October 2005 - 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: Documentation Template Haskell
by Simon Peyton-Jones 16 Nov '05

16 Nov '05

Christoph, and other TH users, | > I have been waiting for a long while for more documentation | > about the use of Template Haskell as it is currently implemented, | > now ghc-6.4.1. The documentation on the web refers to several old You're absolutely right. Template Haskell is appallingly badly documented. The reason I have not done anything about it is that there's very little traffic on the Template Haskell list (or any other, about TH), and that has de-motivated me so far as improving TH is concerned. I tend to pay more attention to things that I think people are actively using and care about. TH seems to have had quite a bit less impact than I originally thought it might, given that it provides something qualitatively new. But maybe I'm wrong. For example, I didn't know that it was important to you. Maybe there are others. In any case, as you may guess I find it difficult to find enough time to do all the GHC related things that GHC's users would like. That's good, because it means GHC is being useful, but bad because some things don't get done. My current hope is to broaden the base of people who are willing to contribute to GHC themselves, with support from me and Simon, so that Simon and I aren't always on the critical path. So here's an offer. If you'd be willing to draft the documentation you would like to see, I would be willing to - review it, - answer questions, and - write parts where you don't know what to say. But you already know a lot, so you are well placed to write an outline. Perhaps TH folk would be willing to help too? RSVP to the TH list (which I'm ccing). I know this won't be quite the reply you were hoping for, but I hope you'll consider it. Simon | -----Original Message----- | From: Christoph Herrmann [mailto:herrmann@uni-passau.de] | Sent: 22 October 2005 18:42 | To: Simon Peyton-Jones | Subject: Re: Documentation Template Haskell | | Dear Simon, | | > Anything more than a list of library function types would do it. | | It would be good if it contains some explanations. Recently | I have found the following example on the mailing list but I can | only guess what, e.g., "tupP" means. | | sel' :: Int -> Int -> ExpQ | sel' i n = lamE [pat] rhs | where pat = tupP (map varP as) | rhs = varE (as !! (i - 1)) | as = map mkName [ ("a__" ++ show j) | j <- [1..n] ] | | Looking up Google for "tupP TH" leads me to some pages about TH | where I can either find some old example TH programs without "tupP" | or a list of library functions where I can only get the type of "tupP". | What I need would be a few English words telling what tupP actually does. | | Also, I'm wondering about the TH function "tuple" below whether it is | an old version, a plan for the future or even supposed to work with | ghc-6.4.1 but I forgot some import declarations. | | tuple :: Int -> Q Exp | tuple n = [|\list -> $(tupE (exprs [|list|])) |] | where | exprs list = [infixE (Just (list::[Char])) | (varE "!!") | (Just (litE $ integerL (toInteger num))) | | num <- [0..(n - 1)]] | | | Christoph Herrmann wrote: | > Dear Simon, | > | > I have been waiting for a long while for more documentation | > about the use of Template Haskell as it is currently implemented, | > now ghc-6.4.1. The documentation on the web refers to several old | > versions of TH and most of the examples don't work any more, | > e.g., the generation of a function which selects a component | > from a tuple. I don't know which of the new features you proposed | > are currently implemented. | > | > I need Template Haskell both for my work on metaprogramming | > and for teaching. Could you please point me to information | > about the current state? It doesn't matter for me how well it is | > written, as far as the definitions are up-to-date. Also, some examples | > which deal with code construction and reification and which work with | > ghc-6.4.1 would be useful for me. Again, I do not care about how they | > look like. Anything more than a list of library function types would do it. | > | > Many thanks in advance | > -- | > Christoph Herrmann | > University of Passau, Germany | > | >

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RE: Re[2]: [Template-haskell] RE: Documentation Template Haskell
by Simon Peyton-Jones 26 Oct '05

26 Oct '05

| well, i will rewrite doc&tutorial in the next few days, then yell here | again to get critique great | | SPJ> | one more suggestion - i think that we must include in GHC | SPJ> | distribution useful TH application libraries: printf, class derivation | SPJ> | (a la DrIFT) | | what you think about that? It'd be great if someone wrote documented libraries. The best way to distribute them would be as a Cabal package, I think Simon

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RE: [Template-haskell] RE: Documentation Template Haskell
by Simon Peyton-Jones 26 Oct '05

26 Oct '05

Good start. I'm not sure whether it would have enough detail to meet Christoph's need, but let's ask him. Maybe he can fill in some of the blanks. Simon | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On | Behalf Of Bulat Ziganshin | Sent: 25 October 2005 15:25 | To: Simon Peyton-Jones | Cc: Christoph Herrmann; template-haskell(a)haskell.org | Subject: Re: [Template-haskell] RE: Documentation Template Haskell | | Hello Simon, | | Tuesday, October 25, 2005, 1:47:38 PM, you wrote: | | SPJ> So here's an offer. If you'd be willing to draft the documentation you | | "TH can be used to automatically generate any Haskell code at the compile | time. This code will be compiled just the same as if it was | hand-written. This allows to create metaprograms - programs, which | itself generate Haskell programs, allowing to use programming | techniques not available in Haskell itself, such as macro | preprocessing, self-optimized algorithms, generic programming, | functions with variable parameters count and so on | | TH code is written as ordinal Haskell functions, which generate AST | (abstract syntax tree) representing Haskell code. This AST represented | by type Exp. Moreover, these functions must function in special monad | Q, which provides abilities to generate unique identifiers for | code builded and to make queries about environment in which this function | is running. | | Type Exp allows to represent any Haskell code: | | data Exp = LitE Lit -- representing literals | | VarE Name -- representing variables | ..... | | Type Lit, used in Exp, represents Haskell literals: | ..... | -- description for other types | | | To make TH programming easier, the above-mentioned constructors are | duplicated by functions which accepts arguments lifted to Q monad and | return results in the same monad. For example, AppE constructor dubbed | by appE function. These functions are defined using types which is | also lifted to Q monad, such as ExpQ = Q Exp and so on. | | To create unique identifiers for generated code you must use the | following functions: .... | | You can also query about environment which runs TH function with the | following functions: .... | | | Also, to simplify building of ASTs, Template Haskell can automatically | translate ordinary Haskell code to this representation by using | [|...|]. This code has type ExpQ, so it is a computation which will | return Exp, representing code inside brackets, on execution in monad Q. | Code inside brackets can be any Haskell code, local identifiers will | be translated to code generating unique identifiers. | | If you need to create expression of type Pat, use form [p| ... |], | type Dec - use [d| ... |]. | | Code, generated by TH functions, can be spilled to hand-written code | by using splice notation: $function or $(function parameters...). You | can use splices in any place where Haskell expression or declaration | can be written. Of course, the spliced expression must return value of | appropriate type - ExpQ or DecQ, respectively. You also can use spills | in definitions of TH functions, inside abovementioned brackets, making | some form of macrogeneration. In other cases, you must import TH | functions in order to spill it, because TH-generating code must be | already compiled in time where current module code isn't even | syntax-processed" | | >END-OF-DOC | | | | this doc doesn't include 'name and ''type, whose explanations i can't | recall. i also wrote TH tutorial, but don't finish it. You can see the | last edition as http://freearc.narod.ru/th.htm | | one more suggestion - i think that we must include in GHC | distribution useful TH application libraries: printf, class derivation | (a la DrIFT) | | | | -- | Best regards, | Bulat mailto:bulatz@HotPOP.com | | | | _______________________________________________ | template-haskell mailing list | template-haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/template-haskell

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RE: [Template-haskell] mkName problem
by Simon Peyton-Jones 26 Oct '05

26 Oct '05

You are quite right. I've fixed the HEAD, and the fix should appear in 6.4.2 Thanks for the report Simon ________________________________ From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On Behalf Of Tomasz Zielonka Sent: 07 October 2005 14:12 To: template-haskell(a)haskell.org Subject: [Template-haskell] mkName problem Hello! The attached code shows a behaviour which I don't understand. This could be a bug in TH or a misunderstanding on my side. The problem is that dynamically bound Names made with mkName sometimes "don't see" the variable. See the code for details. It could be a bit shorter, but I wanted to show why I cat't use the 'var syntax. Try to --make T, then uncomment the marked line in T.hs. Best regards Tomasz

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RE: Documentation Template Haskell
by Simon Peyton-Jones 25 Oct '05

25 Oct '05

| It would be good if it contains some explanations. Recently | I have found the following example on the mailing list but I can | only guess what, e.g., "tupP" means. | | sel' :: Int -> Int -> ExpQ | sel' i n = lamE [pat] rhs | where pat = tupP (map varP as) | rhs = varE (as !! (i - 1)) | as = map mkName [ ("a__" ++ show j) | j <- [1..n] ] tupP has type [Q Pat] -> Q Pat http://www.haskell.org/ghc/docs/latest/html/libraries/template-haskell/L anguage-Haskell-TH-Lib.html#v%3AtupP It's a "smart constructor" for patterns, just like the other "smart constructors" described in the original TH paper (section 6.3) http://research.microsoft.com/%7Esimonpj/Papers/meta-haskell/index.htm [In the paper, the type Expr is now ExpQ, or equivalently Q Exp.] Does that help? Simon

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mkName problem
by Tomasz Zielonka 07 Oct '05

07 Oct '05

Hello! The attached code shows a behaviour which I don't understand. This could be a bug in TH or a misunderstanding on my side. The problem is that dynamically bound Names made with mkName sometimes "don't see" the variable. See the code for details. It could be a bit shorter, but I wanted to show why I cat't use the 'var syntax. Try to --make T, then uncomment the marked line in T.hs . Best regards Tomasz

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