Wildcard self identifiers

[baronfel]

Submitted by Daniel Robinson on 9/29/2014 12:00:00 AM
9 votes on UserVoice prior to migration

Two underscores are frequently used in member definitions to denote an ignored "self" identifier. This seems like a hack given that the language already provides a wildcard pattern that represents an unused value.

Original UserVoice Submission
Archived Uservoice Comments

[dsyme]

I’m marking this as approved in principle for F# 4.×. you are invited to submit a quality and tested implementation would be needed, to be submitted to http://github.com/Microsoft/visualfsharp.
See http://fsharp.github.io/2014/06/18/fsharp-contributions.html for details about contributing to the F# language and core library

FWIW I’ve actually taken a look at this once or twice and it was surprisingly invasive to implement. But by all means give it a go and ask if you need help.

Don Syme BDFL F# Language/Core Library Evolution

[eugbaranov]

@dsyme does this require an RFC?
Any pointers to where is it the best to start?

[dsyme]

@Regent In theory an RFC is required but it can be ultra short unless you discover corner-cases in the implementation

For implementation - starting with this:

type C() = 
   member _.P = 1

and compiling using fsc.exe a.fs we get a syntax error. Syntactically _.P is a pattern. Looking through the parsing rules in pars.fsy for memberCore I eventually get down to atomicPatternLongIdent as the thing being parsed

So, the first thing to start is to add _ to the parser in the appropriate place for long identifiers in patterns, e.g.

atomicPatternLongIdent:
  | UNDERSCORE DOT pathOp  { let (LongIdentWithDots(lid,dotms)) = $3 in (None,LongIdentWithDots(ident("_",rhs parseState 1)::lid, rhs parseState 2::dotms)) }

or something like that. One possible problem is that this use of UNDERSCORE might cause conflicts with this:

atomicPattern:
   ...
  | UNDERSCORE 

If so that would be trickier to resolve

[gusty]

I tried adding the rule @dsyme suggested at the top level position of atomicPatternLongIdent and it seems to be working fine. See dotnet/fsharp#6829

CC @cartermp

[pblasucci]

@gusty This is syntax I’ve wanted since 2007 🤪

[gusty]

Yes, me too, long time.

It's all green already, either it was easier than what we thought, or a failing test is missing.

[cartermp]

Closing as this is in for F# 4.7