Haskell / GHCi - loading modules from different directories

Question

My haskell application has the following directory structure:

src/     utils/Utils.hs     subsystem/Subsystem.hs 

The Subsystem module imports Utils module. I would like to hand test this code in GHCi.

The problem is GHCi seems to be only looking for modules available in '.' (current directory), so I copied Utils.hs to subsystem folder and was able to hand-test Subsytem.hs. Is there a better way to do this? For example I would like to start GHCi in the src directory and let it search for modules in ./utils and ./subsystem directories. Can I specify a module path to GHCi?

Answer 1

You can tell GHCi where to search for modules by using the -i option:

ghci Foo.Bar -isrc 

This will load src/Foo/Bar.hs into GHCi. This way, you can also specify two different directories like this:

ghci Bar.hs -i.:config  

It will look for the dependencies in ./ and ./config/ .

See the GHC user's guide for more information about the module search path.

Answer 2

By default, when GHC looks for modules, it interprets Foo.Bar as Foo/Bar.hs. So if you have a single project, you could have a module Utils as Utils.hs in the top-level directory, and a module Utils.Fishcakes as Utils/Fishcakes.hs. Note that Utils.hs can exist alongside a directory named Utils, or both can exist independently. A common style tends to be using the top-level module to simply re-export things from modules below it in the hierarchy, but this isn't required. The GHC User Guide covers the above behavior, as well as describing what other options are supported.

As far as I know, in most cases code will either use the above default structure, will use some other structure specified as part of a cabal build, or will expect to be installed as a library.