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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