Zygote has a problem of derivation in the loss function

New to Julia
,
1

I have a problem with Flux. Unfortunately Zygote cannot derive my loss function and I am not sure what it is. It would be nice if someone could help me with this problem.
My loss function looks like this:

function loss_ungelabelt2(X,Y)
    
    a_list = Float32[]
    b_list = Float32[]
    mix_res = Float32[]
    
    λ::Float32 = 0.3
    loss::Float32 = 0
    
    for i in 1:2  #eigentlich size(X,4)  
        a = X[:,:,:,rand(1:size(X,4))] 
        b = X[:,:,:,rand(1:size(X,4))]
        
        mix = λ .* a .+ (1 - λ) .* b
        println(length(mix))
        println(size(mix))
        println("NEU")
        mix_res=reshape([mix_res...,mix...],size(mix)...,:) # einzelne Elemente 
        a_list=reshape([a_list...,a...],size(a)...,:)
        b_list=reshape([b_list...,b...],size(b)...,:)
        println(size(mix_res))
        println("neueRunde")
        
        
    end
    y_mix = model(mix_res)
    y_a = model(a_list)
    y_b = model(b_list)
    println("a", size(y_a))
    println("b", size(y_b))
    y_mix_ab =  λ .* y_a .+ (1 - λ) .* y_b 
    println(length(y_mix_ab))
    
    quadrat_loss = sum(abs2,y_mix .- y_mix_ab)
    
    return quadrat_loss
    
end

I get this error:

MethodError: no method matching size(::NTuple{784,Float32})
Closest candidates are:
  size(::Tuple, !Matched::Integer) at tuple.jl:22
  size(!Matched::Flux.OneHotVector) at /Users/lisa/.julia/packages/Flux/IjMZL/src/onehot.jl:8
  size(!Matched::ZMQ.Message) at /Users/lisa/.julia/packages/ZMQ/R3wSD/src/message.jl:95
  ...

Stacktrace:
 [1] unbroadcast(::Array{Float32,3}, ::NTuple{784,Float32}) at /Users/lisa/.julia/packages/Zygote/seGHk/src/lib/broadcast.jl:53
 [2] (::Zygote.var"#1107#1109"{NTuple{784,Float32}})(::Array{Float32,3}) at /Users/lisa/.julia/packages/Zygote/seGHk/src/lib/broadcast.jl:74
 [3] map(::Zygote.var"#1107#1109"{NTuple{784,Float32}}, ::Tuple{Array{Float32,3},Array{Float32,3}}) at ./tuple.jl:158
 [4] (::Zygote.var"#1106#1108"{Tuple{Array{Float32,3},Array{Float32,3}}})(::NTuple{784,Float32}) at /Users/lisa/.julia/packages/Zygote/seGHk/src/lib/broadcast.jl:74
 [5] (::Zygote.var"#3852#back#1110"{Zygote.var"#1106#1108"{Tuple{Array{Float32,3},Array{Float32,3}}}})(::NTuple{784,Float32}) at /Users/lisa/.julia/packages/ZygoteRules/6nssF/src/adjoint.jl:49
 [6] loss_ungelabelt2 at ./In[13]:14 [inlined]
 [7] (::typeof(∂(loss_ungelabelt2)))(::Float32) at /Users/lisa/.julia/packages/Zygote/seGHk/src/compiler/interface2.jl:0
 [8] (::Zygote.var"#41#42"{typeof(∂(loss_ungelabelt2))})(::Float32) at /Users/lisa/.julia/packages/Zygote/seGHk/src/compiler/interface.jl:45
 [9] gradient(::Function, ::Array{Float32,4}, ::Vararg{Any,N} where N) at /Users/lisa/.julia/packages/Zygote/seGHk/src/compiler/interface.jl:54
 [10] top-level scope at In[14]:1
 [11] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1091

I would be very grateful for an answer.

2

what abt this size(::NTuple{N,T}) where {N, T} = N

3

What exactly do you mean by this?
Because I am not quite sure where this mistake comes from. The sizes in the code are multidimensional. And only the lengths are 784.

4

the size function does not support arguments of type NTuple{N,Float32}. @xiaodai 's solution is to add a method to size as: size(::NTuple{N,T}) where {N, T} = N. It should solve the problem reported i.e. missing a size method.

5

Ah okay. Thanks a lot!
Sry, I don’t usually work with Julia and I am a little helpless.
But if I add the line Base.size(::NTuple{N,T}) where {N, T} = N I get an argumentError: invalid tuple dimension 2

6

You could do that, it might fix it. (Or it might not)
Its not the real solution though, and it is kind of a bad thing to do, it is type piracy (overloading a function you don’t own on a set of types none of which you own).

This is a bug in Zygote. you should report it on the Zygote issue tracker.
Its not something you have done wrong.

I did some poking
The error is this line https://github.com/FluxML/Zygote.jl/blob/ed366f32c0f520567526040d9f8acaf0d83613c3/src/lib/broadcast.jl#L52
where zygote tries to take size(x̄)
x̄ is turning out to be a Tuple which don’t have size defined on them.
Only length.
However, that is just a symptom, as that line should be fine,
because if x is an Array,
then x̄ should also be an Array.

So something is going wrong somewhere else that is causing it to be a Tuple.
It needs to be fixed in Zygote.

7

Thank you for the effort! I have already reported it and I hope it can be fixed without much effort and time.