Functors.jl provides tools to express a powerful design pattern for dealing with large/ nested structures, as in machine learning and optimisation. For large machine learning models it can be cumbersome or inefficient to work with parameters as one big, flat vector, and structs help manage complexity; but it is also desirable to easily operate over all parameters at once, e.g. for changing precision or applying an optimiser update step.

Functors.jl provides fmap to make those things easy, acting as a 'map over parameters':

julia> using Functors

julia> struct Foo
         x
         y
       end

julia> @functor Foo

julia> model = Foo(1, [1, 2, 3])
Foo(1, [1, 2, 3])

julia> fmap(float, model)
Foo(1.0, [1.0, 2.0, 3.0])

It works also with deeply-nested models:

julia> struct Bar
         x
       end

julia> @functor Bar

julia> model = Bar(Foo(1, [1, 2, 3]))
Bar(Foo(1, [1, 2, 3]))

julia> fmap(float, model)
Bar(Foo(1.0, [1.0, 2.0, 3.0]))

The workhorse of fmap is actually a lower level function, functor:

julia> xs, re = functor(Foo(1, [1, 2, 3]))
((x = 1, y = [1, 2, 3]), var"#21#22"())

julia> re(map(float, xs))
Foo(1.0, [1.0, 2.0, 3.0])

functor returns the parts of the object that can be inspected, as well as a re function that takes those values and restructures them back into an object of the original type.

To include only certain fields, pass a tuple of field names to @functor:

julia> struct Baz
         x
         y
       end

julia> @functor Baz (x,)

julia> model = Baz(1, 2)
Baz(1, 2)

julia> fmap(float, model)
Baz(1.0, 2)

Any field not in the list will not be returned by functor and passed through as-is during reconstruction. This is done by invoking the default constructor, so structs that define custom inner constructors are expected to provide one that acts like the default.

It is also possible to implement functor by hand when greater flexibility is required. See here for an example.

For a discussion regarding the need for a cache in the implementation of fmap, see here.

Use exclude for more fine-grained control over whether fmap descends into a particular value (the default is exclude = Functors.isleaf):

julia> using CUDA

julia> x = ['a', 'b', 'c'];

julia> fmap(cu, x)
3-element Array{Char,1}:
 'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)
 'b': ASCII/Unicode U+0062 (category Ll: Letter, lowercase)
 'c': ASCII/Unicode U+0063 (category Ll: Letter, lowercase)

julia> fmap(cu, x; exclude = x -> CUDA.isbitstype(eltype(x)))
3-element CuArray{Char,1}:
 'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)
 'b': ASCII/Unicode U+0062 (category Ll: Letter, lowercase)
 'c': ASCII/Unicode U+0063 (category Ll: Letter, lowercase)

• StructWalk.jl