Flexible active pattern syntax

[jariji]

julia> @active *{l}(r) begin
       Some(r/l)
       end

julia> @match 60 begin
           *{10}(x) => x
       end
6.0

This syntax is a little cumbersome. Would it be possible to have a pattern like 10 * x => x?

julia> @active l * r begin
       Some(r/l)
       end
ERROR: LoadError: malformed active pattern definition: l * r

[Taine Zhao]

You prefer the first operand of * to be a value, and the second is a pattern, which doesn't seem a good practice to me.

However, having such patterns in your special context is not bad at all, you can use custom patterns:

julia> using MLStyle

# As a pattern, `MyContext(f(a, b))` is equal to `f{a}(b)`:
julia> struct MyContext end
julia> function MLStyle.pattern_uncall(::Type{MyContext}, to_pattern, _, _, arguments)
           @match arguments begin
               [Expr(:call, f, fst, snd)] => to_pattern(:($f{$fst}($snd)))
           end
       end

julia>  @active *{l}(r) begin
              Some(r/l)
              end

julia> @match 10 begin
           MyContext(5.0 * a) => a
       end
2.0

[jariji]

The special context idea with pattern_uncall works for me.

You prefer the first operand of * to be a value, and the second is a pattern, which doesn't seem a good practice to me.

I didn't understand this point. Why would operand order matter for patterns?

[Taine Zhao]

I didn't understand this point. Why would operand order matter for patterns?

5.0 in 5.0 * a is a value, but generally, like in the following code, 5.0 becomes a pattern.

@match value begin
     5.0 => ...
end

Without any special annotations, the meaning of 5.0 * a seems a little vague to me.

[jariji]

Ah yes, I see, it is potentially ambiguous. It is assumed in 5.0 * a that 5.0 is a value and a is a pattern but it could be otherwise. 5.0 could be a pattern and a could be a name in the enclosing scope. Even * could be a pattern.

[jariji]

Thank you.