Why is convert not called for parametric type?

General Usage
,
1

For parametric type, does the constructor not call convert automatically?

julia> struct Foo3{T}
           a::T
           b::Float64
       end

julia> Foo3(1,2)
ERROR: MethodError: no method matching Foo3(::Int64, ::Int64)
Closest candidates are:
  Foo3(::T, ::Float64) where T at REPL[25]:2
Stacktrace:
 [1] top-level scope at none:0

julia> Foo3(1,2.0)
Foo3{Int64}(1, 2.0)

It works properly for regular types:

julia> struct Foo2
           x::Int
           y::Float64
       end

julia> Foo2(1,2)
Foo2(1, 2.0)
2

The constructor DOES call convert.

julia> Foo3{Int}(1, 2)
Foo3{Int64}(1, 2.0)

You can see clearly from the error message and the backtrace that the error happens on the call of Foo3, not inside the constructor. There simply isn’t a constructor defined in this case to guess the type parameter for you.

3 Likes
3

https://github.com/JuliaLang/julia/issues/17186

1 Like
4

Yes, it would be nice if it just works without having to specify the type parameter.

5

You could manually define an outer constructor though:
Foo3(a::T, b) where {T} = Foo3{T}(a, b)

6

That’s a good idea! If such outer constructor were “auto-generated” then it would be super-awesome.