Add @stack which stacks multiple structs into one. Closes #4

[tamasgal]

The @stack macro concatenates the types of the given structs and creates a new one.

Let me know what you think about this. It directly defines the structs, maybe you want it to provide a "template" as an intermediate step first, as proposed in https://github.com/rafaqz/Mixers.jl/issues/4 ?

julia> using Mixers

julia> struct Foo
         x::Int32
       end

julia> struct Bar
         y::Int64
       end

julia> @stack Baz Foo Bar

julia> fieldnames(Baz)
(:x, :y)

julia> fieldtypes(Baz)
(Int32, Int64)

[tamasgal]

Hmm, that ERROR: LoadError: LoadError: UndefVarError: fieldtypes not defined in Julia 1.0 is annoying, not sure how to get around it yet...

[tamasgal]

Alright, fieldtype is there...

[tamasgal]

Not sure about the AppVeyor error though, I have no idea about Windows stuff.

[tamasgal]

I am currently trying to get something like @with_kw @stack Baz Foo Bar to work but no success yet. This is actually something which I would need too 😉

[rafaqz]

This is cool. Does it handle type parameters on Foo? Probably that and allowing macros like @with_kw would be requirements.

Also maybe @stack needs a dumber name to fit in with the rest of the package ;)

[tamasgal]

Yes I was unsure about the naming, I thought stacks is ok as reference to stacking cups, but no idea 🙈

Here are some ideas:

• @highball
• @mash
• @commonwealth (a cocktail with 71 ingredients)
• @longisland (not that many ingredients but more popular)
• @layered reference to layered drinks

Type parameters work like this (I will add tests):

julia> using Mixers, Parameters

julia> struct Foo
                x::Int32
              end

julia> struct Bar
                y::Int64
              end

julia> struct Fjoord{T}
                z::T
              end

julia> @stack Narf{T} Foo Bar Fjoord

julia> Narf{Bool}(1, 2, true)
Narf{Bool}(1, 2, true)

julia> fieldtypes(Narf)
(Int32, Int64, Any)

julia> typeof(Narf{Bool}(1, 2, true))
Narf{Bool}

I however could not get @with_kw accept my struct yet, I tried different quotings but no idea:

julia> using Mixers, Parameters

julia> struct Foo
                x::Int32
              end

julia> struct Bar
                y::Int64
              end

julia> @with_kw @stack Baz Foo Bar
ERROR: LoadError: Only works on type-defs or named tuples.
Make sure to have a space after `@with_kw`, e.g. `@with_kw (a=1,)
Also, make sure to use a trailing comma for single-field NamedTuples.

[rafaqz]

Oh right sorry I missed that - the outer macro runs first!

The trick is to put that macro inside the @stack macro and reapply it after, like I do here: https://github.com/rafaqz/Mixers.jl/blob/master/src/Mixers.jl#L37

Try reusing those methods if possible.

Also for consistency with @mix, we should avoid requiring the T here if we can get it from Fjoord

Narf{T}

But the brackets should stay - it marks the paramtric type and sets up the AST to insert T

[rafaqz]

Also thinking about it I'm not sure how you can get @with_kw to work anyway? it needs the = x?

But allowing macros would allow you to apply some @mix macro inside @stack which would be very cool - you could combine structs and mixins.

[tamasgal]

Sorry for the late response! I am working on it but it's really tricky, especially that I don't have much experience with macros ;) I'll report back when I got something...