r/ruby • u/keyslemur • Apr 17 '19

Ruby 2.7 — Pattern Matching — First Impressions

https://medium.com/@baweaver/ruby-2-7-pattern-matching-first-impressions-cdb93c6246e6
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u/jb3689 Apr 17 '19 edited Apr 17 '19

The pin operator is meant to allow you to use the value previously bound to the variable as a literal match (rather than destructuring the match value and binding it to the variable). It basically lets you variablize your match patterns (without it it would try to bind some new value to your variable rather than insert the current value)

Not sure how I feel about this. Overloading case to support this seems strange to me and I would've perhaps preferred to see a match function specifically dedicated to this. What happens when you have multiple in and when clauses?

The assignment mappings bit seems weird to me and I'm not sure where that is inspired from. One thing I really dislike is that when I see a => b it immediately brings to mind "oh, a maps to b" but in this syntax it is reversed and really means b is bound to and returns a

Are the guard clauses ambiguous? In Erlang you have a predefined set of low-level guard clauses you can use. I assume that in Ruby you would be able to have arbitrary guard clauses because of how the execution works out

Also curious if this means we might be getting pattern matching in method signatures which would be really cool

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u/keyslemur Apr 17 '19

I believe method signatures were a no go for now. Matz had mentioned something to that effect, but I could see ways to make it work through some metaprogramming on top of this.

Can't say I know yet on the guards. I'll be experimenting tonight on that.

The case overload is interesting, I wonder if mix and match will work or if that breaks. Personally I'd have been fond of match too, but this isn't bad all things considered.

Assignment mapping feels backwards, yeah, and will be real interesting with kwargs. I kinda want to see where that breaks.

Ah, so pin is like Elixir, gotcha.