---
title: "Getting Started with kerasnip"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Getting Started with kerasnip}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---



## The Core Idea: From Keras Layers to Tidymodels Specs

The `keras3` package allows for building deep learning models layer-by-layer, which is a powerful and flexible approach. However, the `tidymodels` ecosystem is designed around declarative model specifications, where you define *what* model you want and which of its parameters you want to tune, rather than building it imperatively.

`kerasnip` bridges this gap with a simple but powerful concept: **layer blocks**. You define the components of your neural network (e.g., an input block, a dense block, a dropout block) as simple R functions. `kerasnip` then uses these blocks as building materials to create a brand new `parsnip` model specification function for you.

This new function behaves just like any other `parsnip` model (e.g., `rand_forest()` or `linear_reg()`), making it easy to integrate into `tidymodels` workflows.

## Installation

You can install the development version of `kerasnip` from GitHub. You will also need `keras3`.


``` r
install.packages("pak")
pak::pak("davidrsch/kerasnip")
pak::pak("rstudio/keras3")

# Install the backend
keras3::install_keras()
```

We’ll start by loading `kerasnip`, `tidymodels` and `keras3`:


``` r
library(kerasnip)
library(tidymodels)
#> ── Attaching packages ────────────────────────────────────────────────────────────────────────────── tidymodels 1.5.0 ──
#> ✔ broom        1.0.12     ✔ recipes      1.3.2 
#> ✔ dials        1.4.3      ✔ rsample      1.3.2 
#> ✔ dplyr        1.2.1      ✔ tailor       0.1.0 
#> ✔ ggplot2      4.0.3      ✔ tidyr        1.3.2 
#> ✔ infer        1.1.0      ✔ tune         2.1.0 
#> ✔ modeldata    1.5.1      ✔ workflows    1.3.0 
#> ✔ parsnip      1.5.0      ✔ workflowsets 1.1.1 
#> ✔ purrr        1.2.2      ✔ yardstick    1.4.0
#> ── Conflicts ───────────────────────────────────────────────────────────────────────────────── tidymodels_conflicts() ──
#> ✖ purrr::discard() masks scales::discard()
#> ✖ dplyr::filter()  masks stats::filter()
#> ✖ dplyr::lag()     masks stats::lag()
#> ✖ recipes::step()  masks stats::step()
library(keras3)
#> 
#> Attaching package: 'keras3'
#> The following object is masked from 'package:yardstick':
#> 
#>     get_weights
#> The following object is masked from 'package:infer':
#> 
#>     generate
```

## A `kerasnip` MNIST Example

Let’s replicate the classic Keras introductory example, training a simple MLP on the MNIST dataset, but using the `kerasnip` workflow. This will demonstrate how to translate a standard Keras model into a reusable, modular `parsnip` specification.

If you’re familiar with Keras, you’ll recognize the structure; if not, this is a perfect place to start. We’ll begin by learning the basics through a simple task: recognizing handwritten digits from the MNIST dataset.

The MNIST dataset contains 28×28 pixel grayscale images of handwritten digits, like these:

![MINIST](images/MNIST.png){fig-alt="A picture showing grayscale images of handwritten digits (5, 0, 4 and 1)"}

Each image comes with a label indicating which digit it represents. For example, the labels for the images above might be 5, 0, 4, and 1.

### Preparing the Data

This step is identical to any other Keras model. We load the MNIST dataset, reshape the predictors, and convert the outcome to a factor for `tidymodels`.


``` r
mnist <- dataset_mnist()
x_train <- mnist$train$x
y_train <- mnist$train$y
x_test <- mnist$test$x
y_test <- mnist$test$y

# Reshape
x_train <- array_reshape(x_train, c(nrow(x_train), 784))
x_test <- array_reshape(x_test, c(nrow(x_test), 784))
# Rescale
x_train <- x_train / 255
x_test <- x_test / 255

# Convert outcomes to factors for tidymodels
# kerasnip will handle y convertion internally using keras3::to_categorical()
y_train_factor <- factor(y_train)
y_test_factor <- factor(y_test)

# For tidymodels, it's best to work with data frames
# Use I() to keep the matrix structure of x within the data frame
train_df <- data.frame(x = I(x_train), y = y_train_factor)
test_df <- data.frame(x = I(x_test), y = y_test_factor)
```

### The Standard Keras Approach (for comparison)

Before diving into the `kerasnip` workflow, let's quickly look at how this same model is built using standard `keras3` code. This will help highlight the different approach `kerasnip` enables.


``` r
# The standard Keras3 approach
model <- keras_model_sequential(input_shape = 784) |>
  layer_dense(units = 256, activation = "relu") |>
  layer_dropout(rate = 0.4) |>
  layer_dense(units = 128, activation = "relu") |>
  layer_dropout(rate = 0.3) |>
  layer_dense(units = 10, activation = "softmax")

summary(model)

model |>
  compile(
    loss = "categorical_crossentropy",
    optimizer = optimizer_rmsprop(),
    metrics = "accuracy"
  )

# The model would then be trained with model |> fit(...)
```

The code above is imperative: you define each layer and add it to the model step-by-step. Now, let's see how `kerasnip` approaches this by defining reusable components for a declarative, `tidymodels`-friendly workflow.

### Defining the Model with Reusable Blocks

The original Keras example interleaves `layer_dense()` and `layer_dropout()`. With `kerasnip`, we can encapsulate this pattern into a single, reusable block. This makes the overall architecture cleaner and more modular.


``` r
# An input block to initialize the model.
# The 'model' argument is supplied implicitly by the kerasnip backend.
mlp_input_block <- function(model, input_shape) {
  keras_model_sequential(input_shape = input_shape)
}

# A reusable "module" that combines a dense layer and a dropout layer.
# All arguments that should be tunable need a default value.
dense_dropout_block <- function(model, units = 128, rate = 0.1) {
  model |>
    layer_dense(units = units, activation = "relu") |>
    layer_dropout(rate = rate)
}

# The output block for classification.
mlp_output_block <- function(model, num_classes) {
  model |> layer_dense(units = num_classes, activation = "softmax")
}
```

Now, we use `create_keras_sequential_spec()` to generate our `parsnip` model function.


``` r
create_keras_sequential_spec(
  model_name = "mnist_mlp",
  layer_blocks = list(
    input = mlp_input_block,
    hidden_1 = dense_dropout_block,
    hidden_2 = dense_dropout_block,
    output = mlp_output_block
  ),
  mode = "classification"
)
```

### Building and Fitting the Model

We can now use our new `mnist_mlp()` function. Notice how its arguments, such as `hidden_1_units` and `hidden_1_rate`, were automatically generated by `kerasnip`. The names are created by combining the name of the layer block (e.g., `hidden_1`) with the arguments of that block's function (e.g., `units`, `rate`).

To replicate the `keras3` example, we'll use both `hidden` blocks and provide their parameters.


``` r
mlp_spec <- mnist_mlp(
  hidden_1_units = 256,
  hidden_1_rate = 0.4,
  hidden_2_rate = 0.3,
  hidden_2_units =  128,
  compile_loss = "categorical_crossentropy",
  compile_optimizer = optimizer_rmsprop(),
  compile_metrics = c("accuracy"),
  fit_epochs = 30,
  fit_batch_size = 128,
  fit_validation_split = 0.2
) |>
  set_engine("keras")

# Fit the model
mlp_fit <- fit(mlp_spec, y ~ x, data = train_df)
```


``` r
mlp_fit |>
  extract_keras_model() |>
  summary()
#> Model: "sequential"
#> ┌─────────────────────────────────────────────────────┬────────────────────────────────────────┬───────────────────────
#> │ Layer (type)                                        │ Output Shape                           │               Param # 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dense (Dense)                                       │ (None, 256)                            │               200,960 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dropout (Dropout)                                   │ (None, 256)                            │                     0 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dense_1 (Dense)                                     │ (None, 128)                            │                32,896 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dropout_1 (Dropout)                                 │ (None, 128)                            │                     0 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dense_2 (Dense)                                     │ (None, 10)                             │                 1,290 
#> └─────────────────────────────────────────────────────┴────────────────────────────────────────┴───────────────────────
#>  Total params: 470,294 (1.79 MB)
#>  Trainable params: 235,146 (918.54 KB)
#>  Non-trainable params: 0 (0.00 B)
#>  Optimizer params: 235,148 (918.55 KB)
```


``` r
mlp_fit |>
  extract_keras_model() |>
  plot(show_shapes = TRUE)
```

![model](images/model_plot_shapes_gs.png){fig-alt="A picture showing the model shape"}


``` r
mlp_fit |>
  extract_keras_history() |>
  plot()
```

![plot of chunk model-fit-history](figure/model-fit-history-1.png)

### Evaluating Model Performance

The `keras_evaluate()` function provides a straightforward way to assess the model's performance on a test set, using the underlying `keras3::evaluate()` method. It returns the loss and any other metrics that were specified during the model compilation step.


``` r
mlp_fit |> keras_evaluate(x_test, y_test)
#> 313/313 - 1s - 2ms/step - accuracy: 0.9816 - loss: 0.0933
#> $accuracy
#> [1] 0.9816
#> 
#> $loss
#> [1] 0.09331781
```

### Making Predictions

Once the model is trained, we can use the standard `tidymodels` `predict()` function to generate predictions on new data. By default, `predict()` on a `parsnip` classification model returns the predicted class labels.


``` r
# Predict the class for the first 5 images in the test set
class_preds <- mlp_fit |>
  predict(new_data = head(select(test_df, x)))
#> 1/1 - 0s - 112ms/step
class_preds
#> # A tibble: 6 × 1
#>   .pred_class
#>   <fct>      
#> 1 7          
#> 2 2          
#> 3 1          
#> 4 0          
#> 5 4          
#> 6 1
```

To get the underlying probabilities for each class, we can set `type = "prob"`. This returns a tibble with a probability column for each of the 10 classes (0-9).


``` r
# Predict probabilities for the first 5 images
prob_preds <- mlp_fit |>
  predict(new_data = head(select(test_df, x)), type = "prob")
#> 1/1 - 0s - 79ms/step
prob_preds
#> # A tibble: 6 × 10
#>     .pred_0   .pred_1  .pred_2  .pred_3   .pred_4  .pred_5  .pred_6  .pred_7  .pred_8  .pred_9
#>       <dbl>     <dbl>    <dbl>    <dbl>     <dbl>    <dbl>    <dbl>    <dbl>    <dbl>    <dbl>
#> 1 5.50 e-17 2.57 e-15 2.56e-11 1.41e- 9 3.52 e-19 1.40e-14 3.42e-27 1   e+ 0 7.46e-19 1.19e-10
#> 2 7.14 e-19 1.02 e-12 1   e+ 0 2.66e-13 4.80 e-25 3.38e-22 8.05e-18 8.01e-20 2.42e-18 2.33e-27
#> 3 1.17 e-11 1.000e+ 0 8.86e- 9 1.80e-11 4.10 e- 8 3.54e-12 2.33e- 9 7.20e- 8 1.40e- 8 8.54e-11
#> 4 1.000e+ 0 5.15 e-11 8.54e- 9 3.70e-12 3.02 e-11 8.76e-10 1.26e- 7 1.23e-10 8.62e-12 9.42e-10
#> 5 3.89 e-11 3.38 e-11 1.01e-11 3.33e-14 1.000e+ 0 1.75e-13 2.33e-11 1.73e- 7 5.30e-14 2.48e- 7
#> 6 1.88 e-12 1.000e+ 0 1.17e-10 1.25e-12 5.62 e- 8 3.72e-14 2.97e-10 2.18e- 8 3.60e- 9 2.09e-11
```

We can then compare the predicted class to the actual class for these images to see how the model is performing.


``` r
# Combine predictions with actuals for comparison
comparison <- bind_cols(
  class_preds,
  prob_preds
) |>
  bind_cols(
    head(test_df[, "y", drop = FALSE])
  )
comparison
#> # A tibble: 6 × 12
#>   .pred_class   .pred_0   .pred_1  .pred_2  .pred_3   .pred_4  .pred_5  .pred_6  .pred_7  .pred_8  .pred_9 y    
#>   <fct>           <dbl>     <dbl>    <dbl>    <dbl>     <dbl>    <dbl>    <dbl>    <dbl>    <dbl>    <dbl> <fct>
#> 1 7           5.50 e-17 2.57 e-15 2.56e-11 1.41e- 9 3.52 e-19 1.40e-14 3.42e-27 1   e+ 0 7.46e-19 1.19e-10 7    
#> 2 2           7.14 e-19 1.02 e-12 1   e+ 0 2.66e-13 4.80 e-25 3.38e-22 8.05e-18 8.01e-20 2.42e-18 2.33e-27 2    
#> 3 1           1.17 e-11 1.000e+ 0 8.86e- 9 1.80e-11 4.10 e- 8 3.54e-12 2.33e- 9 7.20e- 8 1.40e- 8 8.54e-11 1    
#> 4 0           1.000e+ 0 5.15 e-11 8.54e- 9 3.70e-12 3.02 e-11 8.76e-10 1.26e- 7 1.23e-10 8.62e-12 9.42e-10 0    
#> 5 4           3.89 e-11 3.38 e-11 1.01e-11 3.33e-14 1.000e+ 0 1.75e-13 2.33e-11 1.73e- 7 5.30e-14 2.48e- 7 4    
#> 6 1           1.88 e-12 1.000e+ 0 1.17e-10 1.25e-12 5.62 e- 8 3.72e-14 2.97e-10 2.18e- 8 3.60e- 9 2.09e-11 1
```

## Example 2: Tuning the Model Architecture

Now we’ll showcase the main strength of `kerasnip`: tuning the network architecture itself. We can treat the number of layers, and the parameters of those layers, as hyperparameters to be optimized by `tune`.

Using the `mnist_mlp` spec we just created, let's define a tunable model.


``` r
# Define a tunable specification
# We set num_hidden_2 = 0 to disable the second hidden block
# for this tuning example
tune_spec <- mnist_mlp(
  num_hidden_1 = tune(),
  hidden_1_units = tune(),
  hidden_1_rate = tune(),
  num_hidden_2 = 0,
  compile_loss = "categorical_crossentropy",
  compile_optimizer = optimizer_rmsprop(),
  compile_metrics = c("accuracy"),
  fit_epochs = 30,
  fit_batch_size = 128,
  fit_validation_split = 0.2
) |>
  set_engine("keras")

# Create a workflow
tune_wf <- workflow(y ~ x, tune_spec)
```

Next, we define the search space for our tunable parameters using `dials`.


``` r
# Define the tuning grid
params <- extract_parameter_set_dials(tune_wf) |>
  update(
    num_hidden_1 = dials::num_terms(c(1, 3)),
    hidden_1_units = dials::hidden_units(c(64, 256)),
    hidden_1_rate = dials::dropout(c(0.2, 0.4))
  )
grid <- grid_regular(params, levels = 3)
grid
#> # A tibble: 27 × 3
#>    num_hidden_1 hidden_1_units hidden_1_rate
#>           <int>          <int>         <dbl>
#>  1            1             64           0.2
#>  2            2             64           0.2
#>  3            3             64           0.2
#>  4            1            160           0.2
#>  5            2            160           0.2
#>  6            3            160           0.2
#>  7            1            256           0.2
#>  8            2            256           0.2
#>  9            3            256           0.2
#> 10            1             64           0.3
#> # ℹ 17 more rows
```


``` r
# Using only the first 100 rows for speed. The real call
# should be: folds <- vfold_cv(train_df, v = 3)
folds <- vfold_cv(train_df[1:100, ], v = 3)

tune_res <- tune_grid(
  tune_wf,
  resamples = folds,
  grid = grid,
  metrics = metric_set(accuracy),
  control = control_grid(save_pred = FALSE, save_workflow = TRUE)
)
#> 2/2 - 0s - 67ms/step
#> 2/2 - 0s - 82ms/step
#> 2/2 - 0s - 85ms/step
#> 2/2 - 0s - 73ms/step
#> 2/2 - 0s - 66ms/step
#> 2/2 - 0s - 100ms/step
#> 2/2 - 0s - 67ms/step
#> 2/2 - 0s - 79ms/step
#> 2/2 - 0s - 92ms/step
#> 2/2 - 0s - 67ms/step
#> 2/2 - 0s - 80ms/step
#> 2/2 - 0s - 75ms/step
#> 2/2 - 0s - 64ms/step
#> 2/2 - 0s - 77ms/step
#> 2/2 - 0s - 96ms/step
#> 2/2 - 0s - 60ms/step
#> 2/2 - 0s - 88ms/step
#> 2/2 - 0s - 87ms/step
#> 2/2 - 0s - 70ms/step
#> 2/2 - 0s - 78ms/step
#> 2/2 - 0s - 96ms/step
#> 2/2 - 0s - 63ms/step
#> 2/2 - 0s - 72ms/step
#> 2/2 - 0s - 110ms/step
#> 2/2 - 0s - 79ms/step
#> 2/2 - 0s - 76ms/step
#> 2/2 - 0s - 94ms/step
#> 2/2 - 0s - 70ms/step
#> 2/2 - 0s - 71ms/step
#> 2/2 - 0s - 89ms/step
#> 2/2 - 0s - 70ms/step
#> 2/2 - 0s - 76ms/step
#> 2/2 - 0s - 82ms/step
#> 2/2 - 0s - 62ms/step
#> 2/2 - 0s - 86ms/step
#> 2/2 - 0s - 85ms/step
#> 2/2 - 0s - 68ms/step
#> 2/2 - 0s - 76ms/step
#> 2/2 - 0s - 85ms/step
#> 2/2 - 0s - 70ms/step
#> 2/2 - 0s - 74ms/step
#> 2/2 - 0s - 78ms/step
#> 2/2 - 0s - 75ms/step
#> 2/2 - 0s - 81ms/step
#> 2/2 - 0s - 81ms/step
#> 2/2 - 0s - 55ms/step
#> 2/2 - 0s - 77ms/step
#> 2/2 - 0s - 86ms/step
#> 2/2 - 0s - 53ms/step
#> 2/2 - 0s - 79ms/step
#> 2/2 - 0s - 82ms/step
#> 2/2 - 0s - 60ms/step
#> 2/2 - 0s - 82ms/step
#> 2/2 - 0s - 93ms/step
#> 2/2 - 0s - 57ms/step
#> 2/2 - 0s - 69ms/step
#> 2/2 - 0s - 85ms/step
#> 2/2 - 0s - 60ms/step
#> 2/2 - 0s - 65ms/step
#> 2/2 - 0s - 89ms/step
#> 2/2 - 0s - 59ms/step
#> 2/2 - 0s - 62ms/step
#> 2/2 - 0s - 98ms/step
#> 2/2 - 0s - 63ms/step
#> 2/2 - 0s - 71ms/step
#> 2/2 - 0s - 93ms/step
#> 2/2 - 0s - 63ms/step
#> 2/2 - 0s - 69ms/step
#> 2/2 - 0s - 84ms/step
#> 2/2 - 0s - 65ms/step
#> 2/2 - 0s - 73ms/step
#> 2/2 - 0s - 85ms/step
#> 2/2 - 0s - 61ms/step
#> 2/2 - 0s - 66ms/step
#> 2/2 - 0s - 77ms/step
#> 2/2 - 0s - 59ms/step
#> 2/2 - 0s - 68ms/step
#> 2/2 - 0s - 64ms/step
#> 2/2 - 0s - 58ms/step
#> 2/2 - 0s - 68ms/step
#> 2/2 - 0s - 68ms/step
```

Finally, we can inspect the results to find which architecture performed the best. First, a summary table:


``` r
# Show the summary table of the best models
show_best(tune_res, metric = "accuracy")
#> # A tibble: 5 × 9
#>   num_hidden_1 hidden_1_units hidden_1_rate .metric  .estimator  mean     n std_err .config         
#>          <int>          <int>         <dbl> <chr>    <chr>      <dbl> <int>   <dbl> <chr>           
#> 1            3            256         0.2   accuracy multiclass 0.800     3  0.0211 pre0_mod25_post0
#> 2            2            256         0.400 accuracy multiclass 0.780     3  0.0220 pre0_mod18_post0
#> 3            3            256         0.400 accuracy multiclass 0.779     3  0.0379 pre0_mod27_post0
#> 4            1            160         0.2   accuracy multiclass 0.770     3  0.0213 pre0_mod04_post0
#> 5            1            160         0.400 accuracy multiclass 0.770     3  0.0213 pre0_mod06_post0
```

Now that we've identified the best-performing hyperparameters, our final step is to create and train the final model. We use `select_best()` to get the top parameters, `finalize_workflow()` to update our workflow with them, and then `fit()` one last time on our full training dataset.


``` r
# Select the best hyperparameters
best_hps <- select_best(tune_res, metric = "accuracy")

# Finalize the workflow with the best hyperparameters
final_wf <- finalize_workflow(tune_wf, best_hps)

# Fit the final model on the full training data
final_fit <- fit(final_wf, data = train_df)
```

We can now inspect our final, tuned model.


``` r
# Print the model summary
final_fit |>
  extract_fit_parsnip() |>
  extract_keras_model() |>
  summary()
#> Model: "sequential_82"
#> ┌─────────────────────────────────────────────────────┬────────────────────────────────────────┬───────────────────────
#> │ Layer (type)                                        │ Output Shape                           │               Param # 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dense_246 (Dense)                                   │ (None, 256)                            │               200,960 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dropout_164 (Dropout)                               │ (None, 256)                            │                     0 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dense_247 (Dense)                                   │ (None, 256)                            │                65,792 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dropout_165 (Dropout)                               │ (None, 256)                            │                     0 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dense_248 (Dense)                                   │ (None, 256)                            │                65,792 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dropout_166 (Dropout)                               │ (None, 256)                            │                     0 
#> ├─────────────────────────────────────────────────────┼────────────────────────────────────────┼───────────────────────
#> │ dense_249 (Dense)                                   │ (None, 10)                             │                 2,570 
#> └─────────────────────────────────────────────────────┴────────────────────────────────────────┴───────────────────────
#>  Total params: 670,230 (2.56 MB)
#>  Trainable params: 335,114 (1.28 MB)
#>  Non-trainable params: 0 (0.00 B)
#>  Optimizer params: 335,116 (1.28 MB)

# Plot the training history
final_fit |>
  extract_fit_parsnip() |>
  extract_keras_history() |>
  plot()
```

![plot of chunk inspect-final-model](figure/inspect-final-model-1.png)

This result shows that `tune` has tested various network depths, widths, and dropout rates, successfully finding the best-performing combination within the search space. By using `kerasnip`, we were able to integrate this complex architectural tuning directly into a standard `tidymodels` workflow.
