July 2003 - template-haskell

reifyType function
by Andre Pang 07 Jul '03

07 Jul '03

Hi all, Sean Seefried and I managed to implement a reifyType function, which we're calling rType since there's already that annoying built-in reifyType identifier ;). It's defined in a ReifyType module which looks like thus: --8<-- module ReifyType where import Data.Dynamic import Language.Haskell.THSyntax class ReifyType a where rType :: a -> Typ instance (ReifyType a, ReifyType b) => ReifyType (a, b) where rType (_ :: (t, u)) = Tapp (Tapp (Tcon (Tuple 2)) (rType (undefined :: t))) (rType (undefined :: u)) instance ReifyType a => ReifyType [a] where rType (_ :: [t]) = Tapp (Tcon List) (rType (undefined :: t)) instance Typeable a => ReifyType a where rType (_ :: t) = (Tcon (TconName typeAsString)) where typeAsString = show $ typeOf (undefined :: t) instance (ReifyType a, ReifyType b) => ReifyType (a->b) where rType (_ :: t -> u) = (Tapp (rType (undefined :: t)) (rType (undefined :: u))) --8<-- An example of how to use it: 18:48 ~/Desktop % ghci -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances ReifyType.hs Type.hs) ... Loading package base ... linking ... done. Compiling ReifyType ( ReifyType.hs, interpreted ) Ok, modules loaded: ReifyType. *ReifyType> rType Char.toLower Loading package haskell98 ... linking ... done. Loading package haskell-src ... linking ... done. Tapp (Tcon (TconName "Char")) (Tcon (TconName "Char")) *ReifyType> rType ((+) :: (Int -> Int -> Int)) Tapp (Tcon (TconName "Int")) (Tapp (Tcon (TconName "Int")) (Tcon (TconName "Int"))) *ReifyType> rType "foobar" Tapp (Tcon List) (Tcon (TconName "Char")) *ReifyType> The major restriction is that you're limited to using it on monomorphic types (since the typeOf function only works on monomorphic types), but otherwise, it seems to work remarkably well. -- % Andre Pang : just.your.average.bounty.hunter

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RE: Template Haskell, [p|...|]
by Simon Peyton-Jones 05 Jul '03

05 Jul '03

There's a separate Template Haskell mailing list, which it'd be worth ccing on this (are you on it)? Ian Lynagh has a home page for Template Haskell, at http://www.haskell.org/th/ [IAN: it'd be worth noting the current unimplemented features of TH on the home page, perhaps. And is the TH page linked from Haskell.org? Couldn't find the link, if it exists.] | trying to adapt the zipN-example in the paper | "Template Meta-programming for Haskell" by | Sheard and Peyton Jones to Language.Haskell.THSyntax | I found that quasi-quotation for patterns does not seem | to work with ghci-6.0, e.g., the following results in | "Parse error in pattern": | | pcons x xs = [p| $x : $xs |] Yes -- pattern splices are not implemented at all. The user manual should say this, but doesn't. There are good reasons for this, as you'll see by looking at the TH mailing list archive. | Trying to use the standard list representation by | | pcons x xs = Pcon "(:)" [x,xs] | | I got the error message | | Data constructor not in scope: `(:)' | tcLookupDataCon: `(:)' is not in scope You need to say "GHC.Base::" to name precisely the (:) defined by GHC. This is clearly Bad and Horrible, and is on the list for a radical clean up. Tim Sheard is working on a design document describing what we have in mind. Tim? Simon

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RE: [Template-haskell] Re: Template Haskell - read function
by Simon Peyton-Jones 03 Jul '03

03 Jul '03

What about passing the type as the driving argument: f :: [String] -> (Int, Bool) f = $(mkReader [t| (Int,Bool) |]) mkReader :: Type -> ExpQ mkReader ty = ... | -----Original Message----- | From: template-haskell-admin(a)haskell.org [mailto:template-haskell-admin@haskell.org] On Behalf Of | Thomas L. Bevan | Sent: 02 July 2003 16:01 | To: template-haskell(a)haskell.org | Cc: haskell(a)haskell.org | Subject: [Template-haskell] Re: Template Haskell - read function | | -----BEGIN PGP SIGNED MESSAGE----- | Hash: SHA1 | | I have got a little further by defining the following functions and I now have a better defined question. | | - --Takes a list and converts it to a n-ary tuple | - --i.e. Generates functions like [String] -> (a,a,a) | listToTuple i = lam [Pvar "xs"] (tup ([ infixE (Just $ var "xs") (var "!!") (Just $ lit (Integer n)) | n <- | [0..(i-1)] ])) | | - --Casts an n-ary tuple of type (String,String,..) | - --i.e. Generates functions like (a,a,a) -> (Int,Bool,Int) | cast i = lam [Ptup pats] (tup [ app (var "read") v | v <- vars]) | where | pats = [ Pvar $ show n | n <- [1..i] ] | vars = [ var $ show n | n <- [1..i] ] | | So, I can now define a function f, | | f :: [String] -> (Int, Bool) | f = $(cast 2) . $(listToTuple 2) | | but I can still see no way to infer the size of the tuple purely from the declared type signature. | | Tom | | | | On Wed, 2 Jul 2003 11:56 am, Thomas L. Bevan wrote: | > Hi, | > | > I'm struggling to write the following function in Template Haskell. | > | > I have a phantom type, | > | > >data HTMLPage a = HTMLPage FilePath | > | > This type points to an HTML file, where 'a' refers to the return type of | > the posted form. | > So, mostly, I would expect types like | > HTMLPage (String, String, Date, Int) | > | > Where this represented a form the returned someone's first name, last name, | > dob and age. | > | > So, I want a function that will take the returned name/value pairs and cast | > them into the matching type. | > | > eg. | > | > >readForm :: Read a => [(String,String)] -> HTMLPage a -> a | > | > Can anyone give me any pointers or am I better off just abandoning this | > approach? | > | > Tom | > | > _______________________________________________ | > Haskell mailing list | > Haskell(a)haskell.org | > http://www.haskell.org/mailman/listinfo/haskell | -----BEGIN PGP SIGNATURE----- | Version: GnuPG v1.2.2 (GNU/Linux) | | iD8DBQE/AvOlYha8TWXIQwoRAju7AKCxXxFQ1gMLTIUt93UpXyIQRVdqMQCg0Ryo | FQK/bFrjQaV9fOyi6naeYEo= | =fuyq | -----END PGP SIGNATURE----- | | _______________________________________________ | template-haskell mailing list | template-haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/template-haskell

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Template Haskell - read function
by Thomas L. Bevan 03 Jul '03

03 Jul '03

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi, I'm struggling to write the following function in Template Haskell. I have a phantom type, >data HTMLPage a = HTMLPage FilePath This type points to an HTML file, where 'a' refers to the return type of the posted form. So, mostly, I would expect types like HTMLPage (String, String, Date, Int) Where this represented a form the returned someone's first name, last name, dob and age. So, I want a function that will take the returned name/value pairs and cast them into the matching type. eg. >readForm :: Read a => [(String,String)] -> HTMLPage a -> a Can anyone give me any pointers or am I better off just abandoning this approach? Tom -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.2 (GNU/Linux) iD8DBQE/AsiKYha8TWXIQwoRAvzuAKDPHD4oztAPLFShEWsLuclT7BaImwCfSB8e 9c2aTbTCSpVwabXVsht62DM= =orX3 -----END PGP SIGNATURE-----

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