[Template-haskell] Re: TH2 bug

[Keean Schupke]

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.
>> | >
>> | >
>>
>>  
>>
>
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