Hi, Am Montag, den 15.07.2013, 19:36 +0200 schrieb Bardur Arantsson:
On 2013-07-15 09:56, Joachim Breitner wrote: A question related to this:
Is it possible for such an "API package" to actually explicitly specify type signatures of its re-exports, perhaps using abstract types where no particular implementation would be dictated?
It seems to me that this would be highly valuable to provide a) documentation which could be shared across all implementations, b) a contract such that a given implementation can tell whether it really is conforming to expected type signatures, and c) a contract such that *users* of base can be certain that they're not unwittingly relying on implementation details in a given implementation of base.
I don’t think so; this would be a very different module system then. What you can do is this: #ifdef API module Foo where -- | Documentation for Bar data Bar -- abstract -- | Documentation for f f :: Int -> Bar f = undefined #else module Foo (Foo) where import "impl-package" Foo #fi so you can compile it with -DAPI to get the specification and the documentation, and compile it without to get the real thing, and then have tool support to compare the two(b). Also, with -DAPI you can get the shared documentation (a). c) can already be achieved now by just exporting the relevant details: Via base, you only can import the Foo type abstractly; if you want to get access to its constructors, you have to import base-impl.
Without such checks I fear that things may end up in a bit of a mess type-wise if there *were* actually to be multiple implementations at some point. (Though it would certainly improve on the curren situtation).
I don’t think the plan is to have different implementations of stuff in base at the same time. If different compilers (fay) replace the base-implementing packages, then there is another implementation, but that would never interfere with a definition on GHC. Or what scenario do you have in mind?
Overall, the proposal sounds like a great idea to start to get a handle on the mess that is base ;).
Thanks, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org