Compile-time recognition of parametric types e.g. K{T} for any K

Usage
#1

Hi,

I’m don’t know how to get the compiler to recognise a type K{T} where K could be any type. For example:

abstract type AbstractGeneric{T} end
#... after defining further types <: AbstractGeneric{T} for example ... #
struct MyType{T} <: AbstractGeneric{T}
  x::T
end

function myMethod(x::MyType{T}) where {T}
  return 2*x.x
end

# Baby example
 function fun(x::K{T}, z::AbstractArray{T}) where {K <: AbstractGeneric, T <: AbstractFloat}
  return sum((myMethod(x) .- z)^2)
end

I specifically want to constrain K{T} with AbstractArray{T} on T. The compiler gives me the error:

ERROR: TypeError: in Type{...} expression, expected UnionAll, got TypeVar

I know that it will compile with:

 function fun(x::AbstractGeneric{T}, z::AbstractArray{T}) where {T <: AbstractFloat}
  return sum((myMethod(x) .- z)^2)
end

But that’s runtime dispatch on myMethod() which I don’t want.

Thanks

#2

This is a type of dispatch which is not allowed in Julia. The correct way to do it is indeed exactly the second thing you mention, but I didn’t understand this part:

Do you mean that in this case its its not inferred, so dispatch will happen at runtime? That’s not what I’m seeing. After a small fix to your fun by changing ^ to a .^, it looks everything is inferred hence dispatch is happening at compile time:

julia> function fun(x::AbstractGeneric{T}, z::AbstractArray{T}) where {T <: AbstractFloat}
         return sum((myMethod(x) .- z).^2)
       end
fun (generic function with 1 method)

julia> @code_warntype fun(MyType(1.), [1.,2,3])
Variables
  #self#::Core.Compiler.Const(fun, false)
  x::MyType{Float64}
  z::Array{Float64,1}

Body::Float64
1 ─ %1 = Main.myMethod(x)::Float64
...
#3

Yes, that’s what I meant. I should remember to use @code_warntype. When I think about it it makes sense or else AbstractArray would be dispatching things at run time which would be slow. It’s just that at a glance it looks like runtime polymorphism in any other language. What would be the runtime dispatch equivalent if it exists?

Thank you.

1 Like
#4

Answering my own question:

abstract type AbstractGeneric end
struct MyType <: AbstractGeneric
  x
end
struct MySecondType <: AbstractGeneric
  x
  y
end

function myMethod(x::MyType)
  return 2*x.x
end

function fun(x::AbstractGeneric, z::AbstractArray)
  return sum((myMethod(x) .- z).^2)
end

@code_warntype fun(MyType(3), rand(5))

Gives

@code_warntype fun(MyType(3), rand(5))
Variables
  #self#::Core.Compiler.Const(fun, false)
  x::MyType
  z::Array{Float64,1}

Body::Any
1 ─ %1 = Main.myMethod(x)::Any
│   %2 = Base.broadcasted(Main.:-, %1, z)::Any
│   %3 = Core.apply_type(Base.Val, 2)::Core.Compiler.Const(Val{2}, false)
│   %4 = (%3)()::Core.Compiler.Const(Val{2}(), false)
│   %5 = Base.broadcasted(Base.literal_pow, Main.:^, %2, %4)::Any
│   %6 = Base.materialize(%5)::Any
│   %7 = Main.sum(%6)::Any
└──      return %7

The compiler substitutes for Any.