Haskell GHC "Could not find module" in CI

GHC tried to import a module and no package in the current build provides it. Either the package that exposes it is not listed in build-depends, or a local module is not listed in the cabal/package file.

What this error means

A build fails with "Could not find module 'X'", often adding "Perhaps you meant ..." or "It is not a module in the current program, or in any known package."

app/Main.hs:3:1: error:
    Could not find module 'Data.Text'
    Perhaps you need to add 'text' to the build-depends in your .cabal file.
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.

Common causes

The providing package is not in build-depends

GHC only sees modules from packages listed under build-depends; omitting the package means its modules are not in scope.

A local module is not listed in the cabal file

A project module exists on disk but is not under other-modules or exposed-modules, so the build does not include it.

How to fix it

Add the package to build-depends

1. Read which module GHC could not find and which package provides it.
2. Add that package to build-depends in the cabal file.
3. Rebuild so the module is in scope.

executable app
  main-is: Main.hs
  build-depends: base, text

List the local module in the cabal file

If the missing module is local, add it to other-modules (or exposed-modules for a library) so the build compiles it.

library
  exposed-modules: MyApp.Config
  other-modules:   MyApp.Internal

How to prevent it

• Keep build-depends in sync with the imports your code uses.
• List every local module under other-modules or exposed-modules.
• Build cleanly from a fresh checkout to catch unlisted modules.

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