April 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/ -------

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Type inference
by Oleg Mürk 25 Apr '06

25 Apr '06

Hello, I have the following problem. I'd like to infer types of local bindings using 'reify': ------------------------------------ testValue = let a = 10 in $(do VarI _ t _ _ <- reify 'a lift (pprint t) ) ------------------------------------- But this code prints only: ------------------------------------- *Debug> testValue "t_0" ------------------------------------- Is this intended behavior? Are there any workarounds? Things work fine if symbol is defined in another module. I do realise that in general $(...) may add additional constraints on local variables. OM

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Question about local bindings
by Andy Gill 18 Apr '06

18 Apr '06

Fellow Templaters, Why does test2 () = runQ [| foldr f z xs |] (f,z,xs) = undefined compile fine, but test2 () = runQ [| foldr f z xs |] where (f,z,xs) = undefined fail with this message: ./Language/Haskell/ER/Equation.hs:108:25: Can't splice the polymorphic local variable `f' In the first argument of `foldr', namely `f' In the first argument of `runQ', namely `[| foldr f z xs |]' In the definition of `test2': test2 () = runQ [| foldr f z xs |] where (f, z, xs) = undefined Andy Gill

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RE: [Template-haskell] quasi quotes and Q monad
by Simon Peyton-Jones 14 Apr '06

14 Apr '06

I've read your message, but I can't figure out what problem you are trying to solve. Can you give a small example that demonstrates it? Simon | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On | Behalf Of Ch. A. Herrmann | Sent: 02 January 2006 16:17 | To: template-haskell(a)haskell.org | Subject: [Template-haskell] quasi quotes and Q monad | | Dear TH experts, | | I have a problem concerning the interaction of quasi quotes and the | quotation monad. Assume a code generating function f (... -> Q Exp) | which is parameterized by a code generating function g (of type | Exp -> Q Exp, or(?) Q Exp -> Q Exp). | | Expressing the problem in the simplest form, the actual instance for g | is (\x -> [| h($x) |]), where h is a toplevel Haskell function working | on arbitrary types, and function f instantiates x | with an expression which consists just of a single variable (VarE). In | order to splice x in the code ($x), the type of x must be (Q Exp). The | reason for that, as mentioned in the 2002 paper | by Sheard and Peyton Jones "Template Metaprogramming for Haskell", is | that the computation of x must be able to access the Q monad. The place | inside f where the actual name for the variable x is generated, has | already access to the Q monad and the *result* of g is | embedded in this monad, no problem. However, I cannot figure out how | this monad can be passed as an *argument* to g and conceptually, there | is no justification to pass this monad: it is just an offspring version | of the one where the lexical scope of g belongs to. | | The value I want to pass for x is of type Exp. Of course, I could turn | this type into (Q Exp) by applying return, but this artificial instance | of the Q monad would come from nowhere, not being connected with the | regular instance used, e.g., for the fresh name generation. | | Especially, I have the following questions: | * Is there a simple solution to this problem? If so, please tell me and | forget about the following questions. | * Is the quasi quote mechanism at all appropriate for what I want to do | or should one better change to the concrete AST representation? That | | would be unfortunate because my aim is to develop Template Haskell | examples which demonstrate ease of use. | * If return is used to turn an expression into monadic form before | splicing, is it possible that | (a) the consistency of fresh name generation is lost, even if one | does the name generation for the spliced expression oneself, | (b) something else goes wrong? | | Many thanks in advance and a Happy New Year | -- | Christoph Herrmann | | | | _______________________________________________ | template-haskell mailing list | template-haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/template-haskell

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RE: [Template-haskell] instance Show Info ?
by Simon Peyton-Jones 10 Apr '06

10 Apr '06

No very good reason. You can always use 'ppr'. Neverthless, since the rest of Exp is in Show, I'll add Info too. Simon | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On | Behalf Of Dimitry Golubovsky | Sent: 05 April 2006 10:32 | To: template-haskell(a)haskell.org | Subject: [Template-haskell] instance Show Info ? | | Why there isn't one? | | At least in "latest" (6.4.x), only instance Ppr Info shows up. | | It might be useful to be able to print raw results of reification. | | -- | Dimitry Golubovsky | | Anywhere on the Web

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RE: [Template-haskell] Renaming in pretty printer
by Simon Peyton-Jones 10 Apr '06

10 Apr '06

I've no idea. It certainly seems that Language.Haskell.TH.PprLib implements a pretty-printing Doc type that does renaming as it goes. I guess that he wanted to have shorter variable names. It's done using a state monad, which looks fragile to me: it appears to rely utterly on printing TH syntax trees with NameUs that do not involve shadowing; which is probably the case, but is not part of the specification. I'm not sure who wrote it: Don Stewart perhaps. Simon | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On | Behalf Of Andy Gill | Sent: 28 March 2006 18:58 | To: template-haskell(a)haskell.org | Subject: [Template-haskell] Renaming in pretty printer | | Template Haskell appears to be doing renaming during pretty printing. | Take this example, that both prints the AST directly, as well as | pretty printing. | | *Main> runQ ([| let x = 1 in x |]) >>= \x -> print (pprint x,x) | ("let x_0 = 1\n in x_0",LetE [ValD (VarP x_7) (NormalB (LitE | (IntegerL 1))) []] (VarE x_7)) | | The var x has name x_7 in the AST, but x_0 in the pretty printed | version. | | Can someone comment on why this is done? | | Andy Gill

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instance Show Info ?
by Dimitry Golubovsky 05 Apr '06

05 Apr '06

Why there isn't one? At least in "latest" (6.4.x), only instance Ppr Info shows up. It might be useful to be able to print raw results of reification. -- Dimitry Golubovsky Anywhere on the Web

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