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Extract common functionality into fold #32

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e4088e0
Extract common functionality into fold
ToucheSir Jan 30, 2022
b3db3e2
Fixup and add cache tests
ToucheSir Jan 30, 2022
94a9b05
cleaner ternary and s/iscachesafe/usecache/
ToucheSir Jan 30, 2022
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87 changes: 63 additions & 24 deletions src/functor.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -26,7 +26,7 @@ function makefunctor(m::Module, T, fs = fieldnames(T))
escargs = map(fieldnames(T)) do f
f in fs ? :(y[$(yᵢ += 1)]) : :(x.$f)
end
escfs = [:($f=x.$f) for f in fs]
escfs = [:($f = x.$f) for f in fs]

@eval m begin
$Functors.functor(::Type{<:$T}, x) = ($(escfs...),), y -> $T($(escargs...))
Expand Down Expand Up @@ -169,7 +169,54 @@ function _default_walk(f, x)
func, re = functor(x)
re(map(f, func))
end
_default_walk(f, ::Nothing, ::Nothing) = nothing
_default_walk(_, ::Nothing, ::Nothing) = nothing

# Side effects only, saves a restructure
function _foreach_walk(f, x)
foreach(f, children(x))
return x
end

### WARNING: the following is unstable internal functionality. Use at your own risk!
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I considered _-prefixing everything but didn't for now because these will probably become part of the public API at some point. Happy to prefix them if that's desired though.

# Wrapper over an IdDict which only saves values with a stable object identity
struct Cache{K,V}
inner::IdDict{K,V}
end
Cache() = Cache(IdDict())

usecache(x) = !isbits(x) && ismutable(x)
# Functionally immutable and observe value semantics, but still `ismutable` and not `isbits`
usecache(::Union{String,Symbol}) = false
# For varargs
usecache(xs::Tuple) = all(usecache, xs)
Base.get!(f, c::Cache, x) = usecache(x) ? get!(f, c.inner, x) : f()

# Passthrough used to disable caching (e.g. when passing `cache=false`)
struct NoCache end
Base.get!(f, ::NoCache, _) = f()

# Encapsulates the self-recursive part of a recursive tree reduction (fold).
# This allows calling functions to remove any self-calls or nested callback closures.
struct Fold{F,L,C,W}
fn::F
isleaf::L
cache::C
walk::W
end
(fld::Fold)(x) = get!(fld.cache, x) do
fld.fn(fld.isleaf(x) ? x : fld.walk(fld, x))
end

# Convenience function for working with `Fold`
function fold(f, x; isleaf = isleaf, cache = false, walk = _default_walk)
if cache === true
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Note that fmap and fcollect are still passing through IdDict() by default. The hope is that new functions either don't allow customization of the cache or use a "safer" default.

cache = Cache()
elseif cache === false
cache = NoCache()
end
return Fold(f, isleaf, cache, walk)(x)
end
### end of unstable internal functionality

"""
fmap(f, x; exclude = Functors.isleaf, walk = Functors._default_walk)
Expand Down Expand Up @@ -253,11 +300,9 @@ Foo(Bar([1, 2, 3]), (40, 50, Bar(Foo(6, 7))))
```
"""
function fmap(f, x; exclude = isleaf, walk = _default_walk, cache = IdDict())
haskey(cache, x) && return cache[x]
y = exclude(x) ? f(x) : walk(x -> fmap(f, x, exclude = exclude, walk = walk, cache = cache), x)
cache[x] = y

return y
return fold(x; cache, walk, isleaf = exclude) do node
exclude(node) ? f(node) : node
end
end

"""
Expand Down Expand Up @@ -296,8 +341,8 @@ fmapstructure(f, x; kwargs...) = fmap(f, x; walk = (f, x) -> map(f, children(x))
fcollect(x; exclude = v -> false)

Traverse `x` by recursing each child of `x` as defined by [`functor`](@ref)
and collecting the results into a flat array, ordered by a breadth-first
traversal of `x`, respecting the iteration order of `children` calls.
and collecting the results into a flat array, ordered by a depth-first,
post-order traversal of `x` that respects the iteration order of `children` calls.
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This is technically breaking (as can be seen from the test changes), but it's unclear to me how many users were relying on the breadth-first iteration order. The one example I found on JuliaHub, AlphaZero, was not. It's also odd that the original worked breadth-first when pretty much all other traversals based on functors are depth-first. So RFC, and happy to back this out if we consider it too breaking.

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I think it's fine to have this breaking change.

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I think that nothing now in Optimisers.jl now cares about the order taken by fmap.

Of course the two halves of destructure care a lot and must match. I haven't completely understood how #31 would have to change; it currently uses fmap for one half and something hand-written for the other.

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The order in fmap has always been stable, things brings fcollect in line as it was the odd one out.


Doesn't recurse inside branches rooted at nodes `v`
for which `exclude(v) == true`.
Expand All @@ -324,33 +369,27 @@ Foo(Bar([1, 2, 3]), NoChildren(:a, :b))

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

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

julia> fcollect(m, exclude = v -> Functors.isleaf(v))
2-element Vector{Any}:
Foo(Bar([1, 2, 3]), NoChildren(:a, :b))
Bar([1, 2, 3])
Foo(Bar([1, 2, 3]), NoChildren(:a, :b))
```
"""
function fcollect(x; output = [], cache = Base.IdSet(), exclude = v -> false)
# note: we don't have an `OrderedIdSet`, so we use an `IdSet` for the cache
# (to ensure we get exactly 1 copy of each distinct array), and a usual `Vector`
# for the results, to preserve traversal order (important downstream!).
x in cache && return output
if !exclude(x)
push!(cache, x)
push!(output, x)
foreach(y -> fcollect(y; cache=cache, output=output, exclude=exclude), children(x))
end
return output
function fcollect(x; output = [], cache = Base.IdDict(), exclude = v -> false)
fold(x; cache, isleaf = exclude, walk = _foreach_walk) do node
exclude(node) || push!(output, node); # always return nothing
end
return output
end

# Allow gradients and other constructs that match the structure of the functor
Expand Down
34 changes: 27 additions & 7 deletions test/basics.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,26 @@ end
end
end

@testset "Folds" begin
arrays = ntuple(i -> [i], 3)
model = Foo(
Foo(arrays[1], arrays[2]),
Foo(arrays[3], arrays[1])
)

total = Ref(0)
Functors.fmap(model, cache = true) do x
total[] += only(x)
end
@test total[] == 6

total = Ref(0)
Functors.fmap(model, cache = false) do x
total[] += only(x)
end
@test total[] == 7
end

@testset "Nested" begin
model = Bar(Foo(1, [1, 2, 3]))

Expand Down Expand Up @@ -72,21 +92,21 @@ end
m2 = 1
m3 = Foo(m1, m2)
m4 = Bar(m3)
@test all(fcollect(m4) .=== [m4, m3, m1, m2])
@test all(fcollect(m4, exclude = x -> x isa Array) .=== [m4, m3, m2])
@test all(fcollect(m4) .=== [m1, m2, m3, m4])
@test all(fcollect(m4, exclude = x -> x isa Array) .=== [m2, m3, m4])
@test all(fcollect(m4, exclude = x -> x isa Foo) .=== [m4])

m1 = [1, 2, 3]
m2 = Bar(m1)
m0 = NoChildren(:a, :b)
m3 = Foo(m2, m0)
m4 = Bar(m3)
@test all(fcollect(m4) .=== [m4, m3, m2, m1, m0])
@test all(fcollect(m4) .=== [m1, m2, m0, m3, m4])

m1 = [1, 2, 3]
m2 = [1, 2, 3]
m3 = Foo(m1, m2)
@test all(fcollect(m3) .=== [m3, m1, m2])
@test all(fcollect(m3) .=== [m1, m2, m3])
end

struct FFoo
Expand Down Expand Up @@ -143,14 +163,14 @@ end
m2 = [1, 2, 3]
m3 = FFoo(m1, m2, (:y, ))
m4 = FBar(m3, (:x,))
@test all(fcollect(m4) .=== [m4, m3, m2])
@test all(fcollect(m4, exclude = x -> x isa Array) .=== [m4, m3])
@test all(fcollect(m4) .=== [m2, m3, m4])
@test all(fcollect(m4, exclude = x -> x isa Array) .=== [m3, m4])
@test all(fcollect(m4, exclude = x -> x isa FFoo) .=== [m4])

m0 = NoChildren(:a, :b)
m1 = [1, 2, 3]
m2 = FBar(m1, ())
m3 = FFoo(m2, m0, (:x, :y,))
m4 = FBar(m3, (:x,))
@test all(fcollect(m4) .=== [m4, m3, m2, m0])
@test all(fcollect(m4) .=== [m2, m0, m3, m4])
end