# Parameters manual

This is a manual by example (examples/ex1.jl).

# Types with default values & keyword constructors

Create a type which has default values using @with_kw:

using Parameters

@with_kw struct PhysicalPara{R}
rw::R = 1000.
ri::R = 900.
L::R = 3.34e5
g::R = 9.81
cw::R = 4220.
day::R = 24*3600.
end

Now the type can be constructed using the default values, or with non-defaults specified with keywords:

# Create an instance with the defaults
pp = PhysicalPara()
pp_f32 = PhysicalPara{Float32}() # the type parameter can be chosen explicitly
# Make one with some non-defaults
pp2 = PhysicalPara(cw=77.0, day= 987.0)
# Make another one based on the previous one with some modifications
pp3 = PhysicalPara(pp2; cw=.11e-7, rw=100.)
# the normal positional constructor can also be used
# (and should be used in hot inner loops)
pp4 = PhysicalPara(1,2,3,4,5,6)

To enforce constraints on the values, it's possible to use @asserts straight inside the type-def. (As usual, for mutables these asserts can be violated by updating the fields after type construction.)

@with_kw struct PhysicalPara2{R}
rw::R = 1000.; @assert rw>0
ri::R = 900.
@assert rw>ri # Note that the placement of assertions is not
# relevant. (They are moved to the constructor.
end

Parameter interdependence is possible:

@with_kw struct Para{R<:Real}
a::R = 5
b::R
c::R = a+b
end
pa = Para(b=7)

Often the bulk of fields will have the same type. To help with this, a default type can be set. Using this feature, the last example (with additional field d) can be written more compactly as:

@with_kw struct Para2{R<:Real} @deftype R
a = 5
b
c = a+b
d::Int = 4 # adding a type overrides the @deftype
end
pa2 = Para2(b=7)

# or more pedestrian
@with_kw struct Para3 @deftype Float64
a = 5
b
c = a+b
d::Int = 4
end
pa3 = Para3(b=7)

Custom inner constructors can be defined as long as:

• one defining all positional arguments is given
• no zero-positional arguments constructor is defined (as that would clash with the keyword constructor)
• no @asserts (as in above example) are used within the type body.

The keyword constructor goes through the inner positional constructor, thus invariants or any other calculation will be honored.

@with_kw struct MyS{R}
a::R = 5
b = 4
MyS{R}(a,b) where {R} = (@assert a>b; new(a,b)) #
MyS{R}(a) where {R} = MyS{R}(a, a-1) # For this provide your own outer constructor:
end
MyS(a::R) where {R} = MyS{R}(a)

MyS{Int}() # MyS(5,4)
ms = MyS(3) # MyS(3,2)
MyS(ms, b=-1) # MyS(3,-1)
try
MyS(ms, b=6) # this will fail the assertion
catch
end

Note that two of the main reasons to have an inner constructor, assertions and simple calculations, are more easily achieved with @asserts and parameter interdependence.

The macro @with_kw defines a show-method which is, hopefully, more informative than the standard one. For example the printing of the first example is:

julia> PhysicalPara()
PhysicalPara{Float64}
rw: Float64 1000.0
ri: Float64 900.0
L: Float64 334000.0
g: Float64 9.81
cw: Float64 4220.0
day: Float64 86400.0

If this show method definition is not desired, for instance because of method re-definition warnings, then use @with_kw_noshow.

## Named Tuple Support

As mentioned in the README, the @with_kw macro can be used to decorate a named tuple and produce a named tuple constructor with those defaults.

These named tuples can be defined as such:

MyNT = @with_kw (f = x -> x^3, y = 3, z = "foo")

And the constructors can be used as follows:

julia> MyNT(f = x -> x^2, z = :foo)
(f = #12, y = 3, z = :foo)

The constructor is not type-locked:

julia> MyNT(f = "x -> x^3")
(f = "x -> x^3", y = 3, z = "foo")

And these named tuples can unpacked in the usual way (see below).

julia> @unpack f, y, z = MyNT()
(f = #7, y = 3, z = "foo")

julia> f
(::#7) (generic function with 1 method)

julia> y
3

julia> z
"foo"

Since the macro operates on a single tuple expression (as opposed to a tuple of assignment expressions),writing @with_kw(x = 1, y = :foo) will return an error suggesting you write @with_kw (x = 1, y = :foo).

# Blocks of constants

Several constants can be defined like so:

@consts begin
a = 1
b = 2
c = 3
end

(if you do the math, you'll need more than three constants in the block to actually save typing.)

# (Un)pack macros

## @unpack and @pack re-exported from UnPack.jl

When working with parameters, or otherwise, it is often convenient to unpack (and pack, in the case of mutable datatypes) some or all of the fields of a type. This is often the case when passed into a function.

The preferred to do this is using the @unpack and @pack! macros from the package UnPack.jl. These are generic and also work with non-@with_kw stucts, named-tuples, modules, and dictionaries. Here one example is given, for more see the README of UnPack. Define a mutable struct MPara:

@with_kw mutable struct MPara{R<:Real}
a::R = 5
b::R
c::R = a+b
end
pa = MPara(b=7)

function fn2(var, pa::MPara)
@unpack a, b = pa # equivalent to: a,b = pa.a,pa.b
out = var + a + b
b = 77
@pack! pa = b # equivalent to: pa.b = b
return out, pa
end

out, pa = fn2(7, pa)

Note that @unpack and @pack! can be customized on types, see UnPack.jl.

## The type-specific (un)pack-all macros (somewhat dangerous)

The @with_kw macro automatically produces type-specific (un-)pack macros of form @unpack_TypeName, @pack_TypeName!, and @pack_TypeName which unpack/pack all fields:

function fn(var, pa::Para)
@unpack_Para pa # the macro is constructed during the @with_kw
# and called @unpack_*
out = var + a + b
b = 77
@pack_Para! pa # only works with mutables
return out, pa
end

out, pa = fn(7, pa)

When needing a new instance, e.g. for immutables, use the no-bang version:

pa2 = @pack_Para

However, note that the (un-)packing macros which unpack all fields have a few pitfalls, as changing the type definition will change what local variables are available in a function using @unpack_*. Examples:

• adding a field pi to a type would hijack Base.pi usage in any function using @unpack_*
• the @unpack_* will shadow an input argument of the function with the same name as a type-fieldname. This I found very perplexing at times.
• they do not work with properties, i.e. they can only pack/unpack the actual fields of types.

Thus, in general, it is probably better to use the @(un)pack(!) macros instead.