Recursive transformations from Functors.jl

Flux models are deeply nested structures, and Functors.jl provides tools needed to explore such objects, apply functions to the parameters they contain, and re-build them.

New layers should be annotated using the Functors.@functor macro. This will enable params to see the parameters inside, and gpu to move them to the GPU.

Functors.jl has its own notes on basic usage for more details. Additionally, the Advanced Model Building and Customisation page covers the use cases of Functors in greater details.

@functor T
@functor T (x,)

Adds methods to functor allowing recursion into objects of type T, and reconstruction. Assumes that T has a constructor accepting all of its fields, which is true unless you have provided an inner constructor which does not.

By default all fields of T are considered children; this can be restricted be restructed by providing a tuple of field names.


julia> struct Foo; x; y; end

julia> @functor Foo

julia> Functors.children(Foo(1,2))
(x = 1, y = 2)

julia> _, re = Functors.functor(Foo(1,2));

julia> re((10, 20))
Foo(10, 20)

julia> struct TwoThirds a; b; c; end

julia> @functor TwoThirds (a, c)

julia> ch2, re3 = Functors.functor(TwoThirds(10,20,30));

julia> ch2
(a = 10, c = 30)

julia> re3(("ten", "thirty"))
TwoThirds("ten", 20, "thirty")

julia> fmap(x -> 10x, TwoThirds(Foo(1,2), Foo(3,4), 56))
TwoThirds(Foo(10, 20), Foo(3, 4), 560)
fmap(f, x, ys...; exclude = Functors.isleaf, walk = Functors.DefaultWalk()[, prune])
fmap(walk, f, x, ys...)

A structure and type preserving map.

By default it transforms every leaf node (identified by exclude, default isleaf) by applying f, and otherwise traverses x recursively using functor. Optionally, it may also be associated with objects ys with the same tree structure. In that case, f is applied to the corresponding leaf nodes in x and ys.


julia> fmap(string, (x=1, y=(2, 3)))
(x = "1", y = ("2", "3"))

julia> nt = (a = [1,2], b = [23, (45,), (x=6//7, y=())], c = [8,9]);

julia> fmap(println, nt)
[1, 2]
[8, 9]
(a = nothing, b = Any[nothing, (nothing,), (x = nothing, y = nothing)], c = nothing)

julia> fmap(println, nt; exclude = x -> x isa Array)
[1, 2]
Any[23, (45,), (x = 6//7, y = ())]
[8, 9]
(a = nothing, b = nothing, c = nothing)

julia> twice = [1, 2];  # println only acts once on this

julia> fmap(println, (i = twice, ii = 34, iii = [5, 6], iv = (twice, 34), v = 34.0))
[1, 2]
[5, 6]
(i = nothing, ii = nothing, iii = nothing, iv = (nothing, nothing), v = nothing)

julia> d1 = Dict("x" => [1,2], "y" => 3);

julia> d2 = Dict("x" => [4,5], "y" => 6, "z" => "an_extra_value");

julia> fmap(+, d1, d2) == Dict("x" => [5, 7], "y" => 9) # Note that "z" is ignored

Mutable objects which appear more than once are only handled once (by caching f(x) in an IdDict). Thus the relationship x.i === x.iv[1] will be preserved. An immutable object which appears twice is not stored in the cache, thus f(34) will be called twice, and the results will agree only if f is pure.

By default, Tuples, NamedTuples, and some other container-like types in Base have children to recurse into. Arrays of numbers do not. To enable recursion into new types, you must provide a method of functor, which can be done using the macro @functor:

julia> struct Foo; x; y; end

julia> @functor Foo

julia> struct Bar; x; end

julia> @functor Bar

julia> m = Foo(Bar([1,2,3]), (4, 5, Bar(Foo(6, 7))));

julia> fmap(x -> 10x, m)
Foo(Bar([10, 20, 30]), (40, 50, Bar(Foo(60, 70))))

julia> fmap(string, m)
Foo(Bar("[1, 2, 3]"), ("4", "5", Bar(Foo("6", "7"))))

julia> fmap(string, m, exclude = v -> v isa Bar)
Foo("Bar([1, 2, 3])", (4, 5, "Bar(Foo(6, 7))"))

To recurse into custom types without reconstructing them afterwards, use fmapstructure.

For advanced customization of the traversal behaviour, pass a custom walk function that subtypes Functors.AbstractWalk. The form fmap(walk, f, x, ys...) can be called for custom walks. The simpler form fmap(f, x, ys...; walk = mywalk) will wrap mywalk in ExcludeWalk then CachedWalk.

julia> struct MyWalk <: Functors.AbstractWalk end

julia> (::MyWalk)(recurse, x) = x isa Bar ? "hello" :
                                            Functors.DefaultWalk()(recurse, x)

julia> fmap(x -> 10x, m; walk = MyWalk())
Foo("hello", (40, 50, "hello"))

julia> fmap(MyWalk(), x -> 10x, m)
Foo("hello", (4, 5, "hello"))

The behaviour when the same node appears twice can be altered by giving a value to the prune keyword, which is then used in place of all but the first:

julia> twice = [1, 2];

julia> fmap(float, (x = twice, y = [1,2], z = twice); prune = missing)
(x = [1.0, 2.0], y = [1.0, 2.0], z = missing)

Return true if x has no children according to functor.


julia> Functors.isleaf(1)

julia> Functors.isleaf([2, 3, 4])

julia> Functors.isleaf(["five", [6, 7]])

julia> Functors.isleaf([])

julia> Functors.isleaf((8, 9))

julia> Functors.isleaf(())

Return the children of x as defined by functor. Equivalent to functor(x)[1].

fcollect(x; exclude = v -> false)

Traverse x by recursing each child of x as defined by functor and collecting the results into a flat array, ordered by a breadth-first traversal of x, respecting the iteration order of children calls.

Doesn't recurse inside branches rooted at nodes v for which exclude(v) == true. In such cases, the root v is also excluded from the result. By default, exclude always yields false.

See also children.


julia> struct Foo; x; y; end

julia> @functor Foo

julia> struct Bar; x; end

julia> @functor Bar

julia> struct TypeWithNoChildren; x; y; end

julia> m = Foo(Bar([1,2,3]), TypeWithNoChildren(:a, :b))
Foo(Bar([1, 2, 3]), TypeWithNoChildren(:a, :b))

julia> fcollect(m)
4-element Vector{Any}:
 Foo(Bar([1, 2, 3]), TypeWithNoChildren(:a, :b))
 Bar([1, 2, 3])
 [1, 2, 3]
 TypeWithNoChildren(:a, :b)

julia> fcollect(m, exclude = v -> v isa Bar)
2-element Vector{Any}:
 Foo(Bar([1, 2, 3]), TypeWithNoChildren(:a, :b))
 TypeWithNoChildren(:a, :b)

julia> fcollect(m, exclude = v -> Functors.isleaf(v))
2-element Vector{Any}:
 Foo(Bar([1, 2, 3]), TypeWithNoChildren(:a, :b))
 Bar([1, 2, 3])
Functors.functor(x) = functor(typeof(x), x)

Returns a tuple containing, first, a NamedTuple of the children of x (typically its fields), and second, a reconstruction funciton. This controls the behaviour of fmap.

Methods should be added to functor(::Type{T}, x) for custom types, usually using the macro @functor.

fmapstructure(f, x; exclude = isleaf)

Like fmap, but doesn't preserve the type of custom structs. Instead, it returns a NamedTuple (or a Tuple, or an array), or a nested set of these.

Useful for when the output must not contain custom structs.


julia> struct Foo; x; y; end

julia> @functor Foo

julia> m = Foo([1,2,3], [4, (5, 6), Foo(7, 8)]);

julia> fmapstructure(x -> 2x, m)
(x = [2, 4, 6], y = Any[8, (10, 12), (x = 14, y = 16)])

julia> fmapstructure(println, m)
[1, 2, 3]
(x = nothing, y = Any[nothing, (nothing, nothing), (x = nothing, y = nothing)])