When Destructuring

The @when is introduced to work with the scenarios where @match is a bit heavy.

It's similar to if-let construct in Rust language.

There're two distinct syntaxes for @when.

Allow Destructuring in Let-Binding

tp = (2, 3)
x = 2

@assert 5 === 
    @when let (2, a) = tp,
                  b  = x
        a + b
    end

@assert nothing ===
    @when let (2, a) = 1,
                   b = x
        a + b
    end

Note that only the binding formed as $a = $b would be treated as destructuring.

@data S begin
    S1(Int)
    S2(Int)
end

s = S1(5)

@assert 500 === 
    @when let S1(x) = s,
              @inline fn(x) = 100x
        fn(x)
    end

In above snippet, @inline fn(x) = 100x is not regarded as destructuring.

Sole Destructuring

However, a let-binding could be also heavy when you just want to solely destructure something.

Finally, we allowed another syntax for @when.

s = S1(5)
@assert 5 === @when S1(x) = s x
@assert 10 === @when S1(x) = s begin
    2x
end
@assert nothing === @when S1(x) = S2(10) x