[Template-haskell] pprDec bug

[Simon Peyton-Jones]

| I'm using GHCi version 6.2.1 under Cygwin.  I'm trying to implement a
| compiler that translates a DSL into Haskell; I'd like to build the
| structure incrementally and then use the pretty printer to output the
| target code.  However, the pretty printer does not always produce
| valid output; sometimes needed parentheses are lacking, as in:
| 
|     Main> pprDec $ DataD [] "M" [] [NormalC "M" [(NotStrict, AppT
(ConT Maybe) (ConT "Int"))]] []
|     data M = M Maybe Int

Good point.  I've fixed this in the HEAD, and on the stable branch, so
it'll be in 6.2.2

Prelude Language.Haskell.TH> ppr $ DataD [] (mkName "M") [] [NormalC
(mkName "M") [(NotStrict, AppT (ConT ''Maybe) (ConT ''Int))]] []
data M = M (Data.Maybe.Maybe GHC.Base.Int)

| Related question: I've defined a utility function
| 
| > pdq decq = runQ decq >>= mapM_ print . map pprDec
| 
| so I can pretty-print declarations in code brackets, viz.:
| 
|     Main> pdq [d| data M = M (Maybe Int) |]
| 
| Is there an easy way to transform the declaration to use the
| short forms of type names, e.g. "Int" instead of "GHC.Base:Int"?

That's harder, because it's not clear what qualifications to omit.  All?
But then the program might be ambiguous.  I guess this is so you can
feed it back into GHC, right?  You can always import GHC.Base,
and then you'll be fine.

Still, one could imagine a flag to pprDec (or, more generally, to ppr)
that, say, printed all top-level names unqualified.  But that'd require
a bit of re-plumbing in the pretty-printer to get the flag to the right
place.  

Simon