Define `@trymatch` and `@tryswitch`

[serenity4]

I think it would be a good addition to have versions of @trymatch and @tryswitch that allow having no matches, like for example

@trymatch x begin
  ::Int => :int
end

# would be equivalent to

@match x begin
  ::Int => :int
  _ => nothing
end

and

@tryswitch x begin
  @case ::Int
    println("int")
end

# would be equivalent to

@switch x begin
  @case ::Int
    println("int")
  @case _
    nothing
end

The main advantage with these is when you have nested @match and @switch statements, where each invocation will require adding this special case for no matches. The name should make it fairly obvious that it will silently "fail".

If it's not already implemented (I looked around, but we never know) I can submit a PR to define these helper macros.

[thautwarm]

Hello, what you said is really a practical feature and we support it via a more concise syntax @when and @otherwise:

julia> @when (a, b, 1) = (1, 2, 1) begin # the left-hand side is a pattern, and the right is a value
           println(a, b)
       end

12

julia> @when (a, b, 1) = (1, 2, 2) begin
           println(a, b)
       end

You can also have more @when clauses and an optional @otherwise clause:

julia> test = (1, 2, 3)

(1, 2, 3)

julia> @when (a, b) = test begin
           println(1)
       @when (a, b, c, d) = test
           println(2)
       @otherwise
           println(3)
       end
3

[serenity4]

It seems very nice when you want to specify successive matches, but doesn't cover the @match/@switch logic where every clause refers to a specific match.

Consider this example with @match and with @when:

@match test begin
  ::String => :string
  ::Int => :int
  ::Bool => :bool
  ::Nothing => :nothing
  _ => nothing
end

@when ::String = test begin;
    :string
    @when ::Int = test; :int
    @when ::Bool = test; :bool
    @when ::Nothing = test; :nothing
end

@match allow referring to test only once and then forgetting about it, while there is a redundancy with @when in having to always specify = test, that could be avoided.

You can take a look at a concrete use case here where it came in really handy.

[thautwarm]

Thanks for this concrete use case! It looks fine to me, and I can add @trymatch later.

Or maybe @Roger-luo could add such functionality to a 3rd-party utility lib?

[serenity4]

I think we can reopen this, since @trymatch still hasn't been implemented yet. But I can take a shot at it, it should be similar to #132.

[thautwarm]

Okay, just reopen it.

[serenity4]

Implemented in #133 and #132.