Changes to Typeable

José Pedro Magalhães jpm at
Mon Oct 29 10:05:58 CET 2012

Sorry, I got a bit lost in this discussion. Let me try to provide a summary.

Current status: I have a local branch with the new poly-kinded Typeable
working fine.
It works as described in [1]. It actually allows deriving Typeable for
things involving
the Constraint kind, but this can be easily disabled. Either way, I think
most of this
is necessary for whatever might follow next. But I'm not sure of how to
push the changes,
because I had to make some changes to these repos: array, containers, dph,
template-haskell, and vector. Worse, I also had to change time, which gets
built from
a tarball. It might not be worth contacting the authors of these packages
for changes
if we're still going to get rid of "deriving Typeable" altogether, so I've
been holding this

It's been proposed to remove the possibility to derive Typeable or write
instances for it.
I'm supposing the way that this would be implemented would be:

7.8: Any uses of "deriving Typeable" would give rise to a warning saying
that it is no longer
necessary. Any instances of Typeable would give rise to a warning saying
that this code
is being ignored, and replaced by an internal Typeable instance. Packages
might break,
or change runtime behaviour due to this change.

7.10: Explicit uses of "deriving Typeable" or instances are an error.

Regarding split :: (a ~ f i) => Dict (Typeable f, Typeable i), I'm not sure
I can judge how
much work that would be. But let's first try to draft a plan for removing
Typeable definitions
from the user, and then consider more extensions.



On Tue, Oct 16, 2012 at 8:32 AM, Emil Axelsson <emax at> wrote:

> 2012-10-15 23:50, Gábor Lehel skrev:
>  On Mon, Oct 15, 2012 at 7:58 AM, Emil Axelsson <emax at> wrote:
>>> I have a use case:
>>> 3/doc/html/Data-DynamicAlt.**html<>
>>> This is a reimplementation of Data.Dynamic to support casting type `a` to
>>> `Dynamic` given a constraint `Typeable (a -> b)`:
>>>    toDyn :: Typeable (a -> b) => P (a -> b) -> a -> Dynamic
>>> With your suggestion, it seems I should be able to use the ordinary
>>> Data.Dynamic instead.
>>> / Emil
>> Great! Do you like my plan? Or perhaps know of a better one?
>> (Relatedly, *does* this have to go through a separate libraries
>> process? Or are we considering Typeable as getting completely
>> replaced, and everything pertaining to it gets discussed here?)
> Your plan certainly seems general enough! But I'm afraid I can't really
> speak about the implications on libraries etc.
> / Emil
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