I don't really know exactly what is going on here, but I'm not sure
equality works like that for types, better to use <:
julia> Array{Int} <: Array{Int,1}
false
julia> Array{Int,1} <: Array{Int}
true
And I don't necessarily think this is wrong - look at the code it generates
f{t,n}(::Type{Array{t,n}},::Array{t,n}) at none:3
f{t,n}(::Type{Array{t,N}},::Array{t,n}) at none:2
There is nothing to distinguish these two from a dispatch perspective - its
just picking the one defined last.
I think you might be right about that code in Base being unreachable.
Which part of the manual is wrong?
Cheers,
Iain
On Saturday, May 31, 2014 10:31:54 AM UTC-4, David Moon wrote:
>
> The manual says Type{xxx} matches only the object xxx, but it does not
> seem to be true. Maybe this is related to my post yesterday "Reflection
> bug for parameterized types?" because this could be another case of
> confusion between the TypeVars declared in a parameterized type
> declaration and TypeVars used to instantiate a parameterized type.
>
> Here is an example, run in Julia 0.3:
>
> julia> begin
> f{t,n}(::Type{Array{t}}, ::Array{t,n}) = (t,n,1)
> f{t,n}(::Type{Array{t,n}}, ::Array{t,n}) = (t,n,2)
> end
> julia> methods(f)
> # 2 methods for generic function "f":
> f{t,n}(::Type{Array{t,n}},::Array{t,n}) at none:3
> f{t,n}(::Type{Array{t,N}},::Array{t,n}) at none:2
>
> julia> f(Array{Int,1}, [1,2,3])
> (Int32,1,2)
>
> julia> f(Array{Int}, [1,2,3])
> (Int32,1,2) *# I was expecting (Int32,1,1) !*
>
> julia> Array{Int} == Array{Int,1}
> false
>
> So Type{Array{t,n}} with t=Int and n=1 matches Array{Int} even though
> Array{Int,1} and Array{Int} are not the same object!
>
> This only happens when the methods have more than one argument and there
> are static parameters shared between the Type{...} argument and another
> argument. Otherwise Type{Array{t}} only matches Array with only one
> parameter specified, as expected.
>
> The first method appears to be unreachable. This means, for example, that
> the predefined method convert{T,n,S}(::Type{Array{T}}, x::Array{S,n}) in
> base/array.jl can never be called; convert{T,n,S}(::Type{Array{T,n}},
> x::Array{S,n}) will be called instead. It doesn't appear that anything
> breaks because of that, but clearly method dispatch is not working the way
> the author of that code expected.
>
> I didn't search for a pre-existing bug about this, because it's kind of
> hard to search when the only search key you have is a generic word like
> "type."
>
> Should I file a bug for this? Or is it supposed to behave like this and
> the manual is just wrong?
>
