Proposal 869: pattern-matching syntax

[Emanon42]

This pull request implements #869

To enable this syntax sugar, set pattern_matching_sugar=true in config

Syntax

It adds syntax of SML-style pattern-matching like that:

fun ack(_,_) switch {
  case (0, n) ->  n + 1
  case (m, 0) -> ack(m - 1, 1)
  case (m,n) ->  ack(m - 1, ack(m, n - 1)) 
}

Semantics

Desugaring the cases defined in pattern-matching function to a Switch phrase and adding a wild card case to raise an error to handle the non-exhaustive case (with position of wrong code).

[dhil]

@Emanon42 while I remember it, for consistency we should probably change all occurrences of MatchFunlit to SwitchFunlit and the match_ prefixed type aliases to have switch_ as a prefix instead.

[jamescheney]

This looks good now. My main suggestion besides whitespace nitpicking is to make get_normal_funlit into a library function and use it in more places to avoid repetition (and save effort if we ever later refactor the funlit cases). Otherwise looks ready to merge.

[Emanon42]

This looks good now. My main suggestion besides whitespace nitpicking is to make get_normal_funlit into a library function and use it in more places to avoid repetition (and save effort if we ever later refactor the funlit cases). Otherwise looks ready to merge.

Looks like put get_normal_funlit in utility.ml could have the problem of the dependency cycle, so I just put it in sugartypes.

[SimonJF]

Thanks for doing all these revisions, @Emanon42! It was good working with you during your internship.

I've got a few final comments, but all of the major things have been addressed now. I'd be happy for this to go in.

[jamescheney]

Hmm, the CI is failing due to the documentation not building for some reason - maybe a transient problem? (this only happens on Travis so it might be a Travis problem).

@dhil, any further remarks before this gets merged? It seems that your most recent review comments have been addressed.

[dhil]

It seems that anonymous switch functions are not being desugared properly:

links> fun(s) switch { case x -> x };
***: Error: "Assert_failure core/desugarModules.ml:322:15"

[dhil]

Looks good to me! I will merge this in shortly.

[dhil]

Note: This pattern/case may be redundant, though, currently we do not check for redundancy or exhaustiveness of patterns.

[SimonJF]

Congratulations, @Emanon42!

[Emanon42]

Congratulations, @Emanon42!

Thanks again for mentoring me! It is really happy to work with you all.
and new avatar is cool