---
title: "Conducting Theory-Based Reproducible Simulation Studies"
output: theorytools:::webex_vignette
bibliography: computational.bib
vignette: >
  %\VignetteIndexEntry{Conducting Theory-Based Reproducible Simulation Studies}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE, warning = FALSE, message = FALSE)
```

This tutorial introduces a reproducible research workflow for computational social science studies,
based on the [Workflow for Open Reproducible Code in Science (WORCS)](https://github.com/cjvanlissa/worcs).

The WORCS workflow, implemented in the `worcs` package, is designed to create fully reproducible research archives in R. 
The `theorytools` R-package is designed to make and work with *FAIR theories*: theories that are Findable in standardized archives (Zenodo, Open Science Framework), Accessible using human- and machine-readable file formats, Interoperable for specific purposes (in this case, conducting computational simulation studies), and Reusable (licensed to be iteratively updated, including by others than the original author).

In this tutorial, we combine `worcs` and `theorytools` to conduct a fully reproducible, theory-driven computational simulation study.

### Before Starting This Tutorial

- [ ] You must have [Set up your computer for WORCS](https://cjvanlissa.github.io/worcs/articles/setup)
- [ ] Running `worcs::check_worcs_installation()` should return all green checkmarks

### Learning Goals

In this tutorial, you will learn how to:

- [ ] Create a reproducible research project using WORCS.
- [ ] Import a "FAIR theory", specified as an augmented DAG.
- [ ] Create a computational simulation study based on the FAIR theory.
- [ ] Add `targets` to your simulation study to reduce redundant compute
- [ ] Add integration testing to your study to ensure your functions work as intended

# Setting up a Reproducible Project with WORCS

## 1. Create a new WORCS Project

WORCS helps automate reproducible workflows by setting up version control, metadata, licenses, and more.

1. Open RStudio.
1. Go to **File > New Project > New Directory > WORCS Project Template**.
    a. Choose a directory name, e.g., `"fair_theory_simulation"`.
    a. *Optionally*, you can create a GitHub repository. If you have GitHub integration set up, all that is required is to specify a GitHub remote repository name.
    a. Keep Manuscript template set to "GitHub document"
    a. Keep Preregistration template set to "None"
    a. Keep License set to "cc0" for a CC-0 copyright waiver
    a. Select "Use renv"
    a. **De-select** "Use targets": we will use `targets` in the next chapter, but we'll add support for it manually for didactic purposes. Once you're familiar with the `targets` workflow, feel free to add it automatically when creating your next project.
    a. Select "Open in new session"
1. Click **Create Project**.

WORCS will now set up a project that implements best practices for reproducibility. A new Rstudio instance should open automatically.

When working through the next sections, add the code chunks to your `"manuscript.Rmd"` file except when the code is labeled with **Interactive:**.
Code labeled with  **Interactive:** should be run only once, in the Console window.
All other code can be inserted in code chunks in your `"manuscript.Rmd"` file; knit it regularly to verify that everything runs as expected.

> What does it mean to run R code *interactively*? `r webexercises::mcq(sample(c(answer = "Executing code immediately in an R session that a human user is interacting with.", "Having one function call another function; these functions are said to be 'interacting'", "Evaluating code by placing it in an Rmarkdown document, and knitting that document.")))`


## 2. Download a FAIR Theory

**Interactive:** You'll need to install some packages to run this tutorial. Run this code:

```{r eval = FALSE}
install.packages("theorytools", prompt = FALSE)
install.packages("dagitty", prompt = FALSE)
install.packages("tidySEM", prompt = FALSE)
```

To your `"manuscript.Rmd"` file, add the following lines in the setup chunk:

```{r eval = FALSE}
library(theorytools)
library(dagitty)
library(tidySEM)
```

We will use a FAIR version of **Self-Determination Theory (SDT)**.
The vignette [Formalizing Self-Determination Theory](https://cjvanlissa.github.io/theorytools/articles/formalizing_sdt.html) describes how this FAIR theory was created based on the original Self-Determination Theory [@deciSelfDeterminationTheory2012].
It was specified as a Directed Acyclic Graph, or DAG: a diagram that specifies the causal relations proposed by a theory.
Such diagrams can be used to derive hypotheses, select control variables for causal inference, and conduct simulation studies (as we will do in this tutorial).

**Interactive:** Run the code below to download this existing FAIR theory, which is stored on Zenodo.
It will create a new subfolder called `"theory/"` for these files, because the theory repository has its own README and LICENSE files; you don't want to overwrite those files from your WORCS repository.
Optionally, you can have a look at the archived version by opening the URL <https://doi.org/10.5281/zenodo.15648655>.

```{r download_theory}
theorytools::download_theory("10.5281/zenodo.15648655", path = "theory")
```

The theory includes a README file, describing the contents of the other files, a LICENSE describing how the theory can be reused, a file called `sdt.txt` containing the DAG, and `definitions.csv`, containing construct definitions.
Note that these definitions require further specification;
as explained [in the Vignette on formalizing SDT](https://cjvanlissa.github.io/theorytools/articles/formalizing_sdt.html)),
the book chapter describing the theory does not explicitly define any of the constructs involved.

> Why is the FAIR theory downloaded into a separate `theory/` subfolder? `r webexercises::mcq(sample(c(answer = "To prevent overwriting the WORCS project's README and LICENSE files", "To avoid downloading unnecessary files", "Because theorytools only works inside a 'theory/' directory", "Because dagitty requires theory files to be placed in a specific folder")))`

## 3. Load the FAIR Theory

This FAIR theory is an augmented DAG, which can be read using the `dagitty` package.

The vignette [Specifying Augmented Directed Acyclic Graphs](https://cjvanlissa.github.io/theorytools/articles/augmented_dags.html) describes what augmented DAGs are.

Such DAGs are particularly useful for computational social science studies, because the `theorytools` package contains methods to convert augmented DAGs to generative models.

Now, let’s load the augmented DAG.
**Note:** when running this code from a code chunk in your Rmarkdown file, you must use the file path `../theory/sdt.txt` to go back to the parent directory, because the manuscript and the theory are both in different subdirectories.
If you run it interactively instead, you are already inside the parent directory, and you can use the path `theory/sdt.txt`.

```{r load_theory, eval = FALSE}
sdt <- dagitty::dagitty(paste(readLines("../theory/sdt.txt"), collapse = "\n"))
```

```{r echo = FALSE}
sdt <- structure("dag {\nexternal_event\nhealthy_development\nintegration\nintrinsic_motivation\nlocus_of_causality\nneeds\nwellbeing\nexternal_event -> intrinsic_motivation\nexternal_event -> locus_of_causality\nexternal_event -> needs\nintegration -> healthy_development\nintegration -> wellbeing\nintrinsic_motivation -> healthy_development\nintrinsic_motivation -> wellbeing\nlocus_of_causality -> intrinsic_motivation\nneeds -> integration\nneeds -> intrinsic_motivation\nneeds -> wellbeing\n}\n", class = "dagitty")

```


Then, we can plot the model using the tidySEM package.
Get a basic plot by running `graph_sem(sdt)`:

```{r echo = TRUE, eval = FALSE}
tidySEM::graph_sem(sdt)
```

```{r echo = FALSE, out.width="60%"}
if(!file.exists('sdt.png')){
  set.seed(1)
  p <- tidySEM::graph_sem(sdt, rect_width = 2, rect_height = 2) + ggplot2::scale_x_continuous(expand = c(.2,0))
  ggplot2::ggsave("sdt.png", p, device = "png")
}
knitr::include_graphics("sdt.png")
```

> According to the DAG, what are the causes of `wellbeing`? `r webexercises::mcq(sample(c(answer = "integration, intrinsic_motivation, needs", "integration, intrinsic_motivation", "needs, intrinsic_ _motivation")))`


## 4. Creating a Computational Model

Computational simulation studies allow us to generate and analyze simulated datasets, for example to:

* Explore what-if scenarios
* Test the implications of model assumptions
* Plan our analyses before data collection
* Conduct power analysis and plan our sample size
* Preregister a fully reproducible analysis pipeline [Preregistration-As-Code, @peikertReproducibleResearchTutorial2021; @vanlissaComplementingPreregisteredConfirmatory2022].

Below is a simple code snippet to generate synthetic data using the `theorytools::simulate_data()` function.
This function samples random values for exogenous variables and computes endogenous variables as functions of their predictors (linear functions, by default).
By default, regression coefficients are randomly sampled in the range `[-0.6, +0.6]` unless specific values are provided by the user.
You might use prior studies or expert knowledge to set more realistic parameter values,
or use the conventional values for null (0), small (.2), or medium (.4) effect sizes.
By default, normal distributions are assumed for exogenous variables and residual error, unless other distributions are specified.
Note that many other functions for simulating data exist,
and some may be better suited to particular use cases.
The tutorial on [Specifying Augmented DAGS](https://cjvanlissa.github.io/theorytools/articles/augmented_dags.html) contains more information about user-defined functional forms and (error) distributions.

```{r simulate-data, eval = FALSE}
set.seed(1)
theorytools::simulate_data(sdt, n = 5)
```
```{r echo = FALSE}
set.seed(1)
knitr::kable(theorytools::simulate_data(sdt, n = 5), digits = 2)
```

This synthetic dataset is consistent with the structure encoded in our FAIR SDT, though the parameter values are arbitrary.

> Which of the following is **not a reason** to use computational simulation studies? `r webexercises::mcq(sample(c(answer = "Fitting a model to real-world data", "Exploring what-if scenarios", "Conducting power analysis", "Preregistering a reproducible analysis pipeline")))`

### Simplifying the Model

Suppose we are interested in studying the effect of intrinsic motivation on well-being.
Even if our research is grounded in SDT, we do not need the entire theory to hypothesize about this specific relationship.
We can use the DAG to derive a restricted version of the theory that includes all variables that might confound the relationship of interest between intrinsic motivation and well-being:

```{r}
sdt_pruned <- theorytools:::prune_dag(sdt,
                                      exposure = "intrinsic_motivation",
                                      outcome = "wellbeing")
sdt_pruned
```

We can now generate a synthetic dataset based on this simplified DAG:

```{r eval = FALSE}
set.seed(1)
df <- theorytools::simulate_data(sdt_pruned, n = 100)
head(df)
```
```{r echo = FALSE}
set.seed(1)
df <- theorytools::simulate_data(sdt_pruned, n = 100)
knitr::kable(head(df), digits = 2)
```


## 5. Analyze the Simulated Data

If our goal is to conduct a computational study, we can manipulate any aspect of the data simulation procedure. 
These manipulations are the independent variables of our computational study.
We can then apply any kind of analysis to the simulated data to obtain the outcome (dependent variable) of our computational study.

One of the most basic types of computational simulation studies is a power analysis,
where we manipulate the true effect size of a parameter (independent variable: effect size) and calculate whether or not we observe a significant result for a test of that parameter in the simulated data (dependent variable: significance).

The analysis function in this case would be a simple significance test of the parameter of interest.
First, we conduct bivariate linear regression (because the DAG indicated that we need to control for the effect of needs):

```{r analysis}
res <- lm(wellbeing ~ intrinsic_motivation + needs, data = df)
summary(res)
```

Then, our outcome of interest is the significance of the effect of `intrinsic_motivation`.
We can extract the p-value as follows, and compare it to the significance level $\alpha < .05$ to get a binary result (significant: TRUE or FALSE):

```{r eval = FALSE}
sum_res <- summary(res)
sum_res$coefficients["intrinsic_motivation", "Pr(>|t|)"] < .05
```

> Run the code yourself, and answer true or false: In this one dataset, the result was significant. `r webexercises::torf(TRUE)`

# Using `worcs` with `targets`: Sustainable Reproducibility

One of the main tenets of open and reproducible science is the ability to re-run all analyses from raw data to published output, to ensure that results are still valid.
However, in computationally intensive projects, re-running unchanged code repeatedly can be redundant, slow, and wasteful.

The `targets` package reduces unnecessary compute by breaking down an analysis pipeline into steps that explicitly reference one another, where each step is only re-computed if either the step changed, or its inputs changed.
Reducing unnecessary computation speeds up the analysis process and reduces the carbon footprint of analyses [@guptaChasingCarbonElusive2021].

By combining `worcs` with `targets`, we get the best of both worlds:

* `worcs` is used to create a fully reproducible research archive;
* `targets` ensures that redundant steps in a reproducible analysis pipeline are not unnecessarily re-computed.

## 6. Add `targets` to the WORCS Project

**Interactive:** Let's install the packages required for this next section:

```{r install_targets, results='hide', eval = FALSE}
install.packages("targets", prompt = FALSE)
install.packages("tarchetypes", prompt = FALSE)
```
**Interactive:** Then, add `targets` integration to the `worcs` project by running:

```{r eval = FALSE}
worcs::add_targets()
```

Note the status messages. The newly created `_targets.R` file specifies the steps in the reproducible workflow.
The newly created `"./R/"` directory should contain functions that are used in that workflow.

## 7. Try Running the Pipeline

Open the file `_targets.R`. Note that the default pipeline (which will look something like the code below) consists of a few placeholder steps:

1. Create `data`
1. Run a `model`
1. Render the `manuscript.Rmd`

```{r eval = FALSE}
list(
  tar_target(
    name = data,
    command = tibble(x = rnorm(100), y = rnorm(100))
  ),
  tar_target(
    name = model,
    command = coefficients(lm(y ~ x, data = data))
  ),
  tarchetypes::tar_render(manuscript, "manuscript/manuscript.Rmd")
)
```

**Interactive:** You can run this pipeline by executing the command `targets::tar_make()`.

### Set a Seed

Computational simulation studies typically rely on random number generation to repeatedly simulate a scenario with some variability.

This dependency on random numbers makes the results non-reproducible (the random numbers will be different upon repeated evaluation).

Fortunately, true random numbers are rarely used: we typically use a sequence of *pseudo* random numbers.

To make the randomness reproducible, we can set a "seed" for the random number generator.
Setting a seed essentially jumps into the sequence of pseudo-random numbers at the same point each time,
so we get the same "random" numbers over and over.

To set a seed, add the command `set.seed(1)` above the `targets` pipeline. You can use any number instead of `1`, which will lead to different results.

### Debugging

Running `targets::tar_make()` might fail; I have observed two modes of failure:

1. It cannot find the file at `../theory/sdt.txt`; changing this to `theory/sdt.txt` seems to work. Perhaps `targets::tar_make()` evaluates code chunks in the parent directory?
1. `targets::tar_make()` fails the first time, but succeeds when re-running it a second time.

## 8. Create Your Own Functions

Now, let's create our own functions for the steps we need in our computational simulation study (power analysis).

### Function to Generate Data

First, we need a function to generate synthetic data, which allows us to manipulate the true effect size.

The `simulate_data()` function we used previously to create a single synthetic dataset can also generate a script for synthetic data generation:

```{r simulate_data}
theorytools::simulate_data(sdt_pruned, n = 100, run = FALSE)
```

Let's write this script to a new file in `./R/`, and edit it.

**Interactive:** Run the following code:

```{r eval = FALSE}
writeLines(
  theorytools::simulate_data(sdt_pruned, n = 100, run = FALSE),
  "R/generate_data.R"
)
```

Open the file `R/generate_data.R`, and edit it as follows (or in any other way you please):

* Wrap the code in a `fuction()` call with arguments `beta, n`
* Comment out the random seed, so that each run will be different
* Comment out `n <- 100` so that `n` is controlled via the function arguments
* Replace the fixed effect size for `intrinsic_motivation` with the argument `beta`, so that effect size is controlled via the function arguments

```
generate_data <- function(beta, n){
# Set random seed
# set.seed(442008606)
# Set simulation parameters
# n <- 100
# Simulate exogenous nodes
needs <- rnorm(n = n)
# Simulate endogenous nodes
intrinsic_motivation <- -(0.19 * needs) + rnorm(n = n)
wellbeing <- beta * intrinsic_motivation - 0.42 * needs + rnorm(n = n)
df <- data.frame(
intrinsic_motivation = intrinsic_motivation,
needs = needs,
wellbeing = wellbeing
)
return(df)
}
```

### *Optional*: The Effect of Needs

Note that the effect of `needs` on `intrinsic_motivation` and `wellbeing` is hard-coded in the code snippet above.
It is possible to draw a random value for these effects each time.
To do so, you could (for example) replace the fixed values with `runif(1, min = -.4, max = .4)`.

### Function to Analyze Data

Now, create another file called `analyze_data.R` to contain a function to perform the analysis procedure we defined in step 5:

```
analyze_data <- function(df){
# Conduct linear regression
res <- lm(wellbeing ~ intrinsic_motivation + needs, data = df)
# Obtain a model summary
sum_res <- summary(res)
# Compare p-value of our coefficient of interest to the significance level, .05
sum_res$coefficients["intrinsic_motivation", "Pr(>|t|)"] < .05
}
```

### Function to Conduct the Study

We will also need a function to conduct the computational simulation study itself. Create a file called `perform_study.R`, containing a function like this (feel free to develop your own):

```
perform_study <- function(study_design, reps = 100){
  # For each row of the study design, execute a function
  pwr <- apply(study_design, 1, function(thisrow){
    # Replicate the row of the study design reps times
    out <- replicate(n = reps, expr = {
      # Simulate data with the beta and n from thisrow
      df <- with(as.list(thisrow), generate_data(beta = beta, n = n))
      # Analyze those data
      analyze_data(df)
    })
    # Calculate the proportion of significant results using mean()
    mean(out)
  })
  # Make a data frame containing the study design and study results (pwr)
  data.frame(study_design, power = pwr)
}
```

## 9. Create Your Own Pipeline

In the file `_targets.R`, we can now define our own pipeline for our computational simulation study.

For each step in the pipeline, we define a `tar_target()`, naming the output with `name`.

Our goal is to create a list that looks something like this:

```
list(
  tar_target(
    name = study_design,
    command = ...
  ),
  tar_target(
    name = study_results,
    command = ...
  ),
  tarchetypes::tar_render(manuscript, "manuscript/manuscript.Rmd")
)
```

### Create Study Design

For a power analysis, we can manipulate the true effect size and sample size.

Let's say we want to design a study where the true effect size $\beta \in [.1, .2, .4]$ and the sample size $n \in [50, 100, 200]$ (for convenience's sake, we omit an analysis of false-positive results where $\beta = 0$ now).

We can use the R-function `expand.grid()` to create all possible combinations:

```{r}
expand.grid(
  beta = c(.1, .2, .4),
  n = c(50, 100, 200)
)
```

Add this to the pipeline as follows:

```
tar_target(
    name = study_design,
    command = expand.grid(
      beta = c(.1, .2, .4),
      n = c(50, 100, 200)
    )
  )
```

Now, add another target to conduct the study, based on `study_design`. Explicitly referencing an object with this `name` will ensure that the code is re-run if `study_design` changes (verify this yourself).

```
tar_target(
    name = study_results,
    command = perform_study(study_design = study_design, reps = 100)
  )
```

## 10. Edit `manuscript.Rmd`

Now, open `manuscript.Rmd` and edit it to load `study_results` and tabulate or plot them.
To load the study results, replace the default line `tar_load(model)` (from the mock pipeline created earlier) with `tar_load(study_results)`.
Using `tar_load()` inside of `manuscript.Rmd` makes the latter dependent on the former.
Thus, if `study_results` changes, the manuscript should be automatically re-run.

**Interactive:** To verify that the interdependencies between pipeline steps are properly tracked, you can run:

```{r eval = FALSE}
install.packages("visNetwork", prompt = FALSE)
targets::tar_visnetwork()
```
```{r echo = FALSE}
knitr::include_graphics("visnetwork.png")
```

Add the following code to the manuscript to tabulate the results:

```{r eval = FALSE}
knitr::kable(study_results, digits = 2)
```

```{r echo = FALSE}
study_results = structure(list(beta = c(0, 0.2, 0.4, 0, 0.2, 0.4, 0, 0.2, 0.4
), n = c(50, 50, 50, 100, 100, 100, 200, 200, 200), power = c(0.09, 
0.22, 0.77, 0.06, 0.56, 0.98, 0.03, 0.74, 1)), class = "data.frame", row.names = c(NA, 
-9L))
knitr::kable(study_results, digits = 2)
```

Or alternatively, add the  following code to plot the study results (this requires running `install.packages("ggplot2", prompt = FALSE)`).


```{r}
library(ggplot2)
df_plot <- study_results
df_plot$beta <- ordered(df_plot$beta)
ggplot(df_plot, aes(x = n, y = power, color = beta, shape = beta)) +
  geom_point() +
  geom_line() +
  theme_bw()
```


## 11. Run the Pipeline

**Interactive:** If you were successful, you should be able to run the pipeline by calling `targets::tar_make()`.

## 12. Add Endpoints

The `worcs` package allows you to specify endpoints for your analysis; this makes it possible to check whether analysis results are identical upon repeated evaluation.

If your analysis is/should be fully deterministic, with any random numbers controlled by `set.seed()` to ensure identical results upon repeated evaluation, then it's possible to specify the entire output document (e.g., `manuscript.html`) as an endpoint.

Alternatively, if your document does depend non-deterministically on random numbers (e.g., you plot the results with some random jitter), then it cannot be exactly reproduced.
In this case, you can either remove all non-deterministic sources of randomness, or simply track deterministic intermediate outputs as endpoints.
For example, you could write the analysis results to a `.csv` file, and track that as an endpoint.

You can add an endpoint by calling `worcs::add_endpoint("manuscript/manuscript.html")`.
If the endpoint is intentionally changed (e.g., because you updated your analysis code),
you can update its record by calling `worcs::snapshot_endpoints()`.

You can call `worcs::reproduce()`, which (in a project with `targets`) calls `tar_make()` and then calls `worcs::check_endpoints()` to verify that the endpoints reproduce after re-running the analysis.

Be very mindful of the fact that `targets` does not re-run pipeline steps when the step, or the inputs of the step, have not changed.
That means that it might look like your code reproduces upon repeat evaluation - but only because the code was not actually re-evaluated, because `targets` retrieved the results from its cache.

To properly evaluate whether your code reproduces, you must first destroy the cache of pipeline results before running `worcs::reproduce()`.
That causes targets to re-run every step of the pipeline.
This might look like:

```{r eval = FALSE}
# Snapshot the current state of the endpoints
worcs::snapshot_endpoints()
# Destroy the cache of targets results
targets::tar_destroy()
# worcs::reproduce() calls targets::tar_make(), then worcs::check_endpoints()
worcs::reproduce()
```

# *Optional:* Use Parallel Computation

Computational studies, like power analyses via Monte Carlo simulation,
often require repeating computations many times, which can be time-consuming.
Modern computers (CPUs) usually have multiple "cores"; by default, a program like Rstudio uses one core - but it's possible to assign a task to multiple cores.
Parallel computing is the practice of distributing a computational task across multiple CPU cores, significantly speeding up execution.

In this section, you’ll learn how to parallelize your simulation study using the `future` package.
This package has a simple interface and can be implemented in your `perform_study()` function with minimal code changes.

## 1. Install and Load Required Packages

There are various ways to implement parallel computing, and some methods are specific to different operating systems. The method below works (at least) on Windows.

**Interactive:** First, install the required packages:

```{r eval = FALSE}
install.packages("future", prompt = FALSE)
```


## 2. Adding Paralellization to `perform_study()`

Paralellization in R requires registering a parallel backend:
essentially, starting an R-session on each of the cores you want to use, and loading the required functions and packages there. 
These cores do not have access to the main R-session you are using interactively; they do not see what variables and functions you have loaded.

To register a parallel backend, you can add the following code to `perform_study()`:

```{r eval = FALSE}
library(future)
plan(multisession, workers = 4L)
```

Instead of hard-coding the number of workers (cores) to `4L` (integer 4),
you should choose the number based on your computer's actual number of cores.
You can determine the number of cores by running `parallelly::availableCores()`.
While you can use all cores for parallel computing,
people often use one or two fewer so they can keep using their computer while the simulation is running.

To do so, you could adapt the code as follows:

```{r eval = FALSE}
library(future)
plan(multisession, workers = parallelly::availableCores()-2L)
```

Note that on Mac or Linux, you might be able to use a more efficient backend by 'forking' processes.
To do so, use `plan(multicore)` instead of `plan(multisession)`.

The package `future.apply` contains parallel equivalents to base R functions like `lapply()`, `sapply()`, and importantly, `replicate()`.
To paralellize the inner loop of our simulation, we can use:

```{r eval = FALSE}
future.apply::future_replicate(n = reps, expr = {
      df <- with(as.list(thisrow), generate_data(beta = beta, n = n))
      analyze_data(df)
    },
    future.seed = TRUE)
```

The argument `future.seed` ensures that random seeds are properly handled so the results will be replicable.
For more information on seeds in parallel computation, see [this blog post](https://www.jottr.org/2020/09/22/push-for-statistical-sound-rng/).

### What to Paralellize?

Computational simulation studies often consist of loops-within-loops.
Deciding *which loop* to paralellize is a bit of an art form, and an area of specialization.
Considerations that weigh in to the decision are:

* What is the overhead (in terms of processing time) to set up the parallel backend?
* Where is the bottleneck: does the simulation fill up working memory, or does it max out your processor? You can [use code profiling](https://adv-r.hadley.nz/perf-measure.html) to assess CPU and memory bottlenecks, but this is beyond the scope of the present tutorial.

In this case, we could paralellize across the 9 conditions of the study, or across the 100 replications of those conditions.
I chose to paralellize across the 9 study conditions, having each core execute one condition 100 times.
Compare the function `perform_study()` with and without paralellization:

#### Without Paralellization

```{r eval = FALSE}
perform_study <- function(study_design, reps = 100){
  # For each row of the study design, execute a function
  pwr <- apply(study_design, 1, function(thisrow){
    # Replicate the row of the study design reps times
    out <- replicate(n = reps, expr = {
      # Simulate data with the beta and n from thisrow
      df <- with(as.list(thisrow), generate_data(beta = beta, n = n))
      # Analyze those data
      analyze_data(df)
    })
    # Calculate the proportion of significant results using mean()
    mean(out)
  })
  # Make a data frame containing the study design and study results (pwr)
  data.frame(study_design, power = pwr)
}
```

#### With Paralellization

```{r eval = FALSE}
perform_study <- function(study_design, reps = 100){
  library(future)
  # Sets up clusters from number of cores
  plan(multisession, workers = parallelly::availableCores()-2L)
  pwr <- apply(study_design, 1, function(thisrow){
    # Replicate the row of the study design reps times
    out <- future.apply::future_replicate(n = reps, expr = {
      # Simulate data with the beta and n from thisrow
      df <- with(as.list(thisrow), generate_data(beta = beta, n = n))
      # Analyze those data
      analyze_data(df)
    },
    future.seed = TRUE)
    # Calculate the proportion of significant results using mean()
    mean(out)
  })
  data.frame(study_design, power = pwr)
}
```

### 3. Re-run the Pipeline

Now rerun your simulation with:

```{r eval = FALSE}
targets::tar_make()
```

Do you notice a speedup? Probably not,
because the overhead associated with setting up the parallel back-end outweighs the time saved by paralellization.
When you are running many repetitions or conducting large computational simulation studies,
there will be time saved.
You can [benchmark your code](https://www.alexejgossmann.com/benchmarking_r/) to estimate how much time will be saved (but this is beyond the scope of the present tutorial).


# *Optional:* Add Integration Tests

Integration testing is a procedure used in software development to ensure that your code works as expected.
In a WORCS project, using integration tests increases scientific rigor and
helps you ensure reproducibility.
This section explains the bare basics of integration testing for reproducible research workflows;
see the [R Packages book](https://r-pkgs.org/testing-basics.html) for more detail.

### Why Write Tests?

You probably already test your functions informally - you write a function, run it in the console, and see if the output looks good.
This is better than nothing - but is it fool-proof?
Will you remember which functions looked good in 6 months?
What if someone changes part of your code?
What if functions are used in a different way then you expected?

**Integration tests:**

* Run automatically and consistently.
* Help catch bugs.
* Provide documentation of expected behavior.
* Encourage modular, testable code.

### When to Test?

* When you write a new function.
* When fixing a bug (add a test to catch it next time).
* Before sharing, submitting, or publishing your analysis.

## 1. Set Up `testthat`

**Interactive**: In your WORCS project, run the following code:

```{r eval = FALSE}
worcs::add_testthat()
```

This creates a `tests/testthat/` folder to hold your tests.
Note the two suggestions printed in the console: 

* Run `usethis::use_test()` to initialize a basic test file and open it for editing.
* Run `worcs::github_action_testthat()` to add a GitHub action that evaluates the integration tests.

## 2. Write a Simple Test

Let's prepare a file to test the behavior of our function `generate_data()` by running `usethis::use_test(name = "generate_data")`.

By default, the test file should look like this, testing some common sense math:

```
test_that("multiplication works", {
  expect_equal(2 * 2, 4)
})
```

Each test file contains:

* A function call to `test_that()`
* A short description, in this case `"multiplication works"`.
* One or more **expectations** that are evaluated, like `expect_equal()` in the example above.

Test files' names must start with `test-`, and they must be saved in `tests/testthat/`.

## 3. Run Tests

In your console or script, you can run:

```{r eval = FALSE}
worcs::test_worcs()
```

Or, to test a single file:

```{r eval = FALSE}
testthat::test_file("tests/testthat/test-generate_data.R")
```

The output should tell you which tests passed or failed, and why they failed.


## 4. Customize Your Test

What do we want to test about `generate_data()`?

It's up to you, but consider testing any assumptions you make about the function, for example:

* It generates a `data.frame`
* All columns are `numeric`
* The number of rows corresponds to `n`
* The number of columns corresponds to the number of variables in the DAG (not tested below)
* At a very high sample size `n`, the regression coefficient of `intrinsic_motivation -> wellbeing` approaches `beta` within a certain tolerance

What would these assumptions look like when tested? For example, like:

```{r eval = FALSE}
test_that("generate_data works", {
  # Run generate_data()
  df <- generate_data(.4, 100)
  # It generates a `data.frame`
  expect_s3_class(df, "data.frame")
  # All columns are `numeric`
  expect_true(all(sapply(df, inherits, what = "numeric")))
  # The number of rows corresponds to `n`
  expect_true(nrow(df) == 100)
  # At high n, the regression coefficient approaches beta within tolerance
  set.seed(1)
  df <- generate_data(.4, 100000)
  res <- lm(wellbeing ~ intrinsic_motivation + needs, data = df)
  expect_equivalent(res$coefficients[2], .4, tolerance = .01)
})
```

Copy-paste any of these tests, or write your own. Note that, instead of grouping all tests, you could also break them up into separate `test_that()` commands:

```{r eval = FALSE}
test_that("generate_data generates a data.frame", {
  # Run generate_data()
  df <- generate_data(.4, 100)
  # It generates a `data.frame`
  expect_s3_class(df, "data.frame")
})

test_that("generate_data returns all numeric columns", {
  df <- generate_data(.4, 100)
  expect_true(all(sapply(df, inherits, what = "numeric")))
})
```

The main advantage of this style is that the output of `testthat::test_dir()` will be documented at a more fine-grained level.

## 5. Run your Tests Online

The development platform 'GitHub' allows you to run code in the cloud (= on their servers).
This is very useful when checking if your `worcs` project reproduces on a different system from your own,
and also, to run your integration tests on a different system.

Doing so avoids the famous meme:

```
It works on my machine ¯\_(ツ)_/¯
```

GitHub Actions is an automation tool offered by GitHub, which allows developers to run workflows directly from their repositories. By using GitHub Actions, we can automate processes such as continuous integration, continuous deployment, testing, and code reviews.

GitHub Actions are triggered by events - for example, when pushing code updates to GitHub.

**Interactive:** Let's set up a GitHub action to run our integration tests:

```{r eval = FALSE}
# Add the appropriate GitHub action:
worcs::github_action_testthat()
```

**Interactive:** The virtual computer environment on GitHub actions will attempt to recreate your local environment based on the `renv` dependency manager.
To ensure that your `renv` records are up to date, run the code below.
It is important to always snapshot your dependencies before reproducing your analyses on another system;
so run `renv::snapshot()` before publishing your code, before triggering a GitHub action, before sharing your code with a collaborator, et cetera.

```{r eval = FALSE}
renv::snapshot()
```

**Interactive:** Now, push your code to GitHub. Among the files that will be updated are instructions for GitHub actions in the `.github/` folder, and your updated `renv.lock` file.

```{r eval = FALSE}
worcs::git_update("add testthat")
```

Since this GitHub action is triggered by pushing code to GitHub, if you now navigate to your GitHub repository.
If you don't remember your GitHub URL, run:

```{r eval = FALSE}
utils::browseURL(gsub(".git", "/actions", gert::git_remote_list()$url, fixed = TRUE))
```

You should see the action running. If all is good, it will show you a green dot (red if there are errors).

Integration testing gives you peace of mind - you know that your code works as expected, and via GitHub actions, can show it to the world too.

## 6. Reproducing your Analysis Online

The `worcs` package also includes functionality to reproduce your entire analysis online via GitHub actions.
However, running a full simulation study is probably not fair use of GitHub's servers, and might raise some eyebrows.

For other, smaller scale studies, [the vignette on endpoints](https://cjvanlissa.github.io/worcs/articles/endpoints.html) explains how to create a GitHub action to reproduce your analyses in the cloud.

# Additional Resources

* [Using targets to Reduce Redundant Computations](https://cjvanlissa.github.io/worcs/articles/targets.html)
* [WORCS documentation](https://cjvanlissa.github.io/worcs/)
* [theorytools documentation](https://cjvanlissa.github.io/theorytools/)
* [FAIR Principles](https://www.go-fair.org/fair-principles/)
* [TOP Guidelines](https://www.cos.io/initiatives/top-guidelines)
* [Paralellizing your Code](https://computing.stat.berkeley.edu/tutorial-dask-future/R-future.html)

# References
