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This is a package I use to handle numerical-model parameters, thus the name. However, it should be useful otherwise too. It has two main features:

Checkout my ten minute JuliaCon 2018 talk.

The macro @with_kw which decorates a type definition to allow default values and a keyword constructor:

julia> using Parameters

julia> @with_kw struct A
           a::Int = 6
           b::Float64 = -1.1
           c::UInt8
       end

julia> A(c=4)
A
  a: 6
  b: -1.1
  c: 4

julia> A()
ERROR: Field 'c' has no default, supply it with keyword.

julia> A(c=4, a = 2)
A
  a: 2
  b: -1.1
  c: 4

The macro also supports constructors for named tuples with default values; e.g.

julia> MyNT = @with_kw (x = 1, y = "foo", z = :(bar))
(::#5) (generic function with 2 methods)

julia> MyNT()
(x = 1, y = "foo", z = :bar)

julia> MyNT(x = 2)
(x = 2, y = "foo", z = :bar)

Unpacking is done with @unpack (@pack! is similar):

struct B
    a
    b
    c
end
@unpack a, c = B(4,5,6)
# is equivalent to
BB = B(4,5,6)
a = BB.a
c = BB.c

Defining several constants

@consts begin
    a = 1
    b = 2.0
    c = "a"
end

The features are:

The keyword-constructor and default-values functionality will probably make it into Julia (# 10146, #533 and #6122) although probably not with all the features present in this package. I suspect that this package should stay usable & useful even after this change lands in Julia. Note that keyword functions are currently slow in Julia, so these constructors should not be used in hot inner loops. However, the normal positional constructor is also provided and could be used in performance critical code.

NEWS.md keeps tabs on updates.

Documentation

Documentation is here: stable and latest.

Related packages

Complementary:

Implementing similar things:

TODO