On Sat, Jun 01, 2002 at 07:56:39PM +0100, Alastair Reid wrote:
> Your Typeable instance looks like this:
>
> > instance Typeable FootnoteData where
> > typeOf _ = mkAppTy (mkTyCon "FootnoteData") [typeOf ("Foo" :: String), typeOf
>(7 :: Int)]
>
> This should be written:
>
> > instance Typeable FootnoteData where
> > typeOf _ = mkAppTy fdtc [typeOf ("Foo" :: String), typeOf (7 :: Int)]
> >
> > fdtc = mkTyCon "FootnoteData"
>
> That is, the TyCon definition has to be a CAF (i.e., a top level
> definition with no arguments). The reason is somewhat grubby but
> comes down to 'things go much faster if you do this').
I see, thanks. I'll try to update the documentation with this
note.
> While we're at it, I'd like to repeat my request that the word
> 'unsafe' be made part of the classname Typeable or the method name
> typeOf. The reason is that a badly written Typeable definition can
> break typesafety. Here's a superficially plausible but broken
> instance for IO which demonstrates it
>
> > instance Typeable (IO a) where
> > typeOf _ = mkAppTy iotc []
> >
> > iotc = mkTyCon "IO"
>
> With this definition
>
> typeOf (return 1 :: IO Int) == typeOf (return () :: IO ())
>
> and so you can coerce back and forth between these types using
> from/toDynamic.
Ahem, this is what I actually did to get my Dynamic types working
in the first place (oops) :). mkAppTy was being given an empty
list of TypeReps, since I was confused about how to generate
those TypeReps.
It would be much neater if you could do something like
instance Typeable Foo where
typeOf _ = mkAppTy constructor [Char, Char, Int]
This is what I expected the Dynamic module interface would look
like, since when do you "typeOf 'a'" in GHCI, it does reply with
"Char".
I guess a possible workaround would be something like
charT = typeOf ('a' :: Char)
intT = typeOf (69 :: Int)
instance Typeable Foo where
typeOf _ = mkAppTy constructor [charT, charT, intT]
which is what I'm doing right now. Maybe these type definitions
can go into the Dynamic module itself?
--
#ozone/algorithm <[EMAIL PROTECTED]> - b.sc (comp. sci+psych)
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