Thanks, the results were interesting enough :)
julia> expand(:(typealias A B))
:(begin
const A
A = B
return B
end)
julia> expand(:(typealias A{T} B{T,1}))
:((AST(:($(Expr(:lambda, Any[:T], Any[Any[Any[:T,:Any,0]],Any[],1,Any[]],
:(begin
const A
GenSym(0) =
(top(TypeConstructor))((top(svec))(T),(top(apply_type))(Main.B,T,1))
Main.A = GenSym(0)
return GenSym(0)
end))))))((top(TypeVar))(:T,top(Any),true)))
Conclusion: Non-parametric type aliases are simple. Parametric type aliases
are not :)
// T
On Friday, October 30, 2015 at 7:31:57 PM UTC+1, Yichao Yu wrote:
On Fri, Oct 30, 2015 at 1:34 PM, Tomas Lycken <[email protected]
> <javascript:>> wrote:
> >> `const A = T` is what `typealias A T` lowered to when it doesn't have
> type
> >> parameters
> >
> > I wanted to inspect the output of this lowering pass, but
> `@code_lowered`
> > was apparently not what I was looking for (at least, `@code_lowered
> > :(typealias Foo Int)` didn't work...).
> >
> > However, I understand your statement as follows:
> >
> > `typealias A T` is equivalent to `const A = T`, while `typealias A{S}
> T{S}`
> > does something more involved? What does `typealias A{S} T{Foo,S}` do? I
> > assume these are not possible using `const <foo> = <bar>` constructs,
> right?
>
> right. Try `expand(:(typealias A B))`
>
> >
> > // T
> >
> > On Friday, October 30, 2015 at 1:45:37 PM UTC+1, Yichao Yu wrote:
> >>
> >> On Fri, Oct 30, 2015 at 8:29 AM, FANG Colin <[email protected]> wrote:
> >> > I have got the same confusion. Any ideas?
> >> >
> >> > I have even seen usage of A = T (no const)
> >> > (http://docs.julialang.org/en/release-0.4/manual/types/#type-unions),
> is
> >> > it
> >> > supposed to be bad (slow) because it is a global variable?
> >> >
> >>
> >> no const will have performance issue since it's a global.
> >> `const A = T` is what `typealias A T` lowered to when it doesn't have
> >> type parameters. In this case, it's just a matter of style. Being
> >> consistent within a file is probably better but you can pick whichever
> >> you prefer.
> >>
> >> >
> >> >
> >> > On Thursday, November 6, 2014 at 3:38:41 PM UTC, Steven G. Johnson
> >> > wrote:
> >> >>
> >> >> Given a type T, what is the difference in practice between
> >> >> typealias A T
> >> >> and
> >> >> const A = T
> >> >> ?
> >> >>
> >> >> There seems to be some disagreement over which one to use (e.g.
> >> >>
> >> >>
> https://github.com/JuliaLang/Compat.jl/commit/8211e38ac7d8448298a2bb3ab36d6f0b6398b577).
>
>
> >> >>
> >> >> My impression is that there is no difference, and that the only
> >> >> advantage
> >> >> of a typealias is that it can be parameterized. Is that right?
> >> >>
> >> >> If they are equivalent, what is the Julian style? Even in Julia
> Base
> >> >> it
> >> >> doesn't seem to be entirely consistent.
>
