GHCI can't load module if both package-conf and source path contains it

Question

I encounter a strange situation in GHCI, don't know if anyone observed similar case. For some module, when I have it in the search path both by -package-conf, and also by -i, GHCI fails when I try to import the module with 'module is not loaded: FooModule'.

• :module loads it fine however
• or I can do :load FooModule, :m to clear the import list, and then import FooModule
• or I can remove the path from -i and then it imports fine

Tracked this to be the otherwise -> modNotLoadedError m loc case in GHC, where otherwise ~ modulePackageId = this_pkg (the meaning of which I don't know).

This is not entirely systematic, there are some module which are both in package and source path, but can be imported.

Answer 1

GHC only knows about packages that are installed. To see which packages are installed, use the ghc-pkg list command:

$ ghc-pkg list
/usr/lib/ghc-6.12.1/package.conf.d:
    Cabal-1.7.4
    array-0.2.0.1
    base-3.0.3.0
    base-4.2.0.0
    bin-package-db-0.0.0.0
    binary-0.5.0.1
    bytestring-0.9.1.4
    containers-0.2.0.1
    directory-1.0.0.2
    (dph-base-0.4.0)
    (dph-par-0.4.0)
    (dph-prim-interface-0.4.0)
    (dph-prim-par-0.4.0)
    (dph-prim-seq-0.4.0)
    (dph-seq-0.4.0)
    extensible-exceptions-0.1.1.0
    ffi-1.0
    filepath-1.1.0.1
    (ghc-6.12.1)
    ghc-prim-0.1.0.0
    haskeline-0.6.2
    haskell98-1.0.1.0
    hpc-0.5.0.2
    integer-gmp-0.1.0.0
    mtl-1.1.0.2
    old-locale-1.0.0.1
    old-time-1.0.0.1
    pretty-1.0.1.0
    process-1.0.1.1
    random-1.0.0.1
    rts-1.0
    syb-0.1.0.0
    template-haskell-2.4.0.0
    terminfo-0.3.1
    time-1.1.4
    unix-2.3.1.0
    utf8-string-0.3.4