‚̧Ninety-nine Pitchers of Malt


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PureScript package manager and build tool powered by Dhall and package-sets.

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The recommended installation method for Windows, Linux and macOS is npm (see the latest releases on npm here):

npm install -g spago

Other installation methods available:

General notes:

  • The assumption is that you already installed the PureScript compiler. If not, get it with npm install -g purescript, or the recommended method for your OS.
  • You might have issues with npm and Docker (e.g. getting the message "Downloading the spago binary failed.." etc) You have two options:
    • either do not run npm as root, because it doesn't work well with binaries. Use it as a nonprivileged user.
    • or use --unsafe-perm: npm install -g --unsafe-perm spago

Super quick tutorial

Let's set up a new project!

$ mkdir purescript-unicorns
$ cd purescript-unicorns
$ spago init

This last command will create a bunch of files:

‚Ēú‚ĒÄ‚ĒÄ packages.dhall
‚Ēú‚ĒÄ‚ĒÄ spago.dhall
‚Ēú‚ĒÄ‚ĒÄ src
‚Ēā¬†¬† ‚ĒĒ‚ĒÄ‚ĒÄ Main.purs
‚ĒĒ‚ĒÄ‚ĒÄ test
    ‚ĒĒ‚ĒÄ‚ĒÄ Main.purs

Let's take a look at the two Dhall configuration files that spago requires:

  • packages.dhall: this file is meant to contain the totality of the packages available to your project (that is, any package you might want to import).

    In practice it pulls in the official package-set as a base, and you are then able to add any package that might not be in the package set, or override existing ones.

  • spago.dhall: this is your project configuration. It includes the above package set, the list of your dependencies, the source paths that will be used to build, and any other project-wide setting that spago will use.

To build your project, run:

$ spago build

This will download the necessary dependencies and compile the sample project in the output/ directory. You can take a look at the content of output/Main/index.js to see what kind of JavaScript has been generated from your new Main.purs file.

You can already see your project running, by doing

$ spago run

..which is basically equivalent to the following command:

$ node -e "require('./output/Main/index').main()"

..which imports the JS file you just looked at, and runs the main with Node.

You can also bundle the project in a single file with an entry point, so it can be run directly (useful for CLI apps):

$ spago bundle-app
$ node .

Design goals and reasons

Our main design goals are:

  • Great UX: you're not supposed to spend your life configuring the build for your project. A good build system just does what's most expected and gets out of the way so you can focus on actually writing the software.
  • Minimal dependencies: users should not be expected to install a myriad of tools on their system to support various workflows. We depend only on git and purs being installed.
  • Reproducible builds: thanks to package sets and Dhall, if your project builds today it will also build tomorrow and every day after that.

Some tools that inspired spago are: Rust's Cargo, Haskell's Stack, psc-package, pulp and purp.

Brief survey of other package managers and build tools available

pulp is excellent, but it is only a build tool. This means that you'll have to use it with either bower or psc-package:

  • If you go for bower, you're missing out on package-sets (that is: packages versions that are known to be working together, saving you the headache of fitting package versions together all the time).

  • If you use psc-package, you have the problem of not having the ability of overriding packages versions when needed, leading everyone to make their own package-set, which then goes unmaintained, etc.

    Of course you can use the package-set-local-setup to solve this issue, but this is exactly what we're doing here: integrating all the workflow in a single tool, spago, instead of having to install and use pulp, psc-package, purp, etc.

Developing and contributing

We'd love your help, and welcome PRs and contributions!

Some ideas for getting started:

For more details see the CONTRIBUTING.md

How do I...

This section contains a collection of workflows you might want to use to get things done with spago

Switch from psc-package

Do you have an existing psc-package project and want to switch to spago?

No problem! If you run spago init, we'll port your existing psc-package.json configuration into a new spago.dhall ūüėé

Note: spago won't otherwise touch your psc-package.json file, so you'll have to remove it yourself.

You'll note that most of the psc-package commands are the same in spago, so porting your existing build is just a matter of search-and-replace most of the times.

Switch from bower

Switching from bower is about the same workflow: just run spago init and we'll try to match the package versions in your bower.json with the ones in the package set, porting the packages to your spago.dhall

Note: spago won't otherwise touch your bower.json file, so you'll have to remove it yourself.

Some packages might not be found or have the wrong version, in which case you'll have to carefully:

See what commands and flags are supported

For an overview of the available commands, run:

$ spago --help

You will see several subcommands (e.g. build, test); you can ask for help about them by invoking the command with --help, e.g.:

$ spago build --help

This will give a detailed view of the command, and list any command-specific (vs global) flags.

Download my dependencies locally

$ spago install

This will download all the transitive dependencies of your project (i.e. the direct dependencies, i.e. the ones listed in the dependencies key of spago.dhall, plus all their dependencies, recursively) to the local .spago folder (and the global cache, if possible).

However, running this directly is usually not necessary, as all commands that need the dependencies to be installed will run this for you.

Build and run my project

We can build the project and its dependencies by running:

$ spago build

This is just a thin layer above the PureScript compiler command purs compile.

The build will produce very many JavaScript files in the output/ folder. These are CommonJS modules, and you can just require() them e.g. on Node.

It's also possible to include custom source paths when building (the ones declared in your sources config are always included):

$ spago build --path 'another_source/**/*.purs'

Note: the wrapper on the compiler is so thin that you can pass options to purs. E.g. if you wish to output your files in some other place than output/, you can run

$ spago build --purs-args '-o myOutput/'

If you wish to automatically have your project rebuilt when making changes to source files you can use the --watch flag:

$ spago build --watch
# or, to clear the screen on rebuild: 
$ spago build --watch --clear-screen

If you want to run the program (akin to pulp run), just use run:

# The main module defaults to "Main" 
$ spago run
# Or define your own module path to Main 
$ spago run --main ModulePath.To.Main
# And pass arguments through to `purs compile` 
$ spago run --main ModulePath.To.Main --purs-args '--verbose-errors'
# Or pass arguments to node 
$ spago run --node-args "arg1 arg2"

Test my project

You can also test your project with spago:

# Test.Main is the default here, but you can override it as usual 
$ spago test --main Test.Main
Build succeeded.
You should add some tests.
Tests succeeded.

Run a repl

As with the build and test commands, you can add custom source paths to load, and pass options to the underlying purs repl via --purs-args.

E.g. the following opens a repl on localhost:3200:

$ spago repl --purs-args '--port 3200'

List available packages

It is sometimes useful to know which packages are contained in our package set (e.g. to see which version we're using, or to search for packages).

You can get a complete list of the packages your packages.dhall imports (together with their versions and URLs) by running:

$ spago list-packages

By passing the --filter flag you can restrict the list to direct or transitive dependencies:

# Direct dependencies, i.e. only the ones listed in spago.dhall 
$ spago list-packages --filter=direct
# Transitive dependencies, i.e. all the dependencies of your dependencies 
$ spago list-packages -f transitive

Add a direct dependency

You can add dependencies that are available in your package set by running:

# E.g. installing Halogen 
$ spago install halogen
# This also supports multiple packages 
$ spago install foreign simple-json

Override a package in the package set with a local one

Let's say I'm a user of the simple-json package. Now, let's say I stumble upon a bug in there, but thankfully I figure how to fix it. So I clone it locally and add my fix.

Now if I want to test this version in my current project, how can I tell spago to do it?

We have a overrides record in packages.dhall just for that!

In this case we override the package with its local copy, which must have a spago.dhall. (it should be enough to do spago init to have the Bower configuration imported)

It might look like this:

let overrides =
      { simple-json = ../purescript-simple-json/spago.dhall as Location

Note that if we list-packages, we'll see that it is now included as a local package:

$ spago list-packages
signal                v10.1.0   Remote "https://github.com/bodil/purescript-signal.git"
sijidou               v0.1.0    Remote "https://github.com/justinwoo/purescript-sijidou.git"
simple-json           local     Local "./../purescript-simple-json"
simple-json-generics  v0.1.0    Remote "https://github.com/justinwoo/purescript-simple-json-generics.git"
smolder               v11.0.1   Remote "https://github.com/bodil/purescript-smolder.git"

And since local packages are just included in the build, if we add it to the dependencies in spago.dhall and then do spago install, it will not be downloaded.

Override a package in the package set with a remote one

Let's now say that we test that our fix from above works, and we are ready to Pull Request the fix.

So we push our fork and open the PR, but while we wait for the fix to land on the next package sets release, we still want to use the fix in our production build.

In this case, we can just change the override to point to some commit of our fork, like this:

let overrides =
    { simple-json =
       // { repo = "https://github.com/my-user/purescript-simple-json.git"
          , version = "701f3e44aafb1a6459281714858fadf2c4c2a977"

Note: you can use a "branch", a "tag" or a "commit hash" as a version. Generally it's recommended that you avoid using branches, because if you push new commits to a branch, spago won't pick them up unless you delete the .spago folder.

Add a package to the package set

If a package is not in the upstream package set, you can add it in a similar way, by changing the additions record in the packages.dhall file.

E.g. if we want to add the facebook package:

let additions =
  { facebook =
      { dependencies =
          [ "console"
          , "aff"
          , "prelude"
          , "foreign"
          , "foreign-generic"
          , "errors"
          , "effect"
      , repo =
      , version =
          "v0.3.0"  -- branch, tag, or commit hash

As you might expect, this works also in the case of adding local packages:


let additions =
  { foobar = ../foobar/spago.dhall as Location

bower link

See how to add local packages or override existing ones

Verify that an addition/override doesn't break the package set

"But wait", you might say, "how do I know that my override doesn't break the package set?"

This is a fair question, and you can verify that your fix didn't break the rest of the package-set by running the verify command.

E.g. if you patched the foreign package, and added it as a local package to your package-set, you can check that you didn't break its dependents (also called "reverse dependencies") by running:

$ spago verify foreign

Once you check that the packages you added verify correctly, we would of course very much love if you could pull request it to the Upstream package-set ‚̧ԳŹ

If you decide so, you can read up on how to do it here.

Automagically upgrade the package set

The version of the package-set you depend on is fixed in the packages.dhall file (look for the upstream var).

You can upgrade to the latest version of the package-set with the upgrade-set command, that will automatically find out the latest version, download it, and write the new url and hashes in the packages.dhall file for you.

Running it would look something like this:

$ spago upgrade-set
Found the most recent tag for "purescript/package-sets": "psc-0.12.3-20190227"
Package-set upgraded to latest tag "psc-0.12.3-20190227"
Fetching the new one and generating hashes.. (this might take some time)
Done. Updating the local package-set file..

If you wish to detach from tags for your package-set, you can of course point it to a specific commit. Just set your upstream to look something like this:

let upstream =


Spago aims to support "monorepos", allowing you to split a blob of code into different "compilation units" that might have different dependencies, deliverables, etc.

A typical monorepo setup in spago consists of:

  • some "libraries" (i.e. packages that other packages will depend on), each having their own spago.dhall
  • some "apps" (i.e. packages that no one depends on), each having their own spago.dhall
  • a single packages.dhall , that includes all the "libraries" as local packages, and that all spago.dhall files refer to - this is so that all packages share the same package set.

So for example if you have lib1, lib2 and app1, you might have the following file tree:

‚Ēú‚ĒÄ‚ĒÄ app1
‚Ēā   ‚Ēú‚ĒÄ‚ĒÄ spago.dhall
‚Ēā   ‚Ēú‚ĒÄ‚ĒÄ src
‚Ēā   ‚Ēā   ‚ĒĒ‚ĒÄ‚ĒÄ Main.purs
‚Ēā   ‚ĒĒ‚ĒÄ‚ĒÄ test
‚Ēā       ‚ĒĒ‚ĒÄ‚ĒÄ Main.purs
‚Ēú‚ĒÄ‚ĒÄ lib1
‚Ēā   ‚Ēú‚ĒÄ‚ĒÄ spago.dhall
‚Ēā   ‚ĒĒ‚ĒÄ‚ĒÄ src
‚Ēā       ‚ĒĒ‚ĒÄ‚ĒÄ Main.purs
‚Ēú‚ĒÄ‚ĒÄ lib2
‚Ēā   ‚Ēú‚ĒÄ‚ĒÄ spago.dhall
‚Ēā   ‚ĒĒ‚ĒÄ‚ĒÄ src
‚Ēā       ‚ĒĒ‚ĒÄ‚ĒÄ Main.purs
‚ĒĒ‚ĒÄ‚ĒÄ packages.dhall


  • the top level packages.dhall might look like this:
let upstream = https://github.com/purescript/package-sets/releases/download/psc-0.13.4-20191025/packages.dhall sha256:f9eb600e5c2a439c3ac9543b1f36590696342baedab2d54ae0aa03c9447ce7d4
let overrides =
  { lib1 = ./lib1/spago.dhall as Location
  , lib2 = ./lib2/spago.dhall as Location
in upstream // overrides
  • lib1/spago.dhall might look something like this:
{ name =
, dependencies =
    [ "effect"
    , "console"
    , "prelude"
, sources =
    [ "src/**/*.purs" ]
, packages =
    ../packages.dhall   -- Note: this refers to the top-level packages file
  • assuming lib1 depends on lib2, lib2/spago.dhall might look something like this:
{ name =
, dependencies =
    [ "effect"
    , "console"
    , "prelude"
    , "lib1"            -- Note the dependency here
, sources =
    [ "src/**/*.purs" ]
, packages =
  • and then app1/spago.dhall might look something like this:
{ name =
, dependencies =
    -- Note: the app does not include all the dependencies that the lib included
    [ "prelude"
    , "simple-json" -- Note: this dep was not used by the library, only the app uses it
    , "lib2"        -- Note: we add `lib2` as dependency
, packages =
    -- We also refer to the top-level packages file here, so deps stay in sync for all packages

To avoid building the same packages over, a shared output folder will be created next to your root packages.dhall.

To disable this behaviour, pass --no-share-output to spago build.

devDependencies, testDependencies, or in general a situation with many configurations

You might have a simpler situation than a monorepo, where e.g. you just want to "split" dependencies.

A common case is when you don't want to include your test dependencies in your app's dependencies.

E.g. if you want to add purescript-spec to your test dependencies you can have a test.dhall that looks like this:

let conf = ./spago.dhall
in conf // {
  sources = [ "test/**/*.purs" ],
  dependencies = conf.dependencies # [ "spec" ]

And then you can run tests like this:

$ spago -x test.dhall test

Bundle a project into a single JS file

For the cases when you wish to produce a single JS file from your PureScript project, there are basically two ways to do that:

1. spago bundle-app

This will produce a single, executable, dead-code-eliminated file:

# You can specify the main module and the target file, or these defaults will be used 
$ spago bundle-app --main Main --to index.js
Bundle succeeded and output file to index.js
# We can then run it with node: 
$ node .

2. spago bundle-module

If you wish to produce a single, dead-code-eliminated JS module that you can require from JavaScript:

# You can specify the main module and the target file, or these defaults will be used 
$ spago bundle-module --main Main --to index.js
Bundling first...
Bundle succeeded and output file to index.js
Make module succeeded and output file to index.js
$ node -e "console.log(require('./index').main)"

Skip the "build" step

When running spago bundle-app and spago bundle-module, Spago will first try to build your project, since bundling requires the project to be compiled first.

If you already compiled your project and want to skip this step you can pass the --no-build flag.

Make a project with PureScript + JavaScript

Take a look at TodoMVC with react-basic + spago + parcel for a working example, or follow one of the next "get started" sections:

Get started from scratch with Parcel (frontend projects)

To start a project using Spago and Parcel together, here's the commands and file setup you'll need:

  1. Follow Spago's "Super quick tutorial"
  2. Initialise a JavaScript/npm project with npm init
  3. Install Parcel as a development-time dependency npm i parcel --save dev
  4. Add a JavaScript file which imports and calls the main function from the output of src/Main.purs. This can be placed in the root directory for your project. Traditionally this file is named index.js. The main function from Main.purs can accept arguments, this is useful since Parcel will replace environment variables inside of JavaScript. It is recommended to read any environment variables in the JavaScript file and pass them as arguments to main. Here is an example JavaScript file:
var Main = require('./output/Main');
function main () {
    Here we could add variables such as
    var baseUrl = process.env.BASE_URL;
    Parcel will replace `process.env.BASE_URL`
    with the string contents of the BASE_URL environment
    variable at bundle/build time.
    A .env file can also be used to override shell variables
    for more information, see https://en.parceljs.org/env.html
    These variables can be supplied to the Main.main function.
    However, you will need to change the type to accept variables, by default it is an Effect.
    You will probably want to make it a function from String -> Effect ()
// HMR setup. For more info see: https://parceljs.org/hmr.html
if (module.hot) {
  module.hot.accept(function () {
    console.log('Reloaded, running main again');
console.log('Starting app');
  1. Add an HTML file which sources your JavaScript file. This can be named index.html and placed in the root directory of your project. Here is an example HTML file:
<!doctype html>
<html lang="en" data-framework="purescript">
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <div id="app"></div>
  <script src="./index.js"></script> 
  1. Add a development script to package.json which will hot-bundle the PureScript code with Spago, and then hot-reload the resulting JavaScript code using Parcel. Here, we'll call this script dev.
  "scripts": {
    "dev": "spago build --watch & parcel index.html",

This script will simultaneously run Spago and Parcel in parallel. NPM scripts allow project dependencies to be treated as if they are on your $PATH. When you run it with npm run dev, Parcel will tell you which port your application is being served on, by default this will be localhost:1234.

If you've followed this guide you can navigate there in a browser and open the JavaScript console, you will see the output of both index.js and the compiled Main.purs file. When you modify any purescript file in ./src, you should see Spago and Parcel rebuild your application, and the browser should execute the new code. For some applications you may adjust the JavaScript function that handles hot modules to fully reload the page with window.location.reload();.

  1. At this point we should be able to test our program by running npm run dev. When you navigate a browser to localhost:1234, you should see 'ūüćĚ' as output in the JavaScript console if this was performed successfully!

  2. When you are ready to build and deploy your application as static html/js/css, you may add a build script to package.json in order to produce a final bundle. This script is usually something like spago build && parcel build index.html.

Get started from scratch with Webpack (frontend projects)

  1. Follow Spago's "Super quick tutorial"
  2. Initialise a JavaScript/npm project with npm init
  3. Add Webpack and purescript-psa as development-time dependencies: npm install --save-dev webpack webpack-cli webpack-dev-server purescript-psa
  4. Install the PureScript loader and HTML plugin for WebPack npm install --save-dev purs-loader html-webpack-plugin. Note that you may require additional loaders for css/scss, image files, etc. Please refer to the Webpack documentation for more information.
  5. Create an HTML file that will serve as the entry point for your application. Typically this is index.html. in your HTML file, be sure to pull in the bundle.js file, which will be Webpack's output. here is an example HTML file:
<!doctype html>
<html lang="en" data-framework="purescript">
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <div id="app"></div>
  <script src="./bundle.js"></script> 
  1. Create a webpack.config.js file in the root of your project. Here is an example webpack configuration:
'use strict';
const path = require('path');
const HtmlWebpackPlugin = require('html-webpack-plugin');
const webpack = require('webpack');
const isWebpackDevServer = process.argv.some(a => path.basename(a) === 'webpack-dev-server');
const isWatch = process.argv.some(a => a === '--watch');
const plugins =
  isWebpackDevServer || !isWatch ? [] : [
      this.plugin('done', function(stats){
module.exports = {
  devtool: 'eval-source-map',
  devServer: {
    contentBase: path.resolve(__dirname, 'dist'),
    port: 4008,
    stats: 'errors-only'
  entry: './src/index.js',
  output: {
    path: path.resolve(__dirname, 'dist'),
    filename: 'bundle.js'
  module: {
    rules: [
        test: /\.purs$/,
        use: [
            loader: 'purs-loader',
            options: {
              src: [
              spago: true,
              watch: isWebpackDevServer || isWatch,
              pscIde: true
        test: /\.(png|jpg|gif)$/i,
        use: [
            loader: 'url-loader',
            options: {
              limit: 8192,
  resolve: {
    modules: [ 'node_modules' ],
    extensions: [ '.purs', '.js']
  plugins: [
    new webpack.LoaderOptionsPlugin({
      debug: true
    new HtmlWebpackPlugin({
      title: 'purescript-webpack-example',
      template: 'index.html'
  1. Add a src/index.js: this file will import and execute the PureScript Main module, and serves as the entry point for the Webpack bundler.

You can also use this file to refer to environment variables which can then be passed to PureScript code. Please refer to the Webpack documentation on environment variable replacement during bundling. Here is an example index.js file:

'use strict';
if (module.hot) {
console.log('app starting');
  1. Add the following development script to package.json:
  "scripts": {
    "webpack:server": "webpack-dev-server --progress --inline --hot"
  1. At this point we should be able to run our program by calling npm run webpack:server. If you point your browser to localhost:4008 you should see ūüćĚ in the JavaScript development console. This means everything went alright!

  2. For production builds, it is recommended to have separate scripts to build and serve. Please refer to the Webpack documentation for more information.

Get started from scratch with Nodemon (backend and/or CLI projects)

  1. Follow Spago's "Super quick tutorial"
  2. Initialise a JavaScript/npm project with npm init
  3. Add Nodemon as a development-time dependency: npm install --save-dev nodemon
  4. Add a JavaScript file which imports and calls the main function from the output of src/Main.purs.

This can be placed in the root directory of your project, and traditionally this file is named index.js.

The main function from Main.purs can accept arguments, and this is useful since the Node runtime will replace environment variables inside of JavaScript. It is recommended to read any environment variables in the JavaScript file and pass them as arguments to main.

Here is an example JavaScript file:

'use strict'
var Main = require('./output/Main');
function main () {
    Here we could add variables such as
    var baseUrl = process.env.BASE_URL;
    Node will replace `process.env.BASE_URL`
    with the string contents of the BASE_URL environment
    variable at bundle/build time.
    These variables can be supplied to the Main.main function,
    however, you will need to change the type to accept variables, by default it is an Effect.
    You will probably want to make it a function from String -> Effect ()
  1. At this point we should be able to run our program by calling spago build followed by node index.js. If you see ūüćĚ as output then this was successful!
  2. Now we want to enable Nodemon, which will watch for file changes in the dependancy tree and reload our Node program every time there is a new change. We'll also tell Spago to watch our PureScript source files so that they are compiled, which in turn will trigger a Nodemon reload.

To configure this, add the following script to your package.json file:

  "scripts": {
    "dev": "spago build --watch & nodemon \"node index.js\"",
  1. You can now run your development environment by calling npm run dev

  2. For a production build, add the following scripts to your package.json:

  "scripts": {
    "build": "spago build && node index.js"
  1. To run a production build, you can now run npm run build!

For publishing CLI programs or NPM modules, please refer to the relevant npm documentation.

Please note that if you are publishing a Node module for consumption by JavaScript users, it is recommended that you pre-compile your PureScript project before distributing it.

Generate documentation for my project

To build documentation for your project and its dependencies (i.e. a "project-local Pursuit"), you can use the docs command:

$ spago docs

This will generate all the documentation in the ./generated-docs folder of your project. You might then want to open the index.html file in there.

If you wish for the documentation to be opened in browser when generated, you can pass an open flag:

$ spago docs --open

To build the documentation as Markdown instead of HTML, or to generate tags for your project, you can pass a format flag:

$ spago docs --format ctags

Get source maps for my project

Quoting from this tweet:

  1. build with --purs-args '-g sourcemaps'
  2. source output (like var someModule = require('./output/Whatever/index.js');) and use something like parcel, to avoid mangling/destroying the sourcemaps
  3. now you can see your breakpoints in action

Use alternate backends to compile to Go, C++, Kotlin, etc

Spago supports compiling with alternate purescript backends like psgo or pskt. To use an alternate backend, add the backend option to you spago.dhall file:

{ name = "aaa"
, backend = "psgo"

The value of the backend entry should be the name of the backend executable.

Publish my library

If you wish to develop a library with spago you can definitely do so, and use it to manage and build your project, until you need to "publish" your library, where you'll need to use pulp.

When you decide you want to publish your library for others to use, you should:

  • run spago bump-version --no-dry-run <BUMP>. This will generate a bower.json in a new commit in Git that is tagged with the version.
  • run pulp publish. This will ensure the package is registered in Bower, push the version tag to Git and upload documentation to Pursuit.

This is because the PureScript ecosystem uses the Bower registry as a "unique names registry". So in order to "publish" a package one needs to add it there, and eventually to package-sets. Consequentially, package-sets requires (full instructions here) that packages in it:

  • are in the Bower registry
  • use spago bump-version or pulp version (because this gives versions with vX.Y.Z)
  • use pulp publish (so that's it's available on the Bower registry and on Pursuit)

All of this will be automated in future versions, removing the need for Pulp.

A library published in this way is purescript-rave.

Get all the licenses of my dependencies

For compliance reasons, you might need to fetch all the LICENSE files of your dependencies.

To do this you can exploit the list-packages command with its --filter flag.

E.g. if you want to print out all the LICENSE files of your direct dependencies:

#!/usr/bin/env bash 
# Note: the `awk` part is to cut out only the package name 
for dep in $(spago list-packages -f direct | awk '{print $1}')
  cat $(find ".spago/${dep}" -iname 'LICENSE')

Know which purs commands are run under the hood

The -v flag will print out all the purs commands that spago invokes during its operations, plus a lot of diagnostic info, so you might want to use it to troubleshoot weird behaviours and/or crashes.

Ignore or update the global cache

There is a global cache that spago uses to avoid re-downloading things - its location will be printed if you call e.g. spago install -v.

It's possible to change the behaviour of the global cache with the --global-cache flag that is accepted by many commands. You can either:

  • skip the cache with --global-cache=skip: in this case the global cache will be ignored and the local project will re-download everything
  • update the cache to the latest version with --global-cache=update: this might be useful if you want to globally cache a tag or commit that is newer than 24h - the time spago will wait before updating its metadata file about "which things are globally cacheable".

Know the output path for my compiled code

As there are now various factors that can affect the output path of compiled code, run spago path output along with any flags you would pass to spago build (like --purs-args or --no-share-output) to return the output path Spago is using. This can be useful for sharing an output folder with webpack, for instance.


Visual Overview: What happens when you do 'spago build'?


Configuration file format

It's indeed useful to know what's the format (or more precisely, the Dhall type) of the files that spago expects. Let's define them in Dhall:

-- The basic building block is a Package:
let Package =
  { dependencies : List Text  -- the list of dependencies of the Package
  , repo = Text               -- the address of the git repo the Package is at
  , version = Text            -- git tag
-- The type of `packages.dhall` is a Record from a PackageName to a Package
-- We're kind of stretching Dhall syntax here when defining this, but let's
-- say that its type is something like this:
let PackageSet =
  { console : Package
  , effect : Package
  ...                  -- and so on, for all the packages in the package-set
-- The type of the `spago.dhall` configuration is then the following:
let Config =
  { name : Text                   -- the name of our project
  , dependencies : List Text      -- the list of dependencies of our app
  , backend : Maybe Text          -- Nothing by default, meaning use purs. If specified, spago will use the executable as the backend
  , sources : List Text           -- the list of globs for the paths to always include in the build
  , packages : PackageSet         -- this is the type we just defined above

Why can't spago also install my npm dependencies?

A common scenario is that you'd like to use things like react-basic, or want to depend on JS libraries like ThreeJS. In any case, you end up depending on some NPM package.

And it would be really nice if spago would take care of installing all of these dependencies, so we don't have to worry about running npm besides it, right?

While these scenarios are common, they are also really hard to support. In fact, it might be that a certain NPM package in your transitive dependencies would only support the browser, or only node. Should spago warn about that? And if yes, where should we get all of this info?

Another big problem is that the JS backend is not the only backend around. For example, PureScript has a C backend and an Erlang backend among the others.

These backends are going to use different package managers for their native dependencies, and while it's feasible for spago to support the backends themselves, supporting also all the possible native package managers (and doing things like building package-sets for their dependencies versions) is not a scalable approach (though we might do this in the future if there's enough demand).

So this is the reason why if you or one of your dependencies need to depend on some "native" packages, you should run the appropriate package-manager for that (e.g. npm).

For examples on how to do it, see next section.

Why we don't resolve JS dependencies when bundling, and how to do it

spago only takes care of PureScript land. In particular, bundle-module will do the most we can do on the PureScript side of things (dead code elimination), but will leave the requires still in.

To fill them in you should use the proper js tool of the day, at the time of writing ParcelJS looks like a good option.

If you wish to see an example of a project building with spago + parcel, a simple starting point is the TodoMVC app with react-basic. You can see in its package.json that a "production build" is just spago build && parcel build index.html.

If you open its index.js you'll see that it does a require('./output/Todo.App'): the files in output are generated by spago build, and then the parcel build resolves all the requires and bundles all these js files in.

Though this is not the only way to include the built js - for a slimmer build or for importing some PureScript component in another js build we might want to use the output of bundle-module.

For an example of this in a "production setting" you can take a look at affresco. It is a PureScript monorepo of React-based components and apps.

The gist of it is that the PureScript apps in the repo are built with spago build (look in the package.json for it), but all the React components can be imported from JS apps as well, given that proper modules are built out of the PS sources.

This is where spago bundle-module is used: the build-purs.rb builds a bundle out of every single React component in each component's folder - e.g. let's say we bundle-module from the ksf-login component and output it in the index.js of the component's folder; we can then yarn install the single component (note it contains a package.json), and require it as a separate npm package with require('@affresco/ksf-login').

How does the "global cache" work?

Every time spago will need to "install dependencies" it will:

  • check if the package is local to the filesystem: if it is then it will skip it as we can just point to the files

  • check if the ref is already in the global cache. If it is, it will just copy it to the project-local cache

  • download a metadata file from the package-sets-metadata repo if missing from the global cache or older 24 hours.

    This file contains the list of tags and commits for every package currently in the package set, updated hourly.

  • check if the tag or commit of the package we need to download is in this cached index, and if it is then this means we can "globally cache" that version - this is because commit hashes are immutable, and tags are "immutable enough"

  • if a version is deemed to be "globally cacheable" then a tarball of that ref is downloaded from GitHub and copied to both the global and the local cache

  • otherwise, the repo is just cloned to the local cache

Note: a question that might come up while reading the above might be "why not just hit GitHub to check commits and tags for every repo while installing?"

The problem is that GitHub limits token-less API requests to 50 per hour, so any decently-sized installation will fail to get all the "cacheable" items, making the global cache kind of useless. So we are just caching all of that info for everyone here.


Spago is failing with some errors about "too many open files"

This might happen because the limit of "open files per process" is too low in your OS - as spago will try to fetch all dependencies in parallel, and this requires lots of file operations.

You can limit the number of concurrent operations with the -j flag, e.g.:

$ spago -j 10 install

To get a ballpark value for the j flag you can take the result of the ulimit -n command (which gives you the current limit), and divide it by four.

Package set caching problems

If you encounter any issues with the hashes for the package-set (e.g. the hash is not deemed correct by spago), then you can have the hashes recomputed by running the freeze command:

$ spago freeze

However, this is a pretty rare situation and in principle it should not happen, and when it happens it might not be secure to run the above command.

To understand all the implications of this I'd invite you to read about the safety guarantees that Dhall offers.

I added a new package to the packages.dhall, but spago is not installing it. Why?

Adding a package to the package-set just includes it in the set of possible packages you can depend on. However, if you wish spago to install it you should then add it to the dependencies list in your spago.dhall.


npm i spago

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