What does #with_kw do in Julia? - julia

Question

I am reading some code which looks like:
#with_kw struct HyperParams
batch_size::Int = 128
latent_dim::Int = 100
epochs::Int = 25
verbose_freq::Int = 1000
output_dim::Int = 5
disc_lr::Float64 = 0.0002
gen_lr::Float64 = 0.0002
device::Function = gpu
end
but it is unclear to me what the #with_kw is doing in this context. Is this still a normal struct? It does not look like the macro is part of base Julia so I am unfamiliar with its usage here.

Answer 1

So it looks like the #with_kw is part of the Parameters package where it provides the ability to define default values for a struct fields and keyword arguments. Per the Julia docs here: https://docs.julialang.org/en/v1/manual/types/#Composite-Types it does not look like you can define default values so this is actually quite useful in that case. Here is an example of what is possible with the keyword arguments and default values:
julia> #with_kw struct HyperParams
batch_size::Int = 128
latent_dim::Int = 100
epochs::Int = 25
verbose_freq::Int = 1000
output_dim::Int = 5
disc_lr::Float64 = 0.0002
gen_lr::Float64 = 0.0002
device::Function
end
HyperParams
julia> hyper = HyperParams(device=cpu)
HyperParams
batch_size: Int64 128
latent_dim: Int64 100
epochs: Int64 25
verbose_freq: Int64 1000
output_dim: Int64 5
disc_lr: Float64 0.0002
gen_lr: Float64 0.0002
device: cpu (function of type typeof(cpu))
julia> HyperParams()
ERROR: Field 'device' has no default, supply it with keyword.
Stacktrace:
[1] error(s::String)
# Base ./error.jl:33
[2] HyperParams()
# Main ~/.julia/packages/Parameters/MK0O4/src/Parameters.jl:493
[3] top-level scope
# REPL[61]:1
julia> hyper = HyperParams(epochs=20, device=cpu)
HyperParams
batch_size: Int64 128
latent_dim: Int64 100
epochs: Int64 20
verbose_freq: Int64 1000
output_dim: Int64 5
disc_lr: Float64 0.0002
gen_lr: Float64 0.0002
device: cpu (function of type typeof(cpu))