Types with default field values, keyword constructors and (un-)pack macros

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Parameters Parameters

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:

  • keyword type constructors with default values, and
  • unpacking and packing of composite types and dicts.

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

julia> using Parameters

julia> @with_kw type A
           a::Int = 6
           b::Float64 = -1.1

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

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

type A
@unpack a, c = A(4,5,6)
# is equivalent to
AA = A(4,5,6)
a = AA.a
c = AA.c

The features are:

  • a keyword constructor for the type
  • allows setting default values for the fields inside the type definition
  • allows assertions on field values inside the type definition
  • a constructor which allows creating a type-instance taking its defaults from another type instance
  • packing and unpacking macros for the type: @unpack_* where * is the type name.
  • generic packing and unpacking macros @pack, @unpack (work with any types).

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 is here.

Related packages

  • QuickTypes.jl also provides key-word constructors and default values. Provides a more compact format. I'd say QuickTypes.jl is more suited for REPL and other write-only code, whereas Parameters.jl may be more suited for code which is also read.
  • SimpleStructs.jl also provides key-word constructors, default values and assertions. But the syntax is less nice than Parameters.jl.


  • do copy of fields on (re-)construct?
  • think about mutables