown:

• Parsedf: aligns with the naming of Parsed, but isn't very readable.

• Like: great as an infix operator, but doesn't convey the right semantics in a pattern match.

• Scanned: works well: it’s a past-tense, result-like word (like Parsed), which fits the active pattern usage. Also it's semantically linked to trySscanf, much like Parsed is to tryParse. Clean and conveys intent clearly.

So far, I’m in favor of Scanned. It clearly communicates what has happened to the input, matches the pattern style of Parsed, and avoids the awkwardness of Like in a pattern match.

FYI: We had originally Parse in this lib, but it was changed by Parsed for the above reasons.

[wallymathieu]

Thanks for the contribution @Happypig375 !