February 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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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] Splice Imports
by Simon Peyton-Jones 06 Feb '06

06 Feb '06

| Could module import splicing be added into Template Haskell? Other than | updating the syntax and pretty printer in Language.Haskell.TH, what else | would need to be done? So you want to be able to say $(f 4) and have that expand to import X or import Y or whatever? That seems convenient, and it's not very hard to implement. Import statements have to become regular declarations, able to appear anywhere; we need TH syntax for them; and so on. It came up a while ago, and Lemmih was quite keen to do it. But I've held off doing it because it'll break ghc -M and ghc --make. It'll mean that GHC can't figure out dependencies until it's actually compiling. This is an issue even for crude Cpp-ery, but we run CPP before ghc -M and ghc --make. That's not possible for TH because you have to typecheck the module before running the splices. One could say that the programmer has to write his own dependencies in the makefile, but that still doesn't help with ghc --make. I don't see an easy path. Anyway that's why it's remained un-done. Simon | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On | Behalf Of Simon Foster | Sent: 03 February 2006 20:45 | To: template-haskell(a)haskell.org | Subject: [Template-haskell] Splice Imports | | Hi, | | Could module import splicing be added into Template Haskell? Other than | updating the syntax and pretty printer in Language.Haskell.TH, what else | would need to be done? | | Thanks, | | -Simon Foster. | | _______________________________________________ | template-haskell mailing list | template-haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/template-haskell

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Splice Imports
by Simon Foster 03 Feb '06

03 Feb '06

Hi, Could module import splicing be added into Template Haskell? Other than updating the syntax and pretty printer in Language.Haskell.TH, what else would need to be done? Thanks, -Simon Foster.

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