Parametric Types with internal constructors should generate an outer helper constructor by default

[tawheeler]

Right now, parametric types with inner constructors require one to specify the data type:

type TypeB{S}
    x::Array{S}
    y::Int
    TypeB(x::Array{S}) = new(x,2)
end
TypeB{Int}([1,2,3])

It would be nice if an outer helper constructor was auto-generated for such cases:

type TypeB{S}
    x::Array{S}
    y::Int
    TypeB(x::Array{S}) = new(x,2)
end
TypeB{S}(x::Array{S}) = TypeB{S}(x)
TypeB([1,2,3]) # <- {Int} not require

[quinnj]

Note if all you need is a default value and don't need to restrict inner constructors, you can do:

type TypeB{S}
    x::Array{S}
    y::Int
end

TypeB(x::Array) = TypeB(x,2)

TypeB([1,2,3])

[tawheeler]

Sorry, the code above was just a dumb example. I was thinking more along the lines of LabelMap used in MLBase:

immutable LabelMap{K}
    vs::Array{K}
    v2i::Dict{K,Int}
end
function LabelMap{K}(vs::Array{K}, v2i::Dict{K,Int})
    length(vs) == length(v2i) || throw(DimensionMismatch("Lengths of vs and v2i mismatch"))
    LabelMap{K}(vs, v2i)
end

[JeffBezanson]

The problem with this is, if you don't want the default outer constructor, how do you get rid of it?

[JeffBezanson]

Another problem with this is that your inner constructor can take any arguments at all, and the auto-generated outer constructor would not know how to call it.

[tawheeler]

Very true. Thanks for considering it.
I'll mark this issue as closed, yes?