November 2004 - template-haskell

Problem with TH
by Keean Schupke 18 Nov '04

18 Nov '04

I am using the new existential datatype support (great!) - but now when I patch in fundep support (more or less as I posted before) adding an extra field to ClassD, I have to modify GHC to use the extra field in ClassD... Now when I include a definition: [d| class Test x |] I get the error: ghc-6.3: panic! (the `impossible' happened, GHC version 6.3): deSugar/DsMeta.hs:925:4-41: Irrefutable pattern failed for pattern [elt_ty] Any idea why this might be happening (why is the error in DsMeta, when the TH stuff is in hsSyn/Convert.lhs and typecheck/TcSplice.lhs? Keean.

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Re: [Template-haskell] Re: template haskell (reify bug and existentials)
by Keean Schupke 08 Nov '04

08 Nov '04

Ian Lynagh wrote: >On Wed, Nov 03, 2004 at 10:17:04AM +0000, Keean Schupke wrote: > > >>Ian Lynagh wrote: >> >> >> >>>>in Syntax.hs: >>>> ... >>>> type Fundep = ([Name],[Name]) -- add to export list >>>> >>>> Have you made a decision regarding FunDeps yet? Keean.

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Re: [Template-haskell] RE: TH2 bug
by MR K P SCHUPKE 03 Nov '04

03 Nov '04

>(i.e. DataD with all the stuff that doesn't occur removed)? Yes, this sounds good to me... effectively we just want the left hand side of the 'data' statement defining them. As they are primitves the right hand side is unknowable (ie they behave as abstract types) Keean.

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RE: TH2 bug
by Simon Peyton-Jones 03 Nov '04

03 Nov '04

OK, now I get it. I'm copying the TH list in the hope that other TH aficionados will chime in. The problem is with reify, not the '' notation. Here it is in a nutshell: what value should be returned by the call (reify ''(->) )? Recall that reify :: Name -> Q Info So reify claims to return an Info, and for a type constructor it currently returns a TyConI; but a TyConI contains a Dec, and there simply is no Dec in the TH syntax for a function type; it's built in, rather like Int# etc. So reify falls over in a bad way. It's obviously unsatisfactory for reify to just fall over in this case, but what would you like? A new constructor in Info for primitive tycons? A new constructor in Dec? And what questions do you want to be able to ask of the value in the Info? For example would it be enough to day data Info = TyConI Dec | PrimTyCon | ClassI Dec ...etc... where the PrimTyCon is returned for (->), Int#, etc? Nothing difficult about fixing this once the design is clear. Simon | -----Original Message----- | From: Keean Schupke [mailto:k.schupke@imperial.ac.uk] | Sent: 27 October 2004 11:14 | To: Simon Peyton-Jones | Subject: Re: TH2 bug | | Yup, using HEAD. Here's an example template: | | ttypeshow :: Name -> Q [Dec] | ttypeshow q = do | r <- reify q | case r of | (TyConI t) -> runIO . putStrLn . show $ t | _ -> return () | return [] | | Then in another file we do: | | $(ttypeshow ''(->)) | | Which fails, as does ''(), and ''(,) etc... | | ''[] works okay however | | Keean. | | | Simon Peyton-Jones wrote: | | >I can't reproduce this. Works for me. What version of GHC are you | >using? The HEAD I assume? | > | >Simon | > | >| -----Original Message----- | >| From: MR K P SCHUPKE [mailto:k.schupke@imperial.ac.uk] | >| Sent: 22 October 2004 18:23 | >| To: glasgow-haskell-users(a)haskell.org; Simon Peyton-Jones | >| Subject: TH2 bug | >| | >| | >| Just reporting a bug, taking types using '' works fine for: | >| | >| ''Int | >| ''[] | >| | >| But when you try: | >| | >| ''(->) | >| | >| you get: | >| | >| No match in record selector TyCon.algTcRhs | >| | >| Please report it as a compiler bug to | >glasgow-haskell-bugs(a)haskell.org, | >| or http://sourceforge.net/projects/ghc/. | >| | >| | >| Keean. | > | >

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template haskell (reify bug and existentials)
by Keean Schupke 03 Nov '04

03 Nov '04

Re: my two earlier postings: 1) existentials in data declarations. I have a project which needs this, Simon PJ thought it was fairly easy to put in and suggested that Ian Lynagh might implement it. Do I need to persuade people further? Will it get done now? Some feedback as to whether it might get done would be nice... Just to clarify I want to do: $(something [d| data A = forall a . A a |] 2) reification of functions (and other primitives)... want to do something like: reify ''(->) Fails because reify/Info does not support reifying primitives. There is already support in the Type type for primitive types (-> (,,) []) so the obvious way to add this is to alter Info to: data Info = ClassI Dec | ClassOpI Name Type Name Fixity | TyConI Dec | PrimTyConI Type | DataConI ... So what are the chances of getting these changes implemented. I need them for a current project, and would appreciate it if someone who knows the inner workings of template-haskell could implement the changes in the CVS HEAD branch of GHC. Failing that I am prepared to have a go myself, but I want to make sure that nobody else is likely to do it first.

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RE: [Template-haskell] Dyamic scoping question
by Simon Peyton-Jones 01 Nov '04

01 Nov '04

Here's your program boiled down a bit foo = 3 baz = $(varE ("fo" ++ "o")) Like any Haskell compiler, GHC does dependency analysis, and type-checks in that order. There no way for it to see that 'baz' depends on 'foo', so it may well typecheck the definition of baz before that of foo. I haven't actually run your example, but I bet that's what's happening. "-ddump-rn" would tell you, because it shows the code after dependency analysis. I wonder if others have tripped over this. There is a tension between dynamic scope and dependency analysis (which is required for type checking) that's hard to resolve. For example, you probably want the example to work even if the definition of 'foo' is after that of 'baz'; at least that's the standard Haskell story. I suppose a workaround is to put 'foo' in another module, but it's not nice. Simon | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On | Behalf Of Bryn Keller | Sent: 25 October 2004 19:48 | To: template-haskell(a)haskell.org | Subject: [Template-haskell] Dyamic scoping question | | Hi folks, | | I'm trying to use TH to generate parsers and writers for arbitrary | datatypes. In the end I'd like every part of the declaration and | generation of these functions to be automatic, but at this early stage | I'm just trying to get one aspect of all that to work. So at the moment, | all I want is to generate a function that exercises a parsing | computation I've already defined earlier in the module: | | import Text.ParserCombinators.Parsec | | import Splices | | data SomeType = S Int | | sometype_p = do {string "S"; num <- many1 digit; return $ S (read num)} | | readSomeType = $(reifyDecl SomeType >>= mkLineReader) | | | and in Splices.hs: | | module Splices where | | import Language.Haskell.THSyntax | import Text.ParserCombinators.Parsec (parse) | import Data.Char (toLower) | | modifyName f name = concat [modules, ":", f origName] | where | (modules, (_:origName)) = break (==':') name | | lower = map toLower | | mkLineReader :: Dec -> Q Exp | mkLineReader (DataD _ name _ fields _) = do | let parserName = varE $ modifyName ((++"_p").lower) name | [| | \line -> case parse $(parserName) line line of | Left err -> error (show err) | Right val -> val | |] | | Now, when I try to compile this I get a message like so: | | tcLookup: `sometype_p' is not in scope | In the first argument of `parse', namely `sometype_p' | In the scrutinee of a case expression: | parse sometype_p line'0 line'0 | In the case expression: | case parse sometype_p line'0 line'0 of | Left err'1 -> error (show err'1) | Right val'2 -> val'2 | | When I add -ddump-splices, it prints this in addition: | | \ line'0 | -> case | Text.ParserCombinators.Parsec.Prim.parse Main.sometype_p | line'0 | line'0 | of | Data.Either.Left err'1 -> GHC.Err.error (GHC.Show.show err'1) | Data.Either.Right val'2 -> val'2 | | which tells me that the name sometype_p is getting built correctly, but | it's just not finding the sometype_p that I've already defined. I | thought that varE was the way to capture existing variables of this sort | in Template Haskell, was I mistaken? What is the approved way? Or have I | just missed something crucial? | | Thanks, | | Bryn | _______________________________________________ | template-haskell mailing list | template-haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/template-haskell

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