[Haskell-cafe] Re: Can we do better than duplicate APIs? [was: Data.CompactString 0.3]

Benjamin Franksen benjamin.franksen at bessy.de
Wed Mar 28 14:44:00 EDT 2007

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
>> 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
> The data structure interfaces have scores of functions and there are about
> different concrete implementations of various sorts.  Even minor interface 
> changes require a lot of tedious editing to make sure that everything
> 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); 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;
>> be much less complicated and more satisfying to use a language feature
>> 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
> Some sort of in-langauge or extra-language support for mechanicly
> 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

> haddock comments,

I thought all the documentation would be in the API classes, not in the
concrete implementations.

> 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
> wouldn't produce Haddock-consumable source files.  ML-style first class 
> modules might fit the bill, but I'm not sure anyone is seriously
> 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.


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