- template-haskell - Haskell.org

TH bug report (again)
by Sean Seefried 16 Oct '03

16 Oct '03

Hi again, ----------- d_show = [d| data A = A instance Show A where show _ = "A" |] -------------- gives ---------------------- ghc-6.2: panic! (the `impossible' happened, GHC version 6.2): Failed binder lookup: GHCziShow.show {- v rKy -} Please report it as a compiler bug to glasgow-haskell-bugs(a)haskell.org, or http://sourceforge.net/projects/ghc/. ----------------------- This might not be a bug, but if it's not can you please tell me what I am doing wrong? Sean

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TH bug report
by Sean Seefried 16 Oct '03

16 Oct '03

Dear Template Haskell users, I just wrote a short program with this declaration and got the following error message. ----- d_class = [d| class Classy a b where f :: a -> b instance Classy Int Bool where f x = if x == 0 then True else False |] ------ Class `Test.Classy' does not have a method `f' In the instance declaration for `Test.Classy Int Bool' In the definition of `d_class': d_class = [d| class Test.Classy a b where { Test.f :: a -> b {- has default method -}; } instance {Test.Classy Int Bool} where [] f x y = if x == 0 then True else False |] ------ This doesn't seem like correct behaviour to me. Sean

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Pre-Call for Contributions -- HC&A Report (November 2003 edition)
by C.Reinke 02 Oct '03

02 Oct '03

[apologies for multiple copies - experience has shown that not all Haskellers can be reached via the main Haskell list anymore] An entry for your diaries (no other action required right now): ------------------------------Pre-Call-------------------------------- Your contributions to the November 2003 edition of the Haskell Communities & Activities Report will be invited soon. http://www.haskell.org/communities/ Submissions are welcome between 20th and 31st of October ---------------------------------------------------------------------- The real Call for Contributions will follow closer to the submission window, in about two weeks. This is just an early warning. You will need to plan about 15 minutes for composing your contribution, which would be a brief, ASCII-format, description of your Haskell activities (see earlier editions for plenty of examples). Cheers, Claus (current editor of HC&A Reports)

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More useful ones
by Sean Seefried 27 Sep '03

27 Sep '03

Often when analysing expressions I want to get all the arguments for a function. The following function decomposes such an application chain. -- Should only be used on expression of form (App e1 e2) getAppChain :: Exp -> (Exp, [Exp]) getAppChain (AppE e1 e2) = let (f, args) = getAppChain e1 in (f, args ++ [e2]) getAppChain e = (e, []) -- The dual of getAppChain mkAppChain :: (Exp, [Exp]) -> Exp mkAppChain (f, args) = foldl1 AppE (f:args) putExpQLn :: ExpQ -> IO () putExpQLn expQ= do exp <- runQ expQ putStrLn (show exp) -- Two very useful functions. -- And these are just good for printing out code. putQ :: Show a => Q a -> IO () putQ xQ = do x <- runQ xQ putStr (show x) putQLn :: Show a => Q a -> IO () putQLn xQ = do putQ xQ putStrLn ""

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Useful TH library functions
by Alastair Reid 26 Sep '03

26 Sep '03

Here's some functions I found myself needing when I wrote Template Greencard which I think would be useful additions to the TH libraries. The names could probably be a bit better and, in some cases, putting a function in the Q monad (i.e., lift the args and result) might be helpful. -- Functions for building types --| mkIOTy alpha = [| IO alpha |] mkIOTy :: Typ -> Typ mkIOTy ty = ConTyp (ConNameTag "GHC.IOBase:IO") `AppTyp` ty --| mkArrows [t1,...tm] rty = [| t1 -> ... -> tm -> rty |] mkArrows :: [Typ] -> Typ -> Typ mkArrows as r = foldr (\ a as -> tapply (ConTyp ArrowTag) [a,as]) r as --| mkTupleTy [t1, ... tm] = [| (t1, ... tm) |] mkTupleTy :: [Typ] -> Typ mkTupleTy [] = htype_Void mkTupleTy [ty] = ty mkTupleTy tys = tapply (ConTyp (TupleTag (length tys))) tys --| tapply tc [t1,...tm] = [| tc t1 ... tm |] tapply :: Typ -> [Typ] -> Typ tapply f [] = f tapply f (a:as) = tapply (AppTyp f a) as -- Functions for taking types apart --| unarrow [| t1 -> ... tm -> rty |] = ([t1, ... tm], rty) unarrow :: Typ -> ([Typ],Typ) unarrow (AppTyp (AppTyp (ConTyp ArrowTag) ty1) ty2) = let (as,r) = unarrow ty2 in (ty1:as,r) unarrow ty = ([],ty) --| unIO [| IO ty |] = (True, ty) unIO :: Typ -> (Bool,Typ) unIO (AppTyp (ConTyp (ConNameTag "GHC.IOBase:IO")) ty) = (True, ty) unIO ty = (False, ty) --| untuple [| (ty1,...tm) |] = [ty1,..tm] untuple :: Typ -> [Typ] untuple ty = case split ty of (ConTyp (TupleTag n), tys) -> tys (ConTyp (ConNameTag "GHC.Base:()"),[]) -> [] _ -> [ty] -- Functions for building expressions and patterns --| simpleLet v e body = [| let v = e in body |] simpleLet :: Var -> Q Exp -> Q Exp -> Q Exp simpleLet v e body = letE [val (VarPat v) (normal e) []] body --| mkPTuple [p1,...pm] = [| (p1,...pm() |] mkPTuple :: [Pat] -> Pat mkPTuple [p] = p mkPTuple ps = TupPat ps -- Reifications of common types -- (The important thing here is that they are not in the Q monad -- so I can refer to them in guards.) htype_Ptr :: HType htype_Ptr = ConTyp (ConNameTag "GHC.Ptr:Ptr") htype_FunPtr :: HType htype_FunPtr = ConTyp (ConNameTag "GHC.Ptr:FunPtr") htype_StablePtr :: HType htype_StablePtr = ConTyp (ConNameTag "GHC.Stable:StablePtr") htype_Void :: HType htype_Void = ConTyp (ConNameTag "GHC.Base:()")

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RE: [Template-haskell] reifyLocn
by Simon Peyton-Jones 22 Sep '03

22 Sep '03

Yes, it's in the works, but don't hold your breath. The next iteration of TH won't be out for at least a couple of months, I'm afraid. I hope this isn't a show-stopper for you. Simon | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell-bounces@haskell.org] On | Behalf Of Brett Letner | Sent: 19 September 2003 23:14 | To: template-haskell(a)haskell.org | Subject: [Template-haskell] reifyLocn | | Is reifyLocn in the works? | | I'd like to use it to splice into my error messages (similar to what | assert gives you). | | Thanks - | | -- | Brett Letner | Galois Connections, Inc. | http://www.galois.com | mailto:bletner@galois.com | phone:(503)626-6616 ext.110 | | | _______________________________________________ | template-haskell mailing list | template-haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/template-haskell

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reifyLocn
by Brett Letner 19 Sep '03

19 Sep '03

Is reifyLocn in the works? I'd like to use it to splice into my error messages (similar to what assert gives you). Thanks - -- Brett Letner Galois Connections, Inc. http://www.galois.com mailto:bletner@galois.com phone:(503)626-6616 ext.110

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RE: [Template-haskell] RE: Question regarding template Haskell
by Simon Peyton-Jones 18 Sep '03

18 Sep '03

| typeOf :: a -> TypeRep | | rType :: a -> Typ (or :: a -> Type in the new THSyntax definitions). | | The two are conceptually the same, but they return you different data | types... | Come to think of it, now that we have a concrete syntax, perhaps it'd | be worth merging the definitions of THSyntax's Type and Data.Dynamic's | TypeRep? They're conceptually the same thing, but with different data | representations ... That would make perfect sense. The only tricky bit is that TypeRep is hash-cons'd so that we can guaranteed to compare TypeReps in constant time. It would probably do no harm to hash-cons Typ's too, I guess. With Ralf Laemmel I've been working on a sort of extension to the Typeable class that gives more info for algebraic data types. The module signature is below. We're working on a paper. Let's muse on this. Simon module Data.Generics.Basics ( -- * The Data class for processing constructor applications Data( gfoldl, -- :: ... -> a -> c a toConstr, -- :: a -> Constr fromConstr, -- :: Constr -> a dataTypeOf -- :: a -> DataType ), -- * Constructor representations Constr, -- abstract, instance of: Eq, Show ConIndex, -- alias for Int, start at 1 Fixity(..), -- instance of: Eq, Show DataType, -- abstract, instance of: Show -- * Constructing constructor representations mkConstr, -- :: ConIndex -> String -> Fixity -> Constr mkDataType, -- :: [Constr] -> DataType -- * Observing constructor representations conString, -- :: Constr -> String conFixity, -- :: Constr -> Fixity conIndex, -- :: Constr -> ConIndex stringCon, -- :: DataType -> String -> Maybe Constr indexCon, -- :: DataType -> ConIndex -> Constr maxConIndex, -- :: DataType -> ConIndex dataTypeCons, -- :: DataType -> [Constr]

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RE: [Template-haskell] RE: Question regarding template Haskell
by Simon Peyton-Jones 18 Sep '03

18 Sep '03

I missed that message. How does rType differ from typeOf, defined in Data.Typeable? simon | -----Original Message----- | From: ozone(a)algorithm.com.au [mailto:ozone@algorithm.com.au] | Sent: 17 September 2003 11:04 | To: Simon Peyton-Jones | Cc: template-haskell(a)haskell.org; diyu60607(a)yahoo.com | Subject: Re: [Template-haskell] RE: Question regarding template Haskell | | On 17/09/2003, at 7:45 PM, Simon Peyton-Jones wrote: | | > | Hi, I have a question regarding template Haskell: from | > | the documentation, I have the impression that it is | > | impossible to reify the type of the variable passed to | > | the template function, is this true? Or, in other | > | words, can I write a function like: | > | | > | getTupleCount :: a -> ExpQ | > | | > | getTupleCount x= | > | do | > | t <- (reifyType x) | > | --work on t | > | ... | | If your 'x' variable is monomorphic, you can use the rType function | that Sean and I came up with, detailed here: | | | http://www.haskell.org/pipermail/template-haskell/2003-July/000128.html | | That currently handles only 2-tuples, but it shouldn't be too hard to | extend it to work on any-size tuples. Let me know if you're having | problems. | | | -- | % Andre Pang : trust.in.love.to.save

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RE: Question regarding template Haskell
by Simon Peyton-Jones 17 Sep '03

17 Sep '03

[I'm redirecting this to the Template Haskell mailing list, where it more properly belongs.] Interesting question. Tim and I are working on revising the 'reifyType' story, but even in our revised version, your program will be rejected with a staging error. reifyType gives a way to consult the compiler's type environment, at the moment when the ExpQ is run. But at that moment 'x' isn't in scope any more. [Tim: this might make a good example for the design notes you're working on.] You might instead wonder if you can write getup :: ExpQ -> ExpQ which asks what the type of the expression passed to it is. This one has come up before, and I don't think we have a good answer yet. Trouble is (a) although quoted code is type-checked, there's not enough info to do a full type-check (b) it's not obvious how to record whatever the type checker does discover. Still an open question, then. Simon | -----Original Message----- | From: libraries-bounces(a)haskell.org [mailto:libraries-bounces@haskell.org] On Behalf Of Yu Di | Sent: 12 September 2003 03:33 | To: libraries(a)haskell.org | Subject: Question regarding template Haskell | | Hi, I have a question regarding template Haskell: from | the documentation, I have the impression that it is | impossible to reify the type of the variable passed to | the template function, is this true? Or, in other | words, can I write a function like: | | getTupleCount :: a -> ExpQ | | getTupleCount x= | do | t <- (reifyType x) | --work on t | ... | | which returns an expression that is an integer for any | tuple input, but reduces to error for any input that | is not a tuple? | | Thanks! | | Di, Yu | 9.11 | | __________________________________ | Do you Yahoo!? | Yahoo! SiteBuilder - Free, easy-to-use web site design software | http://sitebuilder.yahoo.com | _______________________________________________ | Libraries mailing list | Libraries(a)haskell.org | http://www.haskell.org/mailman/listinfo/libraries

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