'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 (I'm using cabal to build).

This is the directory structure of a simple module:

FooMod1
|- FooMod1.cabal
|- Setup.hs
|- src
  |- FooMod1.hs
  |- FooMod1
    |- C1.hs
    |- T1.hs

The source for FooMod1.hs:

module FooMod1 (
    C1(..) ,
    T1(..) ,
) where 

import FooMod1.C1
import FooMod1.T1

The source for C1.hs:

module FooMod1.C1 (
    C1(..)
) where

class C1 a where
    c1FooFun :: a -> IO ()

The source for T1.hs:

module FooMod1.T1 (
    T1(..)
) where

import FooMod1.C1

data T1 = T1 deriving(Show)

instance C1 T1 where
    c1FooFun T1 = putStrLn "c1FooFun from T1"

The source for the cabal file:

Name:                      FooMod1
Version:                   0.0.1
Cabal-version:             >=1.10
Build-type:                Simple

library 
  build-depends:           base >= 4 && < 5
  if impl(ghc >= 7.0.0)
     default-language:     Haskell2010
  ghc-options:             -Wall
  exposed-modules:         FooMod1

  ghc-options:             -Wall -rtsopts
  hs-source-dirs:          src, src/FooMod1
  default-language:        Haskell2010

and the Setup.hs:

module Main where

import Distribution.Simple

main = defaultMain

I can do

cabal configure
cabal build
cabal install

without any problem. When I start ghci and

import FooMod1

it loads the module and I can see the data constructors. But when I try to get the type of a function for example

:t c1FooFun

or construct a value I get:

Failed to load interface for `FooMod1.C1'
There are files missing in the `FooMod1-0.0.1' package,
try running 'ghc-pkg check'.
Use -v to see a list of the files searched for.
In the expression: c1FooFun

'ghc-pkg check' reveals nothing.

What am I missing? I looked it up in the Haskell 2010 Standard (http://www.haskell.org/onlinereport/haskell2010/haskellch5.html) and i can't find the error. So my questions are

1) Why am I getting this error?

2) Is structuring a hierarchical modules like that good practice? (assume considerably larger programs)

Many thanks in advance!

Jules

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.