jmtd → log → Generic Haskell

When I did the work described earlier in template haskell, I also explored generic programming in Haskell to solve a particular problem. StrIoT is a program generator: it outputs source code, which may depend upon other modules, which need to be imported via declarations at the top of the source code files.

The data structure that StrIoT manipulates contains information about what modules are loaded to resolve the names that have been used in the input code, so we can walk that structure to automatically derive an import list. The generic programming tools I used for this are from Scrap Your Boilerplate (SYB), a module written to complement a paper of the same name. In this code snippet, everything and mkQ are from SYB:

extractNames :: Data a => a -> [Name]
extractNames = everything (++) (\a -> mkQ [] f a)
     where f = (:[])

The input must be any type which implements typeclass Data, as must all its members (and their members etc.): This holds for the Template Haskell Exp types. The output is a normal list of Names. The utility function has a more specific type Name -> [Name]. This is all that's needed to walk over the heterogeneous data structures and do something specific (f) when we encounter a Name.

Post-processing the Names to get a list of modules is simple

 nub . catMaybes . map nameModule . concatMap extractNames

Unfortunately, there's a weird GHC behaviour relating to the module names for some Prelude functions that makes the above less useful in practice. For example, the Prelude function words :: String -> [String] can normally be used without an explicit import (since it's a Prelude function). However, once round-tripped through a Name, it becomes GHC.OldList.words. Attempting to import GHC.OldList fails in some contexts, because it's a hidden module or package. I've been meaning to investigate further and, if necessary, file a GHC bug about this.

For this reason I've scrapped all the above and gone with a different plan. We go back to requiring the user to specify their required import list explicitly. We then walk over the Exp data type prior to code generation and decanonicalize all the Names. I also use generic programming/SYB to do this:

unQualifyNames :: Exp -> Exp
unQualifyNames = everywhere (\a -> mkT f a)
     where f :: Name -> Name
           f n = if n == '(.)
            then mkName "."
            else (mkName . last . splitOn "." . pprint) n

I've had to special-case composition (.) since that code-point is also used as the delimiter between package, module and function. Otherwise this looks very similar to the earlier function, except using everywhere and mkT (make transformation) instead of everything and mkQ (make query).