* On Tuesday, March 23 2010, Gwern Branwen wrote: ...
I brought this technique up in #haskell today, where aavogt pointed out that the issue about not being able to access config parameters inside the TH splice could be worked around as long as the TH splice returned a partially applied function which wanted the necessary parameters and would use them appropriately inside itself - that is (very loosely), instead of $(check fookeymap), it'd be more $(check (\x y -> fookeymap x y)). The TH would only inspect the keys of the tuples, and not the functions inside the second space in the tuple.
That idea didn't work out using regular TH splices (those taking a Q Exp), because there is no Lift instance for functions, but maybe one can be written. The duplicates found using the QuasiQuoter are only those that have syntactic equality: getDupes compares all the Exp that are the first elements of each pair that makes a keybinding. For EZConfig-style keys where the key is indicated by a string, it is less of a problem, but note that it can still fail with the following:
main = xmonad $ defaultConfig `additionalKeysP` M.toList -- could be avoided by removing the M.fromList in fromUniqueList [$fromUniqueList| ("M1-a",spawn "aaaaaa"), ("M-a", spawn "bbbbbb") |]
The conversion done in additionalKeysP could be built into the quasiquoter to avoid that problem, though that code cannot be reused without a bit of rearrangement (specifically the XConfig arguments can be replaced with a default modmask). The `fromUniqueList' that I've written so far could be a bit smarter with respect to showing the duplicates and other kinds of invalid input (which are pattern match failures for now).
fromUniqueList = QuasiQuoter { quoteExp = either fail (\(input::Exp) -> do ListE input' <- return input duplicates <- liftM getDupes $ forM input' $ \x -> do TupE [key,_] <- return x return key
unless (null duplicates) $ fail ("Keys overlap:" ++ show duplicates) [| M.fromList $(return input) |] ) . parseExp . ('[':) . (++"]") , quotePat = error "KM.fromUniqueList: quotePat" }
getDupes :: Eq a => [a] -> [a] getDupes xs = xs \\ nub xs ---snip---
So, with a basic solution working, we ought to consider whether to use it. (I assume there's some way to hide the TH splice inside xmonad-core so we don't require any user-visible changes.) This is additional static checking, and it removes one unfortunate feature of list syntax, so it seems good to me.
A quasi-quote cannot be hidden like that: this little bit of extra checking takes the String that's inside the quote, which isn't accessible to functions in the xmonad library. But the syntax is pretty light anyhow. Possibly a concern for us is that QuasiQuote syntax may be improved sometime: http://www.haskell.org/pipermail/glasgow-haskell-users/2010-February/018335.... Another drawback is the dependency on the haskell-src-meta library, whose maintainer has not been reachable. A version that works with ghc-6.12 can found at one of: darcs get http://moonpatio.com/repos/haskell-src-meta_NEW_TH/ # 'official' darcs get http://code.haskell.org/~aavogt/haskell-src-meta/ # a couple small 'improvements' Maybe my analysis is overly negative here, but we can do better than this alternative or the current setup. -- Adam