Changes to Data.Typeable

Simon Marlow marlowsd at
Thu Jul 14 16:56:10 CEST 2011

On 11/07/2011 15:25, Gábor Lehel wrote:
> 2011/7/11 Simon Marlow<marlowsd at>:
>> On 08/07/2011 17:36, Gábor Lehel wrote:
>>> 2011/7/7 Simon Marlow<marlowsd at>:
>> Yes, it's implementation-defined but non-varying.  I know some people have
>> objected to these things being outside the IO monad before, but there is
>> already plenty of precedent (System.Info.os, size of Int, isIEEE...).
>> However, if we take it out of IO then it may limit the possible
>> implementations.  Would the previous implementation, in which keys were
>> assigned at runtime, still be valid?  It is still implementation-defined and
>> non-varying, but only over a single run.
> That's the question. It's in IO now because, while the keys don't vary
> over a single run, they do vary between them. Presumably the new
> version should be 'pure' if and only if that's no longer true. The
> upsides (of not being in IO) are obvious, but unfortunately I don't
> know much at all about the potential downsides in terms of limiting
> implementations.

After talking with Simon Peyton Jones about this, I decided to deprecate 
typeRepKey and add Ord instances to TypeRep and TyCon.  Given that 
typeRepKey isn't returning an Int any more, it isn't adding anything 
over a direct Ord instance on TypeRep (we didn't do this before because 
the Ord instance would vary from run to run).

So you can now make a Map with TypeRep as the key, however to do this 
efficiently you probably want to use a hash map, and make TypeRep 
Hashable (easy, just take a chunk of the fingerprint).


>>> Albeit, the use case I had in mind was using Template Haskell to
>>> construct a case statement over the literal Int values of the keys as
>>> determined at compile time (hopefully compiling down to something like
>>> a C switch statement), and I'm not sure if that's going to work if the
>>> keys are no longer Ints. (That it wouldn't compile down to a switch
>>> statement is one thing, but I'm not sure if the code would literally
>>> be possible to write. Maybe it'd need a Lift instance?) Anyway, I
>>> don't think it would hurt to take it out of IO if given the
>>> opportunity, either way.
>> The keys are 128-bit hashes, so it might still be possible to do something
>> like this, but you would need access to the internal representations.  I'm
>> planning to expose these via Data.Typeable.Internal (no guarantees about
>> stability of this API, however).
> I was going to suggest that a Lift instance could be provided in
> Language.Haskell.TH.Syntax, but I see now that there's quite a few
> types which could have an instance and don't, so that probably belongs
> in a separate proposal. Just having the internals available will
> hopefully be 'good enough' for the use case I mentioned (which itself
> is not that important, just a nice optimization).
>> Cheers,
>>         Simon

More information about the Libraries mailing list