Proposal: Don't require users to use undefined

Axel Simon Axel.Simon at
Wed Oct 27 03:24:50 EDT 2010

Hi all,

On 27.10.2010, at 09:01, Bas van Dijk wrote:

> On Wed, Oct 27, 2010 at 7:57 AM, Michael Snoyman  
> <michael at> wrote:
>> On Wed, Oct 27, 2010 at 7:51 AM, Bas van Dijk  
>> <v.dijk.bas at> wrote:
>>> On Tue, Oct 26, 2010 at 9:09 PM, Henning Thielemann
>>> <lemming at> wrote:
>>>> 4. In the future you may deprecate the immediate use of 'sizeOf'  
>>>> and
>>>> 'alignment'.
>>> We could deprecate sizeOf immediately in favor of sizeOfTagged.
>>> Deprecating it doesn't break any code but does gives warnings so  
>>> that
>>> users are encouraged to upgrade. I don't see a reason to wait.
>> We can't immediately switch to using sizeOfTagged since many people
>> will still be using old versions of base. If we waited one cycle to
>> deprecate the old functions, it won't annoy people. By the next  
>> cycle,
>> in theory a majority of people will already have sizeOfTagged, and it
>> would be feasible to switch our code to sizeOf.
> Yes, that's what I meant. We should {-# DEPRECATE #-} sizeOf now and
> provide sizeOfTagged as an alternative. In a further cycle we can
> remove sizeOf.

I think this whole proposal is a bad idea. There is a cost-benefit  
tradeoff in every library change.

I do not see the problem that anybody is every going to accidentally  
evaluate sizeOf or alginment and get an unexpected "undefined" value  
back. I don't see the compiler optimization problem - it may exists  
but if the argument of Storage is fixed (and it always is in any use  
of Storage) then the compiler should be able to inline the 4 or 8 or  
whatever alignment evaluates to.

So the only benefit I see is that code becomes slightly prettier. But  
it is not code that is read a lot, it's boilerplate code that is used  
internally. No sane Haskell library that interfaces to C exports Ptr  
values and expects the user to use sizeOf and alignment to operate on  
the C data structure. Instead C data structures are accessed using a  
Haskell interface. So the danger of library users  calling sizeOf and  
getting an "undefined" is really zero. Even as a library maintainer,  
I've never managed to evaluate sizeOf by accident.

In Gtk2Hs, we create a lot of Storable instances. If we were to  
implement the proposed change, we would have to manually edit a lot of  
code. There is no benefit for the user. None. There is only a lot of  
work. For future code, the benefit is also near to zero since, from my  
past experience, the problems that motivated this proposal do not  
exist in practice.


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