'Failed to load interface' error when loading compiled modules in ghci

Question

Hello Haskell community,

I\'m new to Haskell and ran into a problem when I tried to structure my first bigger project.

Here\'s the minimal example of the problem

Answer 1

Edit: September 2016

Since I originally answered this question there is a growing practice of defining Foo.Internal modules that are still exposed. In the original answer below I suggested using the other-modules field. A practice that is now popular is to define Foo.Internal.* modules that are exposed but explicitly not part of the supported API. The rational for this pattern is explained in the answers to this question.

---

As noted in the comments your .cabal file is missing the other-modules line. I think cabal install then only installs FoodMod1 since that is all it's been told about.

This is a nice way to create internal modules with, for instance, types that are used throughout your cabal package which you don't want to expose in the package API. Since the other-modules modules cannot be imported from outside your package it allows you to create package private functionality.