#5568: Add Show and Binary instances for TypeRep
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Reporter: simonmar | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.4.1
Component: libraries/base | Version: 7.2.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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(was a re-opening of #3480; copied into a new ticket so we can leave #3480
dead and buried)
Unfortunately the new !TypeRepKey type is abstract and provides no
operations apart from Eq and Ord instances. Hence, there is still no way
to write a type representation into a file or across the wire. Jush
showing a !TypeRep won't cut it, since that loses the distinctions between
similarly named types in different modules.
So, I think we need a Show instance for !TypeRepKey, or better yet, a
Binary instance. Moreover, a non-deprecated pure function `TypeRep ->
TypeRepKey` is in place, since the keys have an actual use as a compact,
efficient, and serializable injection of the typereps.
Alternatively, the Show instance of !TypeRep could be made injective by
printing out the package and module.
=== Commentary from #3480 ===
simonmar says:
I've exposed the representation via the Data.Typeable.Internal module.
Of course that's not guaranteed to be a stable API, but I'm happy to add a
stable API if we can agree on what it should be. Lennart Augustsson and
Neil Mitchell are doing exactly this (serialising !TypeRep) - I think
they're using the Data.Typeable.Internal API right now.
So what shall we do?
Lennart says:
For older versions of ghc I just used (show . typeOf). When ghc 7.2
broke this I had to start using Internal, so now I use tyConModule and
tyConName to emulate the behaviour of the old show.
lealanko says:
I'm sorry for messing up the ticket. I had assumed that the missing Show
instance had been a simple oversight. If so, this would suffice:
{{{
instance Show TypeRepKey where
show (TypeRepKey (Fingerprint hi lo)) = printf "%016x%016x" hi lo
}}}
A Binary instance is more difficult: where would it be put? It cannot be
in Typeable.hs (as base cannot depend on binary), but it cannot be in
Binary.hs either (as !TypeRepKey is opaque and not exposed from Typeable).
I don't think it's a good idea to make TypeReps themselves publicly
deserializable, as that would allow the creation of fake typereps. A
separate one-way injection to a serializable !TypeRepKey seems much more
secure.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5568>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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