"Could not find module" after spago install — why doesn't the new package resolve?

Answer first

With PureScript 0.15.x and a recent Spago release, spago install normally updates the manifest so the package is on the compiler search path. The "Could not find module Effect.Aff" you see after spago install purescript-aff is therefore not the normal "install downloads but manifest stays stale" drift. The most common cause in this setup is a name / manifest file mismatch.

Confirmed facts

• Spago builds from the dependencies declared in the manifest and the sources under src. If a package is not listed as a dependency, its modules are not on the search path.
• In current registry-based Spago the manifest is spago.yaml with a lock file spago.lock. Legacy dhall-based Spago used spago.dhall and packages.dhall as the package set.
• spago install in both generations updates the manifest dependencies. It does not leave the manifest unchanged by design.
• PureScript package names in manifests are short names, e.g. aff, not purescript-aff. The module you import is Effect.Aff from the package aff.

Likely explanation for your case

You mention spago.dhall while also stating PureScript 0.15.x and "recent Spago". Recent Spago for 0.15.x is registry-based and uses spago.yaml, not spago.dhall. If a spago.dhall file is present, Spago will ignore it and use spago.yaml, so an install that appears to succeed may update the wrong file.

Separately, running spago install purescript-aff uses the npm-style name. Spago expects the short name aff. An install with the wrong name can fetch nothing useful or create a dependency entry that does not resolve to the module you import.

Steps needed for this case

1. Confirm which Spago generation you are on:

   spago --version

   If the version is 0.93 or newer, the project uses spago.yaml + spago.lock. If you still have spago.dhall, you are on legacy Spago which is not the supported pairing for purs 0.15.x.
2. Check the active manifest:

   ls spago.yaml spago.dhall

   Open the file that exists and inspect the dependencies section.
3. Verify the dependency name is correct:

   grep -n "aff" spago.yaml

   You should see an entry like aff under dependencies. If you see purescript-aff, remove it and add the correct short name.
4. Reinstall with the correct short name if needed:

   spago install aff

   This updates the manifest and lock file together.
5. Build:

   spago build

   The module Effect.Aff should now resolve.

Keeping CI and local identical

Commit the manifest and lock file that your Spago generation uses: spago.yaml and spago.lock for registry Spago, or spago.dhall and packages.dhall for legacy. CI should run spago build which respects those committed files. Do not rely on ad-hoc spago install runs in CI.

One missing diagnostic that changes the recommendation: which manifest file your project actually uses and the output of spago --version. With that I can confirm whether you are on legacy dhall Spago or registry Spago and give the exact edit path.