Spago Explained

What is it?

A way to use specific versions of libraries that are known to compile together without problems, as verified by CI.

Why Use It?

spago only allows you to use dependencies that compile together on a specific PureScript release. You do not have to track down which version of a DependencyA to use to ensure it compiles when you also use DependencyB. Moreover, you don't have to verify that DependencyA at v1.0.0 works on PureScript release 0.13.8 instead of 0.11.7.

When a new PureScript release with breaking changes occurs, using bower is painful until the ecosystem "catches up." Since a new release draws in a lot of people, their initial exploration of PureScript when using bower can be horrible.

spago also allows you to

  • 'patch' a dependency with your own version
    • fix a bug in its implementation
    • update a library to a newer PS release if it hasn't been done yet
    • update a library's transitive dependency to a newer release without needing to submit a PR
  • add local or cloud-based dependencies not found in the official package set
    • a project you use frequently, like a custom Prelude library.
    • a project with your preferred aliases to functions/values (i.e. using <!> for map instead of <$>)

How does it work?

Spago Terms

A package in this context is 4 things:

  1. a Git repo
  2. a tag in that repo
  3. a set of its dependencies (which are also packages).
  4. a name to refer to the combination of the above three things

Thus, a package is a unique named repo-tag-dependencies combination (e.g. prelude could indicate the Prelude repo at the 'v4.1.1' tag).

A package set consists of a set of packages. It's a JSON-like file that maps a package name to its corresponding repo-tag-dependencies combination. A package set gets verified to ensure that its set of packages compiles together on a given PureScript compiler release. Once verified, they are considered "immutable."

A package set includes all dependencies: direct ones and their transitive dependencies. For example, if the set includes the package, PackA, which depends on the package, PackB, the package set must include both packages:

  • PackA
    • Version: v1.0.0
    • Repo:
    • Dependencies: ["PackB"] (spago will look up "PackB" in the package set to resolve it)
  • PackB
    • Version: v1.0.0
    • Repo:
    • Dependencies: [] (no dependencies)

The Process It Uses

Here's a "big picture" flowchart for what a person does and how it fits into their developer workflow:


Problem Points?

  • Major
    • You cannot use this workflow to develop libraries. Use pulp and bower for that.
  • Minor / has workarounds
    • Just like bower, you still need to use npm to install JavaScript libraries for any PureScript bindings. To understand why, see