On Mar 28, 2007, at 2:44 PM, Benjamin Franksen wrote:
Robert Dockins wrote:
After taking a look at the Haddock docs, I was impressed by the
amount of
repetition in the APIs. Not ony does Data.CompactString duplicate
the
whole
Data.ByteString interface (~100 functions, adding some more for
encoding
and decoding), the whole interface is again repeated another four
times,
once for each supported encoding.
I'd like to mention that as maintainer of Edison, I face similar
difficulties.
The data structure interfaces have scores of functions and there
are about
20
different concrete implementations of various sorts. Even minor
interface
changes require a lot of tedious editing to make sure that everything
stays
in sync.
But... you have the type of all functions nailed down in classes.
Thus, even
if a change in the API means a lot of tedious work adapting the
concrete
implementations, at least the compiler helps you to check that the
implementations will conform to the interface (class);
This is true.
and users have to
consult only the API docs, and not every single function in all 20
implementations. With ByteString and friends there is (yet) no common
interface laid down anywhere. All the commonality is based on
custom and
good sense and the willingness and ability of the developers to
make their
interfaces compatible to those of others.
One could use code
generation or macro expansion to alleviate this, but IMO the
necessity to
use extra-language pre-processors points to a weakness in the
language;
it
be much less complicated and more satisfying to use a language
feature
that
avoids the repetition instead of generating code to facilitate it.
I've considered something like this for Edison. Actually, I've
considered
going even further and building the Edison concrete
implementations in a
theorem prover to prove correctness and then extracting the Haskell
source.
Some sort of in-langauge or extra-language support for mechanicly
producing
the source files for the full API from the optimized core API
would be
quite welcome. Handling export lists,
How so? I thought in Edision the API is a set of type classes.
Doesn't that
mean export lists can be empty (since instances are exported
automatically)?
No. Edison allows you to directly import the module and bypass the
typeclass APIs if you wish. Also, some implementations have special
functions that are not part of the general API, and are only
available via the module exports.
One could make typeclasses the only way to access the main API, but I
rather suspect there would be performance implications. I get the
impression that typeclass specialization is less advanced than
intermodule inlining (could be wrong though).
haddock comments,
I thought all the documentation would be in the API classes, not in
the
concrete implementations.
It is now, but I've gotten complaints about that (which are at least
semi-justified, I feel). Also, the various implementations have
different time bounds which must documented in the individual
modules. Ideally, I'd like to have the function documentation string
and the time bounds on each function in each concrete
implementation. I've not done this because its just too painful to
maintain manually.
typeclass instances,
etc, are quite tedious.
I have to admit, I'm not sure what an in-language mechanism for doing
something like this would look like. Template Haskell is an
option, I
suppose, but its pretty hard to work with and highly non-
portable. It
also
wouldn't produce Haddock-consumable source files. ML-style first
class
modules might fit the bill, but I'm not sure anyone is seriously
interested
in bolting that onto Haskell.
As I explained to SPJ, I am less concerned with duplicated work when
implementing concrete data structures, as with the fact that there
is still
no (compiler checkable) common interface for e.g. string-like
thingies,
apart from convention to use similar names for similar features.
Fair enough. I guess my point is that typeclasses (ad per Edison)
are only a partial solution to this problem, even if you can stretch
them sufficiently (with eg, MPTC+fundeps+whatever other extension) to
make them cover all your concrete implementations.
Cheers
Ben
Rob Dockins
Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
-- TMBG
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