# \`Chain\` vs \`foldl\` in Flux.jl
**URL:** https://discourse.julialang.org/t/chain-vs-foldl-in-flux-jl/60136
**Category:** Machine Learning
**Created:** [April 28, 2021, 12:00am UTC](https://discourse.julialang.org/t/chain-vs-foldl-in-flux-jl/60136 "2021-04-28T00:00:47Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@dbg](https://discourse.julialang.org/u/dbg)
#### Post date: [April 28, 2021, 12:00am UTC](https://discourse.julialang.org/t/chain-vs-foldl-in-flux-jl/60136/1 "2021-04-28T00:00:47Z")
I am new to both ML and Julia, so this might be a silly question. I am trying to use `foldl` instead of `Chain` to create a model with multiple layers, but I am not able to train the model created using `foldl`. Does anyone know what I am doing wrong?
I have a Jupyter notebook with the following cells:
**Cell 1** : Setting up the data - create two annular clouds of points to classify
```
using Plots, Interact, Flux
using Flux: mse, shuffle, throttle
function random_circular_coordinates(radius, num_points, σ=0.1)
# angles evenly spaced around unit circle
angles = range(0; stop=(2 * π), length=num_points)
# randomness added based on the Normal distribution
points = radius .+ σ * randn(num_points)
# random coordinates "wrapped" around unit circle
coordinates = [points .* cos.(angles) points .* sin.(angles)]
return permutedims(coordinates)
end
num_points = 100
radius_1 = 2
circle_1 = random_circular_coordinates(radius_1, num_points)
radius_2 = 0.5
circle_2 = random_circular_coordinates(radius_2, num_points)
X_train = [circle_1 circle_2]
y1, y2 = -1, 1
Y_train_column = [fill(y1, num_points); fill(y2, num_points)]
Y_train = permutedims(Y_train_column)
τ = 0.0
@show size(X_train)
@show size(Y_train)
iters = 1000
dataset = ((X_train, Y_train) for _ in 1:iters)
opt = ADAM()
```
**Cell 2** : Create a model using `Chain` and train it.
```
m1 = Chain( Dense(size(X_train,1), 32, relu),
Dense(32, 1) )
loss1(x, y) = mse(m1(x), y)
evalcb1() = @show(loss1(X_train, Y_train))
Flux.train!(loss1, params(m1), dataset, opt; cb=throttle(evalcb1, 0.01))
```
**Cell 3** : Create the same model using `foldl` and train it.
```
layers = [ Dense(size(X_train,1), 32, relu),
Dense(32, 1) ]
m2(x) = foldl((x, m2) -> m2(x), layers, init = x)
loss2(x, y) = mse(m2(x), y)
evalcb2() = @show(loss2(X_train, Y_train))
Flux.train!(loss2, params(m2), dataset, opt; cb=throttle(evalcb2, 0.01))
```
The `Chain` approach seems to work, but the `foldl` approach doesn’t.
Does anyone know what I am doing wrong?
---
### Author:  [@ToucheSir](https://discourse.julialang.org/u/ToucheSir)
#### Post date: [April 28, 2021, 12:16am UTC](https://discourse.julialang.org/t/chain-vs-foldl-in-flux-jl/60136/2 "2021-04-28T00:16:39Z")
> [@dbg](#):
>
> `Flux.train!(loss2, params(m2), dataset, opt; cb=throttle(evalcb2, 0.01))`
This should be `Flux.train!(loss2, params(layers[1], layers[2]), dataset, opt; cb=throttle(evalcb2, 0.01))`. `m2` is just a function and not a model struct or array, hence calling `params` on it returns nothing. In contrast, `m1` is a model struct because its type (`Chain`) implements the `functor` interface that Flux uses to deconstruct and retrieve parameter arrays.
---
### Author:  [@dbg](https://discourse.julialang.org/u/dbg)
#### Post date: [April 28, 2021, 12:31am UTC](https://discourse.julialang.org/t/chain-vs-foldl-in-flux-jl/60136/3 "2021-04-28T00:31:01Z")
Thanks a lot! Yes, that worked; in fact, the following works (where I put in the array of layers as the argument to `params`):
