'Failed to load interface' error when loading compiled modules in ghci
ghc, haskell, module
Solution
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.
Problem
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