Run stale tests on file change in Elixir.

Mix is an awesome tool but most Elixir beginners are not aware of all its features. mix test --stale is one of them and can make your workflow much smoother. It tells mix to run only tests which reference modules that changed from the latest test run.

Literally, this means that if you have changed some code in your application, the mix will run only the tests that touch this code or its dependents. This saves a lot of time if you are trying to follow TDD in a mid-to-large sized project.

But to run this command we still have to switch between a code editor and a console. Fortunately, mix test supports --listen-on-stdin flag that configures mix to read stdin and restart the tests after receiving a newline. Now we only need a way to watch for the file system events.

Here is a simple bash script for that (tested on macOS):

#!/bin/bash

command -v fswatch >/dev/null 2>&1 || { echo >&2 "fswatch is not installed. To install it use 'brew install fswatch'. Aborting."; exit 1; }
command -v mix >/dev/null 2>&1 || { echo >&2 "mix is not installed. Aborting."; exit 1; }

PROJECT_PATH=$(git rev-parse --show-toplevel)
[[ "$?" != "0" ]] && echo "Stale test watch works only within git project." && exit 1;

declare -a IGNORED_PATHS
declare -i throttle_by=2

# Ignore all paths from gitignore and ~/.gitignore_global
IGNORED_PATHS+=('.git' '_build' 'docs' 'deps' 'cover')

function read_gitignore() {
  while IFS='' read -r line || [[ -n "$line" ]]; do
    IGNORE_PATH=${line%%#*}
    if [[ "${IGNORE_PATH}" != "" ]]; then
      IGNORED_PATHS=("${IGNORED_PATHS[@]}" ${IGNORE_PATH})
    fi
  done < "$1"
}

[ -f "${PROJECT_PATH}/.gitignore" ] && read_gitignore "${PROJECT_PATH}/.gitignore"
[ -f "$HOME/.gitignore_global" ] && read_gitignore "$HOME/.gitignore_global"

function filter_ignored_paths() {
  while read match; do
    local changes_detected=1
    for e in "${IGNORED_PATHS[@]}"; do
      [[ "$match" == "${PROJECT_PATH}/$e"* || "$match" == *"$e" ]] && changes_detected="0" && break;
    done

    [[ "${changes_detected}" == "1" ]] && echo "Changes detected in $match";
  done
}

ARGS="$@"
[[ "${ARGS}" == "" ]] && ARGS="--stale"

fswatch $PWD | filter_ignored_paths | mix test --listen-on-stdin ${ARGS}

It utilizes fswatch to read the file system events, it filters out the files ignored by git along with some common directories that we are not interested in. Then it writes a new line to stdout.

If you run into odd dependency resolution (e.g., renames or large refactors), reset state with a full run: mix test.

Faster iteration makes better tests - and better code. Keep the loop tight.