Re: Dynamic types: GHCI works, GHC doesn't?

just as a note, the new version of DrIFT (which i now maintain) has the ability to derive 'Typeable', which is used for the Strafunski generic programming representation (which it also can derive..) the homepage is now at 'http://repetae.net/john/computer/haskell/DrIFT' I sort of fudged the Typeable derivation rule, if someone wanted to robustify it, i would gladly accept patches. I dont think that 'unsafe' need be added to Typeable since hopefully people will never be creating instances themselves, with the instances hidden behind DrIFT and/or 'deriving' clauses then they can be assured of being correct (well... as correct as anything is :)) . Typeable itself is not inherently unsafe, it is broken implementations of it which are. John On Sun, Jun 02, 2002 at 12:11:49PM +1000, Andre Pang wrote:

On Sat, Jun 01, 2002 at 07:56:39PM +0100, Alastair Reid wrote:

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Your Typeable instance looks like this:

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instance Typeable FootnoteData where typeOf _ = mkAppTy (mkTyCon "FootnoteData") [typeOf ("Foo" :: String), typeOf (7 :: Int)]

This should be written:

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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.

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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

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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?

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