How to extract the specific type from an instance of a generic type in julia?

Question

How do I get the subtype of an instance of an parametric type in julia? For example:

immutable Dog{T <: Number}
    snout::T
end

dog = Dog(5.)
typeof(dog)

...returns Dog{Float64}. Is there a way to get at the type Float64 from the variable dog without referring explicitly to the field snout?

Answer 1

It depends on your use case. If you are interested in a specific case like this, a good way is to define a function

dogtype{T}(::Dog{T}) = T

Then dogtype(Dog(.5)) will give you Float 64.

This is the kind of pattern that is used to implement the eltype function in base Julia.

Answer 2

This works for me:

julia> VERSION
v"0.4.0-dev+5733"

julia> immutable Dog{T <: Number}
           snout::T
       end

julia> dog = Dog(0.5)
Dog{Float64}(0.5)

julia> typeof(dog).parameters[1]
Float64