And let me add that, as far as I know, a call as ModInt{i}(...) is always a
call to a inner constructor of the type ModInt{i}. We cannot call external
constructors like that.
Moreover, external constructors seem be be ordinary generic functions and
one can even do stuff unrelated to the type (somehow also true for inner
constructors):
julia> ModInt{n}(a::Array{Float64, n}) = "stuff unrelated to ModInt type"
julia> ModInt([1., 2.])
"stuff unrelated to ModInt type"
While this
julia> ModInt{1}([1., 2.])
no method convert(Type{Int64}, Array{Float64,1})
is just a (unsuccessful) call to the inner constructor of ModInt{1}.
On Wednesday, April 2, 2014 3:14:46 AM UTC+1, Leah Hanson wrote:
>
> I do not understand (numeric?) type parameters, and the manual has not
> enlightened me.
> As an exercise, I'm trying to write a ModInt type.
>
> I found https://github.com/JuliaLang/julia/blob/master/examples/modint.jl,
> which does work.
>
> However, I don't understand what the difference is between the version
> used there and the version I was trying.
>
> The example:
> ~~~
>
> immutable ModInt{n} <: Integer
>
> k::Int
>
>
> ModInt(k) = new(mod(k,n))
>
> end
>
> ~~~
>
> My failing attempt:
> ~~~
> immutable ModInt{n} <: Integer
> k::Int
> ModInt(k,n) = new{n}(mod(k,n))
> end
> ~~~
>
> At first, I was confused about how the example knows what `n` is in the
> constructor. I realize that this is constructed as `ModInt{2}(5)`, but the
> `n` is just magically shared?
> This part is clarified in the manual, if you assume that numeric type
> parameters work exactly as normal ones. (
> http://docs.julialang.org/en/release-0.2/manual/constructors/#parametric-constructors
> )
>
> In the my attempted version, the type is accepted, but constructing things
> doesn't work:
> ~~~
> julia> five = ModInt(5,2)
> ERROR: no method ModInt{n}(Int64, Int64)
>
> julia> five = ModInt{2}(5)
> ERROR: no method ModInt{2}(Int64)
> ~~~
>
> The second instantiation works for the example, which is what I expected.
> However, I still don't understand why the first instantiation doesn't work
> for my version. What does the compiler think is going on?
>
> Thanks,
> Leah
>
