August 2006 - template-haskell

automatic instances for pretty printing and parsing
by Johannes Waldmann 09 Sep '06

09 Sep '06

Dear all, I use Text.PrettyPrint.HughesPJ and Text.ParserCombinators.Parsec heavily so I made some DrIFT rules for deriving the "obvious" instances for class ToDoc a where toDoc :: a -> Doc class Reader a where readerPrec :: Int -> Parser a http://141.57.11.163/cgi-bin/cvsweb/drift/src/UserRuleToDoc.hs?rev=1.5 Still I'd like to avoid external preprocessors (for hygienic reasons, and because they do not seem to fit well with the Cabal model) so I'd like to investigate whether I could get the same effect with Template Haskell (in ghc) - and what this would cost. Perhaps someone has solved a similar problem before. On the other hand, HaXml does need DrIFT it seems. -- -- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 -- ---- http://www.imn.htwk-leipzig.de/~waldmann/ -------

3 2

Small TH demo: generating instances
by Rene de Visser 01 Sep '06

01 Sep '06

I am hoping that this example is small, simple and well commented enough to help people generate instances with TH. The instance generation is used to create instances to render haskell code to TH code that generates the original haskell code. there is an example in Test.hs The instance generation function is in Base.hs. TH_Render.hs is where the instances are derived. 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2 2

Fix pprinting global names
by Einar Karttunen 31 Aug '06

31 Aug '06

Hello The recent commit added package names to pprinting global names. The attached patch reverts that - which is nice for actually using the pprinted representation and agrees with the comments in the file. e.g. base:GHC.Show.Show -> GHC.Show.Show Patch attached. - Einar Karttunen

1 0

Tool to preprocess template haskell definitions?
by Einar Karttunen 30 Aug '06

30 Aug '06

Hello I remember someone having written a small utility to preprocess a Haskell source file with TH definitions and generate a new file with the TH bits replaced by their resulting slices. If anyone knows the utility or a way to do this pointers would be nice. I am trying to use Haddock to create documentation, but datatypes are created with TH... - Einar Karttunen

2 1

GADTs
by Ian Lynagh 14 Aug '06

14 Aug '06

Hi all, Currently we do not have GADT support in Template Haskell, but I need it for something I'm working on. The relevant datastructures currently look like this: data Dec = ... | DataD Cxt Name [Name] [Con] [Name] | ... data Con = NormalC Name [StrictType] | RecC Name [VarStrictType] | InfixC StrictType Name StrictType | ForallC [Name] Cxt Con I propose altering Dec thus: data Dec = ... | DataD Cxt Name [Name] Cons [Name] | ... (changing [Con] to Cons) and adding new datastructures: data Cons = NormalCons [Con] | GadtCons [GadtCon] deriving( Show, Eq ) data GadtCon = GadtC [Name] Cxt Name [StrictType] Type deriving( Show, Eq ) (this follows the style used to add pattern guards to the datastructures). Complete patches for GHC and the template-haskell library are at http://urchin.earth.li/~ian/th-gadts/ If you have any objections or comments then please let me know; otherwise I think it would be great to get this into GHC 6.6. Thanks Ian

5 13

Internal type consistency of quoted expressions
by Liyang HU 01 Aug '06

01 Aug '06

(oops. Sorry about the double-post. Sent it with a wrong From: line and it doesn't seem to have come through...) Hello all, Long story, but one thing after another led me to resort to TH as a means to (re-)implement Conor McBride's (applicative) idiom brackets[0], where: [[ g x0 x1 ... xn ]] expands to pure g <*> x0 <*> x1 <*> ... <*> xn (pure and <*> come from Control.Applicative.) While I can't use [[ and ]], I can define a CPP macro: #define II(e) $( idiom [| e |] ) where the idiom function does the above rewriting at compile time. II( ... ) looks acceptable as a replacement for [[ ... ]]. Good. (Just as well, since the type-level implementation of the iI Ii brackets given in [1] will no longer work as of GHC 6.5, but is used fairly extensively in the Epigram 2 source code...) Even though (g x0 x1 ... xn) won't typecheck outside of a quotation, I can nevertheless get away with [| g x0 x1 ... xn |] because it is /internally/ consistent. (Template Meta-programming for Haskell, §7.1 paragraph 3; figure 2, rule BRACKET.) Yesterday, Conor pointed out that the if and case constructs are pure 'things', and since my implementation of the above idiom function seemed capable of this sort of syntax mangling, why not treat them as one more case in the applicative/monadic idiom bracket. (I've always wanted monadic if and case expressions. Hurrah!) The idea being that: II( if c then t else e ) and II( case d of { ... } ) would expand to c >>= \ b -> if b then t else e and d >>= \ x -> case x of { ... } respectively. I'd already written some of this up, and just got around to implementing the changes earlier tonight, but I'm stumped by the internal type consistency restriction: c has type (Monad m => m Bool), which means [| if c then t else e |] isn't 'internally consistent'. With the case-expression, d has type (Monad m => m a); (x :: a); and the elided patterns in { ... } match values of type a. So [| case d of { ... } |] won't get past the supposed 'sanity check'[2] either. :( My current want-it-now 'solution' is to comment out the sole tcSimplifyBracket invocation in ghc/compiler/typecheck/TcSplice.lhs. Clearly this is the wrong approach, seeing as other people would like to use this too. (I presume this will do what I want. I'm still waiting for stage1 to finish building...) I guess a more constructive approach would be to hack in an alternative unchecked quotation bracket, [s| ... |] say ('s' stands for 'syntax'). If upstream is willing to consider such an extension, I'll get started... Any alternative suggestions before I do? :-/ Thanks for listening, /Liyang [0] http://cs.nott.ac.uk/~ctm/IdiomLite.pdf [1] http://cs.nott.ac.uk/~ctm/Idiom.pdf [2] I am sane, right? I mean, is there any other reason for the internal type-consistency check other than to detect /potential/ errors /earlier/?

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