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 <tomas....@gmail.com > <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 <coli...@gmail.com> 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. >