# Flux.jl MethodError ... (::Dense{typeof(identity),... on simple Chain model

**URL:** <https://discourse.julialang.org/t/flux-jl-methoderror-dense-typeof-identity-on-simple-chain-model/29247>\
**Category:** Machine Learning\
**Created:** [September 27, 2019, 9:24pm UTC](https://discourse.julialang.org/t/flux-jl-methoderror-dense-typeof-identity-on-simple-chain-model/29247 "2019-09-27T21:24:42Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![m\_Tucci](https://avatars.discourse-cdn.com/v4/letter/m/67e7ee/32.png) [@m\_Tucci](https://discourse.julialang.org/u/m_Tucci)\
**Post date:** [September 27, 2019, 9:24pm UTC](https://discourse.julialang.org/t/flux-jl-methoderror-dense-typeof-identity-on-simple-chain-model/29247/1 "2019-09-27T21:24:43Z")

</div>

Hi, I’m new to using Flux and I keep getting a data type error when running the following code:

```julia
using LinearAlgebra, Flux, Base.Iterators, Statistics
using CuArrays
using Flux: onehotbatch, onecold, crossentropy, throttle, Tracker

trainX = Tracker.data(rand(19,348)) |> gpu
trainY = Tracker.data(rand(19,348)) |> gpu

m = Chain(
  Dense(19,32),
  #LSTM(32,32),
  Dense(32,19),
  softmax) |> gpu

loss(x, y) = Flux.mse(m(x), y)

accuracy(x, y) = mean(onecold(m(x)) .== onecold(y))

dataset = zip(trainX, trainY)
evalcb = () -> @show(loss(trainX, trainY))
opt = ADAM()

Flux.train!(loss, params(m), dataset, opt)

```

Running `train!` results in:

```julia
MethodError: no method matching (::Dense{typeof(identity),TrackedArray{…,Array{Float32,2}},TrackedArray{…,Array{Float32,1}}})(::Float64)
Closest candidates are:
  Dense(!Matched::AbstractArray{T<:Union{Float32, Float64},N} where N) where {T<:Union{Float32, Float64}, W<:(AbstractArray{T,N} where N)} at /home/m_tucci/.julia/packages/Flux/dkJUV/src/layers/basic.jl:110
  Dense(!Matched::AbstractArray{#s107,N} where N where #s107<:AbstractFloat) where {T<:Union{Float32, Float64}, W<:(AbstractArray{T,N} where N)} at /home/m_tucci/.julia/packages/Flux/dkJUV/src/layers/basic.jl:113
  Dense(!Matched::AbstractArray) at /home/m_tucci/.julia/packages/Flux/dkJUV/src/layers/basic.jl:98
applychain(::Tuple{Dense{typeof(identity),TrackedArray{…,Array{Float32,2}},TrackedArray{…,Array{Float32,1}}},Dense{typeof(identity),TrackedArray{…,Array{Float32,2}},TrackedArray{…,Array{Float32,1}}},typeof(softmax)}, ::Float64) at basic.jl:31
(::Chain{Tuple{Dense{typeof(identity),TrackedArray{…,Array{Float32,2}},TrackedArray{…,Array{Float32,1}}},Dense{typeof(identity),TrackedArray{…,Array{Float32,2}},TrackedArray{…,Array{Float32,1}}},typeof(softmax)}})(::Float64) at basic.jl:33
loss(::Float64, ::Float64) at Player_Flux.jl:45
#14 at train.jl:72 [inlined]
gradient_(::getfield(Flux.Optimise, Symbol("##14#20")){typeof(loss),Tuple{Float64,Float64}}, ::Tracker.Params) at back.jl:97
#gradient#24(::Bool, ::typeof(Tracker.gradient), ::Function, ::Tracker.Params) at back.jl:164
gradient at back.jl:164 [inlined]
macro expansion at train.jl:71 [inlined]
macro expansion at progress.jl:119 [inlined]
#train!#12(::getfield(Flux.Optimise, Symbol("##16#22")), ::typeof(Flux.Optimise.train!), ::Function, ::Tracker.Params, ::Base.Iterators.Zip{Tuple{Array{Float64,2},Array{Float64,2}}}, ::ADAM) at train.jl:69
train!(::Function, ::Tracker.Params, ::Base.Iterators.Zip{Tuple{Array{Float64,2},Array{Float64,2}}}, ::ADAM) at train.jl:67
top-level scope at Player_Flux.jl:53
include_string(::Module, ::String, ::String, ::Int64) at eval.jl:30
(::getfield(Atom, Symbol("##124#129")){String,Int64,String})() at eval.jl:94
withpath(::getfield(Atom, Symbol("##124#129")){String,Int64,String}, ::String) at utils.jl:30
withpath at eval.jl:46 [inlined]
#123 at eval.jl:93 [inlined]
with_logstate(::getfield(Atom, Symbol("##123#128")){String,Int64,String}, ::Base.CoreLogging.LogState) at logging.jl:395
with_logger at logging.jl:491 [inlined]
#122 at eval.jl:92 [inlined]
hideprompt(::getfield(Atom, Symbol("##122#127")){String,Int64,String}) at repl.jl:77
macro expansion at eval.jl:91 [inlined]
macro expansion at dynamic.jl:24 [inlined]
(::getfield(Atom, Symbol("##121#126")))(::Dict{String,Any}) at eval.jl:86
handlemsg(::Dict{String,Any}, ::Dict{String,Any}) at comm.jl:164
(::getfield(Atom, Symbol("##19#21")){Array{Any,1}})() at task.jl:268

```

I feel like I’m missing something obvious here but I’m too new to this to figure it out. I found another thread that seemed similar and suggested adding `Tracker.data()` to training data sets, but that (obviously) didn’t work.  
Any suggestions?

---

<div class="post-metadata">

**Author:** ![improbable22](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/improbable22/32/5464_2.png) [@improbable22](https://discourse.julialang.org/u/improbable22)\
**Post date:** [September 27, 2019, 11:06pm UTC](https://discourse.julialang.org/t/flux-jl-methoderror-dense-typeof-identity-on-simple-chain-model/29247/2 "2019-09-27T23:06:09Z")

</div>

If you look at `first(dataset)`, you’ll see a tuple of numbers. And if you put those into `loss`, or just take `m(0.5)`, then you get the same error — Dense wants to act not on a number but on a matrix.

What you need is something of the form `dataset = [(trainX, trainY), (trainX, trainY)]`, so that each element of `dataset` forms good arguments for `loss`.

Mine explicitly copies the data twice, which you can avoid by calling some iterator instead, as some examples do. But it may be clearer just to write out the loop. The source code of `train!` is clearer than the function, really.

---

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**Author:** ![m\_Tucci](https://avatars.discourse-cdn.com/v4/letter/m/67e7ee/32.png) [@m\_Tucci](https://discourse.julialang.org/u/m_Tucci)\
**Post date:** [September 28, 2019, 5:45pm UTC](https://discourse.julialang.org/t/flux-jl-methoderror-dense-typeof-identity-on-simple-chain-model/29247/3 "2019-09-28T17:45:05Z")

</div>

You’re definitely right. I got a chain with Dense layers to work but when I uncomment `LSTM` I get the following:

```julia
`back!` was already used
back_(::Tracker.Call{Missing,Tuple{}}, ::Array{Float64,1}, ::Bool) at back.jl:42
back(::Tracker.Tracked{Array{Float32,1}}, ::Array{Float64,1}, ::Bool) at back.jl:58
#13 at back.jl:38 [inlined]
foreach at abstractarray.jl:1921 [inlined]
back_(::Tracker.Call{getfield(Tracker, Symbol("#back#548")){4,getfield(Base.Broadcast, Symbol("##2#4")){getfield(Base.Broadcast, Symbol("##8#10")){getfield(Base.Broadcast, Symbol("##8#10")){getfield(Base.Broadcast, Symbol("##1#3")),getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##7#9"))}},getfield(Base.Broadcast, Symbol("##11#12")){getfield(Base.Broadcast, Symbol("##11#12")){getfield(Base.Broadcast, Symbol("##13#14"))}},getfield(Base.Broadcast, Symbol("##15#16")){getfield(Base.Broadcast, Symbol("##15#16")){getfield(Base.Broadcast, Symbol("##17#18"))}},typeof(*)},getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##7#9"))}},getfield(Base.Broadcast, Symbol("##11#12")){getfield(Base.Broadcast, Symbol("##11#12")){getfield(Base.Broadcast, Symbol("##13#14"))}},getfield(Base.Broadcast, Symbol("##15#16")){getfield(Base.Broadcast, Symbol("##15#16")){getfield(Base.Broadcast, Symbol("##17#18"))}},typeof(*)},typeof(+)},NTuple{4,TrackedArray{…,Array{Float32,1}}}},NTuple{4,Tracker.Tracked{Array{Float32,1}}}}, ::Array{Float64,1}, ::Bool) at back.jl:38
back(::Tracker.Tracked{Array{Float32,1}}, ::Array{Float64,1}, ::Bool) at back.jl:58
#13 at back.jl:38 [inlined]
foreach at abstractarray.jl:1921 [inlined]
back_(::Tracker.Call{getfield(Tracker, Symbol("#back#548")){2,getfield(Base.Broadcast, Symbol("##2#4")){getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##8#10")){getfield(Base.Broadcast, Symbol("##1#3")),getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##7#9"))},getfield(Base.Broadcast, Symbol("##11#12")){getfield(Base.Broadcast, Symbol("##13#14"))},getfield(Base.Broadcast, Symbol("##15#16")){getfield(Base.Broadcast, Symbol("##17#18"))},typeof(tanh)}},typeof(*)},Tuple{TrackedArray{…,Array{Float32,1}},TrackedArray{…,Array{Float32,1}}}},Tuple{Tracker.Tracked{Array{Float32,1}},Tracker.Tracked{Array{Float32,1}}}}, ::Array{Float64,1}, ::Bool) at back.jl:38
back(::Tracker.Tracked{Array{Float32,1}}, ::Array{Float64,1}, ::Bool) at back.jl:58
(::getfield(Tracker, Symbol("##13#14")){Bool})(::Tracker.Tracked{Array{Float32,1}}, ::Array{Float64,1}) at back.jl:38
foreach(::Function, ::Tuple{Tracker.Tracked{Array{Float32,2}},Tracker.Tracked{Array{Float32,1}}}, ::Tuple{Array{Float64,2},Array{Float64,1}}) at abstractarray.jl:1921
back_(::Tracker.Call{getfield(Tracker, Symbol("##509#510")){TrackedArray{…,Array{Float32,2}},TrackedArray{…,Array{Float32,1}}},Tuple{Tracker.Tracked{Array{Float32,2}},Tracker.Tracked{Array{Float32,1}}}}, ::Array{Float64,1}, ::Bool) at back.jl:38
back(::Tracker.Tracked{Array{Float32,1}}, ::Array{Float64,1}, ::Bool) at back.jl:58
#13 at back.jl:38 [inlined]
foreach at abstractarray.jl:1921 [inlined]
back_(::Tracker.Call{getfield(Tracker, Symbol("#back#548")){2,getfield(Base.Broadcast, Symbol("##2#4")){getfield(Base.Broadcast, Symbol("##8#10")){getfield(Base.Broadcast, Symbol("##1#3")),getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##7#9"))}},getfield(Base.Broadcast, Symbol("##11#12")){getfield(Base.Broadcast, Symbol("##11#12")){getfield(Base.Broadcast, Symbol("##13#14"))}},getfield(Base.Broadcast, Symbol("##15#16")){getfield(Base.Broadcast, Symbol("##15#16")){getfield(Base.Broadcast, Symbol("##17#18"))}},typeof(+)},typeof(identity)},Tuple{TrackedArray{…,Array{Float32,1}},TrackedArray{…,Array{Float32,1}}}},Tuple{Tracker.Tracked{Array{Float32,1}},Tracker.Tracked{Array{Float32,1}}}}, ::Array{Float64,1}, ::Bool) at back.jl:38
back(::Tracker.Tracked{Array{Float32,1}}, ::Array{Float64,1}, ::Bool) at back.jl:58
foreach at back.jl:38 [inlined]
back_(::Tracker.Call{getfield(Tracker, Symbol("##511#512")){TrackedArray{…,Array{Float32,1}}},Tuple{Tracker.Tracked{Array{Float32,1}}}}, ::Array{Float64,1}, ::Bool) at back.jl:38
back(::Tracker.Tracked{Array{Float32,1}}, ::Array{Float64,1}, ::Bool) at back.jl:58
#13 at back.jl:38 [inlined]
foreach at abstractarray.jl:1921 [inlined]
back_(::Tracker.Call{getfield(Tracker, Symbol("#back#548")){4,getfield(Base.Broadcast, Symbol("##2#4")){getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##8#10")){getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##1#3"))},getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##5#6")){getfield(Base.Broadcast, Symbol("##7#9"))}},getfield(Base.Broadcast, Symbol("##11#12")){getfield(Base.Broadcast, Symbol("##11#12")){getfield(Base.Broadcast, Symbol("##13#14"))}},getfield(Base.Broadcast, Symbol("##15#16")){getfield(Base.Broadcast, Symbol("##15#16")){getfield(Base....

```

---

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**Author:** ![KK\_JuliaNoob](https://avatars.discourse-cdn.com/v4/letter/k/9de0a6/32.png) [@KK\_JuliaNoob](https://discourse.julialang.org/u/KK_JuliaNoob)\
**Post date:** [October 17, 2019, 12:38pm UTC](https://discourse.julialang.org/t/flux-jl-methoderror-dense-typeof-identity-on-simple-chain-model/29247/4 "2019-10-17T12:38:38Z")

</div>

I am getting a similar ` `back!` was already used` error.  
Were you able to figure this out?