December 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/ -------

3 2

TH tutorial
by softpro＠gmx.net 20 Dec '05

20 Dec '05

hi, i've recently written a tutorial on template haskell located at (http://www.haskell.org/hawiki/TemplateHaskellTutorial) as i have very little _real_ experience with template haskell i would very much appreciate any feedback or corrections in the wiki. thanks a lot, Johannes Ahlmann

1 0

RE: [Template-haskell] How to get list of instances of a given class?
by Simon Peyton-Jones 16 Dec '05

16 Dec '05

| BTW is "Dec" for "Declaration"? All I would need is the fact that a | particular instance was declared, nothing else. that's right

2 1

RE: [Template-haskell] How to get list of instances of a given class?
by Simon Peyton-Jones 16 Dec '05

16 Dec '05

| Is it possible to publish a list of what is possible to | access/introspect, and what is not possible? In 6.2.2? In 6.4.1? It's hard for me to do that. Except to say: look at the Info type http://www.haskell.org/ghc/docs/latest/html/libraries/template-haskell/L anguage-Haskell-TH-Syntax.html#t%3AInfo which tells you what reify returns. But I'm entirely willing to answer qns. | Just for the future: from the original Template Haskell paper [2], I | got the impression that the ultimate goal was to be able to access the | whole tree of objects internally known to the compiler at the moment | when the TH constructs are involved, wasn't it? However I understand | that the priority is low. The priority is low because few people ask, and because few people volunteer. It's a shame because it's a bit chicken-and-egg-ish. If TH was more complete, more people might use it. Many of the things that need doing do not require new design work -- they just involve filling out the interface -- and hence could be done by someone other than me! A particularly easy thing to do is to fill out the Wiki Simon

3 2

RE: [Template-haskell] How to get list of instances of a given class?
by Simon Peyton-Jones 16 Dec '05

16 Dec '05

There's currently no way to get the list of instances of a class. It would not be very difficult to add this. Any volunteers? Or I could do it if there is serious demand. How much difference would it make? I've added a note to the Hawiki page about TH, and pointed to it from GHC's wiki. A really desirable thing would be for someone to write Haddock documentation for the Template Haskell API. Would someone be willing to do that? Simon | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On | Behalf Of Dimitry Golubovsky | Sent: 13 December 2005 14:38 | To: template-haskell(a)haskell.org | Subject: [Template-haskell] How to get list of instances of a given class? | | Bulat kindly pointed me to the SerTH example. I also have been trying | to browse the GHC documentation trying to find the answer to my | quesitons. | | I see, for example, what can be gotten for a class (provided that the | name of the class is known): | | data Dec | ... | ClassD Cxt String [String] [Dec] -- or ClassD Cxt Name [Name] | [FunDep] [Dec] in | --6.4.x | InstanceD Cxt Type [Dec] | ... | | The documentation (neither for 6.2.2 nor for the latest) says nothing | about what is in [Dec] for a class. Are instances of the class known | at the moment of slplicing contained there? | | I may be asking too common questions. I just need to see whether my | task is accomplishable via TH. If it is not, then I'll do this via | some plugins to HSFFIG, but the latter is not much desirable. | | Please advise. | | Thanks. | | -- | Dimitry Golubovsky | | Anywhere on the Web

2 1

How to get list of instances of a given class?
by Dimitry Golubovsky 13 Dec '05

13 Dec '05

Bulat kindly pointed me to the SerTH example. I also have been trying to browse the GHC documentation trying to find the answer to my quesitons. I see, for example, what can be gotten for a class (provided that the name of the class is known): data Dec ... ClassD Cxt String [String] [Dec] -- or ClassD Cxt Name [Name] [FunDep] [Dec] in --6.4.x InstanceD Cxt Type [Dec] ... The documentation (neither for 6.2.2 nor for the latest) says nothing about what is in [Dec] for a class. Are instances of the class known at the moment of slplicing contained there? I may be asking too common questions. I just need to see whether my task is accomplishable via TH. If it is not, then I'll do this via some plugins to HSFFIG, but the latter is not much desirable. Please advise. Thanks. -- Dimitry Golubovsky Anywhere on the Web

2 1

Need specific examples of TH use
by Dimitry Golubovsky 12 Dec '05

12 Dec '05

Could someone please point me to working examples of TH usage to manipulate with class instances and data structures? A typical example of what I want to try: collect specific subset of instances of some class and construct some code iterating over each instance in the subset. To clarify: I want to try to add some TH capabilities to HSFFIG: subset of instances represents an imported C structure; a code to generate may be e. g. peeker/poker for a structure, or a serializer, or anything else. I am hoping, that's what metaprogramming is for ;) Thanks. -- Dimitry Golubovsky Anywhere on the Web

1 0

RE: [Template-haskell] Accessing module imports in TH
by Simon Peyton-Jones 12 Dec '05

12 Dec '05

Currently no. Asking whether "x" or "M.x" is in scope would be quite easy to implement, although it doesn't exist at the moment. Hmm. Actually I think you could do qRecover (qreify (mkName "M.x")) to get the effect, couldn't you? the reify will fail if the name isn't in scope, but you can catch the failure with the recover. Asking what is *exported* would be harder. Ghc doesn't process the export list until after processing all top-level splices, because the things to be exported might not be in scope until then. S | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On | Behalf Of Robin Bate Boerop | Sent: 11 December 2005 18:10 | To: template-haskell(a)haskell.org | Subject: [Template-haskell] Accessing module imports in TH | | Is there a way to determine within Template Haskell code at compile | time whether the module being compiled imports particular functions? | Also, is there a way to obtain a list of function names exported by | the module being compiled? | | -- | Robin Bate Boerop | http://robin.bateboerop.name | | _______________________________________________ | template-haskell mailing list | template-haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/template-haskell

1 0

Accessing module imports in TH
by Robin Bate Boerop 11 Dec '05

11 Dec '05

Is there a way to determine within Template Haskell code at compile time whether the module being compiled imports particular functions? Also, is there a way to obtain a list of function names exported by the module being compiled? -- Robin Bate Boerop http://robin.bateboerop.name

1 0

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 | | | |

3 4