In my experience using Guix and attempting to make contributions, I've
noticed that the vast majority of times when a library breaks, it's
because one of its dependencies changed version. For instance,
referencing something like `rust-my-lib-1`, where "1" refers to the
semver "1.x" of the package, e.g., "1.0.32", and `rust-foo` depends on
`rust-my-lib == 1.0.32`. However, in some other package got updated to
"1.0.34" so `rust-foo` will break. I've seen this happen a lot with
Haskell and Rust libraries.

Many libraries in different languages don't follow semver, which can
lead to cases like `rust-serde-json`, which, between versions "1.0.97"
and "1.0.98," changed its dependency from `indexmap` "1.x" to "2.x,"
causing several packages like rust-analyzer to break. I've also observed
this in Haskell with packages like "text."

This is problematic because:

    - Over time, it becomes more vulnerable to libraries/packages
      breaking.

    - It makes reproducible software more challenging, as "1.x" can
      encompass many versions.

    - Debugging becomes difficult since that package could be a deep
      dependency in the system package dependency chain, such as
      Rust/Haskell/NPM, etc.

    - It makes it more likely that if a dependency changes, many
      packages will need to be updated/rebuilt due to that change.

For these reasons, I believe that pinned versions should be a
requirement in libraries, always specifying the exact dependency, for
example, `rust-serde-json-1.0.98`.

This brings the following benefits:

    - Fewer packages will be prone to rebuilding when changing the
      definition of a library.

    - Reduced likelihood of libraries/packages breaking.

    - Easier maintenance of packages and libraries without fear of
      breaking others or having to update many.

There could be some potential disadvantages:

    - The list of library versions may grow larger, making it harder to
      detect orphaned or unused versions.

Additionally, I believe that a command to list the dependency tree of a
package would be ideal for easier debugging.

Regards!