Make `match` work with function results

[skelly37]

Related problem

I don't really know how to describe it but sometimes we keep global constants as functions returning the value. It would be nice to have match supporting it instead of having the else if ladder.

Describe the solution you'd like

The function return values should be evaluated

This match statement should match log CRITICAL_LEVEL_PREFIX, now it evaluates the default _ option

Describe alternatives you've considered

if-else works just fine but match is sometimes the cleaner way to do something

Additional context and details

No response

[amtoine]

the following does not seem to work with records

~ ❯ > let foo = {a: 1, b: 2}
~ ❯ > match 1 {
∙     $foo.a => { print "$foo.a" },
∙     $foo.b => { print "$foo.b" },
∙     _ => { print "default" },
∙ }
Error: nu::parser::parse_mismatch

  × Parse mismatch during operation.
   ╭─[entry #9:1:1]
 1 │ match 1 {
 2 │     $foo.a => { print "$foo.a" },
   ·     ───┬──
   ·        ╰── expected valid variable name
 3 │     $foo.b => { print "$foo.b" },
   ╰────

the error is not very helpfull here...
that might be a first improvement? 😋

[sophiajt]

Patterns are describing a shape and then the holes in that shape you want to grab the value out. They do support constants, as well.

The design should keep to traditional pattern matching because it will improve learn-ability for folks who are used to traditional pattern matching (including allowing folks who are used to nushell to use other languages with pattern matching) as well as open up opportunities in the future for engine optimisations around matching of patterns.

Running commands and getting the results are not part of traditional pattern matching.match isn't intended to replace a chain of else if but rather to allow you to "destructure" a value to get components inside of it out. That's why @amtoine's example also doesn't work, because that example doesn't use any destructuring. It's trying to reach into a value. This works in if but it's not part of how pattern matching works.