Sorry guys for the OT, but let me conclude the answer to F.B.:
g(v::Point{Integer}) = "Point... parametric type is Integer abstract class"
can be used to do
g(Point{Integer}(int64(1),int32(2)))
"Point... parametric type is Integer abstract class"
It seems to me, who knows nothing of computer science, that in fact Julia
has in fact a powerful multiple dispatch system (though only on types not
on values).
On Sunday, November 17, 2013 1:53:56 AM UTC, Cristóvão Duarte Sousa wrote:
>
> I'm not a Julia specialist but I think that for the last case one must do:
>
> julia> g{T<:Integer}(v::Point{T}) = "Point... parametric type is Integer
> abstract class"
> g (generic function with 1 method)
>
> julia> g(p)
> "Point... parametric type is Integer abstract class"
>
> Anyway, I think that there must be a reason to allow the function
> definition the way you have put, or else it should not be allowed in the
> first place.
> We may ask that on Julia users group...
>
> On Tuesday, November 12, 2013 10:55:09 PM UTC, F. B. wrote:
>>
>> I had a try at this with Julia... unfortunately there is a problem there
>> because parametric type constructors are invariant, while they should be
>> covariant for this particular case:
>>
>> julia> immutable Point{T}
>> x::T
>> y::T
>> end
>>
>> julia> p = Point{Int64}(3, 5)
>> Point{Int64}(3,5)
>>
>> julia> p2 = Point{Float64}(3.0, 5.0)
>> Point{Float64}(3.0,5.0)
>>
>> julia> p.x
>> 3
>>
>> julia> p.y
>> 5
>>
>> julia> f(v::Point{Float64}) = "Point... parametric type Float64"
>> f (generic function with 3 methods)
>>
>> julia> f(v::Point) = "Point... no parametric type"
>> f (generic function with 1 method)
>>
>> julia> f(p2)
>> "Point... parametric type Float64"
>>
>> julia> f(p)
>> "Point... no parametric type"
>>
>> julia> f(v::Point{Int64}) = "Point... parametric type Int64"
>> f (generic function with 2 methods)
>>
>> julia> f(p)
>> "Point... parametric type Int64"
>>
>> If methods with detail are not matched, the more generic one is matched:
>>
>> julia> p3 = Point{String}("hello", "world")
>> Point{String}("hello","world")
>>
>> julia> f(p3)
>> "Point... no parametric type"
>>
>>
>> The problem is here, parametric types break all inheritance relations:
>>
>> julia> g(v::Point{Integer}) = "Point... parametric type is Integer
>> abstract class"
>> g (generic function with 1 method)
>>
>> julia> g(p)
>> ERROR: no method g(Point{Int64})
>>
>> In Julia may approach is not possible, or at least it has to be handled
>> in another way.
>>
>> Another language, Scala, has a similar behavior as Julia, but allows to
>> put markers on parametric type to keep inheritance relations for derived
>> types, + for covariant, - for invariant.
>>
>> In my suggestion, this problem should be considered.
>>
>>
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