- template-haskell - Haskell.org

FW: TH page
by Simon Peyton-Jones 06 Jan '06

06 Jan '06

Anyone willing to volunteer? Not a big task TH Hawiki page: http://www.haskell.org/hawiki/TemplateHaskell TH page to merge into it: http://www.haskell.org/th Simon -----Original Message----- From: Ian Lynagh [mailto:igloo@earth.li] Sent: 06 January 2006 02:22 To: Simon Peyton-Jones Subject: Re: TH page Hi Simon, On Fri, Dec 16, 2005 at 02:44:52PM -0000, Simon Peyton-Jones wrote: > > I rather think that haskell.org/th could be merged into the Hawiki page > for TH. What do you think? I'm happy for someone to merge it. Thanks Ian

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quasi quotes and Q monad
by Ch. A. Herrmann 05 Jan '06

05 Jan '06

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

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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

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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

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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

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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

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How to get list of instances of a given class?
by Dimitry Golubovsky 14 Dec '05

14 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

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Need specific examples of TH use
by Dimitry Golubovsky 13 Dec '05

13 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

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RE: [Template-haskell] Accessing module imports in TH
by Simon Peyton-Jones 13 Dec '05

13 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

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Accessing module imports in TH
by Robin Bate Boerop 12 Dec '05

12 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

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