time library dependencies

Edward Kmett ekmett at gmail.com
Tue Jun 2 17:30:00 EDT 2009

I would personally would argue that solution 2 (content depends on compiler)
is the least offensive solution. Just incorporate any Typeable and Data
derivings behind CPP guards, and just set a flag in the cabal file that
defaults to true, which adds the existing DeriveDataTypeable language
feature. Its self-contained and exists precisely for this purpose, and
doesn't require all of the power of Rank2Types or

Admittedly with 2, you get different contents on different platforms, but
the only content you are missing in a minimalist "Haskell 98 + addenda +
CPP"-only environment is instances that probably can't exist on that
platform anyways, since Data.Data.Data is full of Rank-2 types.

One could argue that a hand-implemented rank 2 Data.Data.Data instance is
more portable than a DeriveDataTypeable provided Data.Data.Data instance,
but at that point I think maintainability becomes a factor, plus you can
further counter-argue that you don't actually need Rank-2 types to define a
Data.Data.Data instance, merely to define the class, and the arguments could
go back and forth for days.

Heck the existing containers library is so gauche about this as to just
guard its definitions for Data behind an #if __GLASGOW_HASKELL__ block, so
by comparison the DerivingDataTypeable approach is fairly compassionate to
third party distributions and provides a clear path for them to belly up to
the bar if they want the instances. =)

-Edward Kmett

On Tue, Jun 2, 2009 at 1:42 PM, Ashley Yakeley <ashley at semantic.org> wrote:

> Ashley Yakeley wrote:
>> Malcolm Wallace wrote:
>>> OK, I object to the proposed change.  The time library is pretty core,
>>> and should continue to be buildable by compilers and interpreters other
>>> than GHC.  This does not necessarily mean it must be strictly H'98
>>> compliant, but that would be a good approximation of the common
>>> language.
>> What is that "good approximation"?
>> These are the options:
> I forgot, time already requires CPP...
> 1. time to require Haskell 98 + FFI + CPP + Rank2Types, include Data
> instances
> 2. time to require Haskell 98 + FFI + CPP, content depends on compiler
> 3. time to require Haskell 98 + FFI + CPP, a time-extras package with
> orphan Data instances
> --
> Ashley Yakeley
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