Flux.destructure gives MethodError when used with non-trainable parameters

[ctessum]

Hello,

I've experienced an error (using Flux 0.11.6) with the following code:

using Flux

struct train_part
    a
    b
end

function (a::train_part)(x) 
    a.a * a.b * x
end

Flux.@functor train_part (a,) # Specify that only the 'a' matrix is trainable.

m = Chain(
    Dense(2, 2, tanh),
    train_part(zeros(2,2), zeros(2,2)),
)

Flux.destructure(m)

This is the error message:

ERROR: MethodError: no method matching train_part(::Matrix{Float64})
Closest candidates are:
  train_part(::Any, ::Any) at REPL[12]:2
Stacktrace:
  [1] (::var"#3#4")(y::NamedTuple{(:a,), Tuple{Matrix{Float64}}})
    @ Main ~/.julia/packages/Functors/YlETM/src/functor.jl:12
  [2] fmap1(f::Function, x::train_part)
    @ Functors ~/.julia/packages/Functors/YlETM/src/functor.jl:30
  [3] #fmap#13
    @ ~/.julia/packages/Functors/YlETM/src/functor.jl:35 [inlined]
  [4] (::Functors.var"#14#15"{IdDict{Any, Any}, Flux.var"#33#35"{Zygote.Buffer{Any, Vector{Any}}}})(x::train_part)
    @ Functors ~/.julia/packages/Functors/YlETM/src/functor.jl:35
  [5] map
    @ ./tuple.jl:214 [inlined]
  [6] fmap1(f::Function, x::Chain{Tuple{Dense{typeof(tanh), Matrix{Float32}, Vector{Float32}}, train_part}})
    @ Functors ~/.julia/packages/Functors/YlETM/src/functor.jl:30
  [7] fmap(f::Flux.var"#33#35"{Zygote.Buffer{Any, Vector{Any}}}, x::Chain{Tuple{Dense{typeof(tanh), Matrix{Float32}, Vector{Float32}}, train_part}}; cache::IdDict{Any, Any})
    @ Functors ~/.julia/packages/Functors/YlETM/src/functor.jl:35
  [8] fmap
    @ ~/.julia/packages/Functors/YlETM/src/functor.jl:34 [inlined]
  [9] destructure(m::Chain{Tuple{Dense{typeof(tanh), Matrix{Float32}, Vector{Float32}}, train_part}})
    @ Flux ~/.julia/packages/Flux/goUGu/src/utils.jl:409
 [10] top-level scope
    @ REPL[16]:1

I think what might be happening is that destructure extracts one value from train_part, because only one of the values in train_part is trainable, and then it tries to reconstruct it with just the one variable, which does not work.

[ToucheSir]

Ah yes, this is a case of FluxML/Functors.jl#6 / FluxML/Functors.jl#3. It should be resolved in Functors.jl 2.x, which you can get by upgrading to Flux 0.12. If you're not able to update Flux, you can implement functor manually per FluxML/Functors.jl#3 (comment).

[ctessum]

Thanks, that fixes it!