@hdgarrood's Thoughts on PureScript package management in 2019.
Below are my thoughts on why we need both. I'm not sure whether this is entirely correct.
Think about what happens when a PureScript release is made that includes breaking changes.
Updating each library in the ecosystem to account for those breaking changes is similar to putting a plant inside a vase with colored water. The colored water will first enter its roots, then go up its branches, and finally appear in every leaf (Kids' experiment explained with photos)
In our above analogy, the
purescript-prelude library and other libraries with no dependencies are the "roots" of the ecosystem. As they get updated, the libraries that depend on them (i.e. the "branches") can now be updated. A "leaf" corresponds to a library which has no dependents.
A package set is immutable. Thus, one cannot add to the package set a package that has been updated to the new release unless all of the packages in the package set can also be updated.
During that transitional time,
spago cannot help. Rather, we must depend on
Bower to slowly update each library to its new version that depends on transitive libraries that have been updated to new versions.
spago is more suited for application developers and
bower is more suited for library developers.