[Template-haskell] Re: TH2 bug

[Keean Schupke]

Cool, that means I can derive instances of TTypeable (type level type rep)
for primitive types...

    Keean.

Simon Peyton-Jones wrote:

>Did I mention that I've committed this change.
>
>  | PrimTyConI 	-- Ones that can't be expressed with a data type 
>		-- decl, such as (->), Int#
>	Name 
>	Int	-- Arity
>	Bool	-- False => lifted type; True => unlifted
>
>
>| -----Original Message-----
>| From: template-haskell-bounces at haskell.org
>[mailto:template-haskell-bounces at haskell.org] On
>| Behalf Of Keean Schupke
>| Sent: 27 October 2004 15:12
>| To: Keean Schupke
>| Cc: template-haskell at haskell.org; Simon Peyton-Jones
>| Subject: Re: [Template-haskell] Re: TH2 bug
>| 
>| Just checked and 'Type' has an entry (ArrowT) for functions.
>| Why not just use the existing Type. IE
>| 
>| reify ''(->)
>| 
>|     data Info =
>|        PrimTyCon Type
>|        TyConI Dec
>| 
>| This would be fine for tuples as well, I wonder if () could be
>| encoded as a tuple of 0?
>| 
>|     Keean.
>| 
>| Keean Schupke wrote:
>| 
>| > Actually I would like it to be a special name... but I guess there
>| > are other possibilities.
>| >
>| > Ideally I would like:
>| >
>| > case info of
>| >    (PrimTyCon name) -> instanceD (cxt []) (appT (conT classname)
>(appT
>| > (appT (conT name) (varT a) (varT b)))
>| >
>| > to generate:
>| >
>| >    instance ClassName (a -> b)
>| >
>| >
>| >    Keean
>| >
>| > Simon Peyton-Jones wrote:
>| >
>| >> 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 at 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 at imperial.ac.uk]
>| >> | >| Sent: 22 October 2004 18:23
>| >> | >| To: glasgow-haskell-users at 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 at haskell.org,
>| >> | >| or http://sourceforge.net/projects/ghc/.
>| >> | >|
>| >> | >|
>| >> | >|     Keean.
>| >> | >
>| >> | >
>| >>
>| >>
>| >>
>| >
>| > _______________________________________________
>| > template-haskell mailing list
>| > template-haskell at haskell.org
>| > http://www.haskell.org/mailman/listinfo/template-haskell
>| 
>| 
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