November 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: [Template-haskell] TH problem
by Simon Peyton-Jones 06 Dec '05

06 Dec '05

There is a bug in GHC: mkName ":" doesn't work right. let pcons x xs = return $ InfixP x (mkName ":") xs The reason is that ":" is built-in syntax, and I wasn't handing that right. You can get around it by replacing the call to mkName by a quoted name, thus: let pcons x xs = return $ InfixP x '(:) xs but I will fix the bug. Simon | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On | Behalf Of Ch. A. Herrmann | Sent: 29 November 2005 16:55 | To: template-haskell(a)haskell.org | Subject: [Template-haskell] TH problem | | Dear TH gurus, | | I'm desperately trying to convert the old TH examples to work with ghc-6.4.1 | and got stuck with a problem I cannot explain. Although the splice seems | to be correct, the compiler complains about the (:) in the gerated | pattern match. | I appended the source files. They are not very nice right now, because all | guidance I have are the types generated by haddock and all effort I spent | was to make them fit somehow. | Please tell me where the error is and how to proceed or, even | better, please send me the corrected source files back. | | Many thanks in advance. | | Christoph | ------------------------------------------------------------------------ ---------------- | Loading package base-1.0 ... linking ... done. | Compiling Zip ( ./Zip.hs, interpreted ) | Compiling Sel ( ./Sel.hs, interpreted ) | Compiling Main ( Main.hs, interpreted ) | Loading package haskell98-1.0 ... linking ... done. | Loading package haskell-src-1.0 ... linking ... done. | Loading package template-haskell-1.0 ... linking ... done. | Main.hs:16:18: | Main.hs:16:18-23: Splicing expression | zipN 3 | ======> | let | zp[a4fV] = \ y1 y2 y3 | -> case (y1, y2, y3) of | (x1 : xs1, x2 : xs2, x3 : xs3) | -> ((x1, x2, x3) `GHC.Base.:` (zp[a4fV] | xs1 xs2 xs3)) | (_, _, _) -> GHC.Base.[] | in zp[a4fV] | In the second argument of `($)', namely `$[splice](zipN 3) as bs cs' | In a 'do' expression: print $ ($[splice](zipN 3) as bs cs) | In the definition of `main': | main = do | let as = ['a' .. 'e'] | bs = map toUpper as | cs = map ord as | print $ ($[splice](zipN 3) as bs cs) | let ds = [1 .. ] | print $ ($[splice](zipN 4) ds as bs cs) | print $ ($[splice](sel 1 3) (2, 'a', 6)) | | Main.hs:16:18: Not in scope: data constructor `:' | | Main.hs:16:18: Not in scope: data constructor `:' | | Main.hs:16:18: Not in scope: data constructor `:' | Failed, modules loaded: Sel, Zip. | -- | Christoph | | | |

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TH problem
by Ch. A. Herrmann 29 Nov '05

29 Nov '05

Dear TH gurus, I'm desperately trying to convert the old TH examples to work with ghc-6.4.1 and got stuck with a problem I cannot explain. Although the splice seems to be correct, the compiler complains about the (:) in the gerated pattern match. I appended the source files. They are not very nice right now, because all guidance I have are the types generated by haddock and all effort I spent was to make them fit somehow. Please tell me where the error is and how to proceed or, even better, please send me the corrected source files back. Many thanks in advance. Christoph ---------------------------------------------------------------------------------------- Loading package base-1.0 ... linking ... done. Compiling Zip ( ./Zip.hs, interpreted ) Compiling Sel ( ./Sel.hs, interpreted ) Compiling Main ( Main.hs, interpreted ) Loading package haskell98-1.0 ... linking ... done. Loading package haskell-src-1.0 ... linking ... done. Loading package template-haskell-1.0 ... linking ... done. Main.hs:16:18: Main.hs:16:18-23: Splicing expression zipN 3 ======> let zp[a4fV] = \ y1 y2 y3 -> case (y1, y2, y3) of (x1 : xs1, x2 : xs2, x3 : xs3) -> ((x1, x2, x3) `GHC.Base.:` (zp[a4fV] xs1 xs2 xs3)) (_, _, _) -> GHC.Base.[] in zp[a4fV] In the second argument of `($)', namely `$[splice](zipN 3) as bs cs' In a 'do' expression: print $ ($[splice](zipN 3) as bs cs) In the definition of `main': main = do let as = ['a' .. 'e'] bs = map toUpper as cs = map ord as print $ ($[splice](zipN 3) as bs cs) let ds = [1 .. ] print $ ($[splice](zipN 4) ds as bs cs) print $ ($[splice](sel 1 3) (2, 'a', 6)) Main.hs:16:18: Not in scope: data constructor `:' Main.hs:16:18: Not in scope: data constructor `:' Main.hs:16:18: Not in scope: data constructor `:' Failed, modules loaded: Sel, Zip. -- Christoph

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RE: [Template-haskell] Re: Documentation Template Haskell
by Simon Peyton-Jones 16 Nov '05

16 Nov '05

| But I thought I would (a) Offer some praise - TH is unused partially | because GHC is so great (b) Explain that part of the reason I don't use | TH is that there is no good documentation, and that's only because more | people don't use TH, which may be only because there is no good | documentation . . . Thanks; good points. As ever, I'm hoping that someone will volunteer to write some documentation. Indeed I think Bulat might be... Simon

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$f syntax in declaration position don't work (in contrast to $(f) )
by Bulat Ziganshin 16 Nov '05

16 Nov '05

Hello template-haskell, $f syntax in declaration position don't work. for example, the following code fails to compile: module Main where import Sub(f) $f if we change last line to the $(f) it works as expected. is it a bug or undocumented feature? ;) -- Best regards, Bulat mailto:bulatz@HotPOP.com

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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: ghc-6.4.1: panic - Prelude.last: empty list
by Simon Peyton-Jones 14 Nov '05

14 Nov '05

Good point. I've fixed this, in the HEAD at least (not sure whether it'll make it into the stable branch). Now the conversion from TH syntax to GHC's syntax is done eagerly, so that no lurking exceptional values can subsequently crash GHC. It also means that any errors found during this conversion process can be reported more cleanly. Simon | -----Original Message----- | From: glasgow-haskell-bugs-bounces(a)haskell.org [mailto:glasgow-haskell-bugs- | bounces(a)haskell.org] On Behalf Of Bulat Ziganshin | Sent: 30 October 2005 20:27 | To: Einar Karttunen | Cc: glasgow-haskell-users(a)haskell.org; glasgow-haskell-bugs(a)haskell.org | Subject: Re: ghc-6.4.1: panic - Prelude.last: empty list | | Hello Einar, | | Sunday, October 30, 2005, 4:20:20 PM, you wrote: | | EK> I am having problems with GHC 6.4.1 dying with the message: | | EK> ghc-6.4.1: panic! (the `impossible' happened, GHC version 6.4.1): | EK> Prelude.last: empty list | | EK> The source is nontrivial and contains template haskell. The error | EK> seems to appear after byte code generation. | | are you will be glad to see trivial source of this problem? :) | | | {-# OPTIONS_GHC -fglasgow-exts -fth #-} | module Main where | main = return () | $( [d| |] >>= return.tail) | | | C:\!\Haskell\!!!!>ghc --make -ddump-splices test-derive.hs | Chasing modules from: test-derive.hs | Compiling Main ( test-derive.hs, test-derive.o ) | Loading package base-1.0 ... linking ... done. | Loading package haskell98-1.0 ... linking ... done. | Loading package template-haskell-1.0 ... linking ... done. | ghc.EXE: panic! (the `impossible' happened, GHC version 6.4.1): | Prelude.tail: empty list | | Please report it as a compiler bug to glasgow-haskell-bugs(a)haskell.org, | or http://sourceforge.net/projects/ghc/. | | | | as you see, TH sometimes just don't caught exceptions in your TH code. | moreover, with -ddump-splices TH can print code it generated and only | then panic on error in generation routine! | | -- | Best regards, | Bulat mailto:bulatz@HotPOP.com | | | | _______________________________________________ | Glasgow-haskell-bugs mailing list | Glasgow-haskell-bugs(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs

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