Ross Paterson wrote: > On Thu, Aug 14, 2008 at 08:00:25PM +0200, Sean Leather wrote: > > Well, the warning is right that you don't need to re-export module B: > > instances are implicitly exported. So you could just remove the > export of > > "module B", unless there's a reason to export it (such as, you might > add > > some exported functions or data types to it later) > > > > Hmm, the disappointing result of removing module B from the export list > > is that now it doesn't show up in the Haddock-generated documentation > > for module A. Not that there was anything specific in B to document, > > because it's only instances. But I do want the user to know that > > importing A also imports B. Sigh... I suppose there's CPP if I really > > want it. > > That shouldn't be necessary. Because instances are implicitly exported > and imported, you'll really want to avoid orphan instances, so that the > user can be sure that if both the class and the data type are in scope, > so is the instance. But then the instance will be listed under both > the class and the type in the Haddock documentation. It won't matter > where the instance was defined, just as it doesn't matter where other > stuff is defined, just where it is in scope. >
In my case, it does matter where instances are in scope. My library requires orphan instances by design. If the programmer imports the top-level module, then s/he gets a default set of instances. But the programmer has the option to import select modules and get a different set of classes and instances. My goal was to put the default set of instances into a separate module and re-export that from a higher-level module. That way, it is more clearly documented where things are located. Unfortunately Haddock does not allow doc comments to be attached to > instances, but that's independent of whether the defining module is shown. > Agreed. I am getting the impression that it would be nice if there were a lot of things that were more explicit about instances, both in Haskell and in Haddock. Sean
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