preserve do syntax in AST

[JeffBezanson]

part of #21774

[ararslan]

Can you remind me on why triage was in favor of preserving do? Thinking about it now, this seems like an annoying thing to have to remember to special-case when writing a macro that handles anonymous functions like this.

[JeffBezanson]

I'm not 100% convinced myself, but I can think of some reasons:

• This allows better pretty-printing of expressions.
• Converting do to its lowered form re-orders expressions, which is a bit unintuitive.
• Keno felt that the do form might be useful for macro DSLs to intercept.

[MikeInnes]

the do form might be useful for macro DSLs to intercept.

Yes, this is generally a good reason to preserve this kind of information. If you don't care it might be an extra concern (though not if you're just walking over all lambdas).

The extra work for do blocks pales in comparison with things like function definitions, which we don't normalise at all. So we need a more general solution there anyway, probably involving exposing some lowering tools in Base.Meta.

[JeffBezanson]

1.0 decision needed.

[MikeInnes]

So right now I'm generating expressions that look like this:

map(((quetzal,)->(Dagger.Thunk)((penguin,)->begin  
        bear = (Array{Any,1})(quetzal)
        for jellyfish = quetzal
            bear[jellyfish] = penguin[jellyfish] + 1
        end
        bear
    end
end, chslice(cat, domain(cat, 1)))), domprod(domain(cat, 1)))

With this change the code would output like this:

map(domprod(domain(cat, 1))) do quetzal
    (Dagger.Thunk)(chslice(cat, domain(cat, 1)))) do penguin 
        bear = (Array{Any,1})(quetzal)
        for jellyfish = quetzal
            bear[jellyfish] = penguin[jellyfish] + 1
        end
        bear
    end
end

Which is a significant readability improvement.

[StefanKarpinski]

We don't really need this change to get that, we just need some decent heuristics on when to use do syntax printing a function call. Just off the top of my head: the function is the first argument and the body of the function definition is multiline.

[StefanKarpinski]

Triage is ambivalent about this. @JeffBezanson, it's your call. Do what your heart tells you.

[StefanKarpinski]

Triage: let's do it. Merge as soon as CI passes.