Convert traitfns to use where-syntax

[mauro3]

Yep, but needs to wait for JuliaLang/julia#23211

[mauro3]

JuliaLang/julia#23211 is a won't fix, but only affects Julia 0.6. So, either this needs to wait for 0.7 or the syntax needs to be changed. A possibility is:

@traitfn f{X; Tr{X}}(x::X) = 1
# would be
@traitfn f(x::X) where trait(Tr{X})<:Tr{X} where {X} = 1

# Multitrait
@traitfn f(x::X, y::Y) where trait(Tr2{X,Y})<:Tr2{X,Y} where {X,Y} = 1

or using the syntax overhauls proposed in #45

@traitfn f(x::X) where Tr(X)<:Tr where {X} = 1

# Multitrait
@traitfn f(x::X, y::Y) where Tr2(X,Y)<:Tr2 where {X,Y} = 1

The nice thing about this is that it clearly states that a computation on the types, e.g. Tr2(X,Y) is done, and that the result of that is then compared to some type. X-ref: https://discourse.julialang.org/t/why-does-julia-not-support-multiple-traits/5278/25, JuliaLang/julia#18457 (comment)

[sbromberger]

@mauro3 - would it be possible to tag a 0.7-only version of SimpleTraits so we can start migrating LightGraphs using the new where syntax?

[mauro3]

Yes, but I would rather wait for feature freeze to happen before spending time chasing 0.7.

[sbromberger]

Makes sense to me :) Thanks.

(Though didn't feature freeze happen on December 15?)

[mauro3]

No, it didn't quite yet, but should soon: https://discourse.julialang.org/t/1-0-feature-freeze-dec-15th/7209/28

[sbromberger]

https://discourse.julialang.org/t/1-0-feature-freeze-dec-15th/7209/34 - any way we might be able to start soon?

[mauro3]

I haven't forgotten this but I have not found the time yet. Sorry! Hopefully the week after next.