[Haskell-cafe] Shrinking the Prelude: The "categorical" approach

Henning Thielemann lemming at henning-thielemann.de
Fri Dec 22 09:55:08 EST 2006

On Thu, 21 Dec 2006, Ross Paterson wrote:

> On Thu, Dec 21, 2006 at 07:16:16PM +0100, Henning Thielemann wrote:
> > About the question, whether functions should be provided in the most 
> > general context, I refer to
> >   http://www.haskell.org/haskellwiki/Slim_instance_declaration
> Certainly we all agree that a module that defines instances should export
> functions it uses that others can use in defining further instances,
> but I don't see the point of this rule as stated.  What would we gain
> if Ratio exported (+) on rationals with a different name?

 (+) is probably not a convincing example. E.g. the Functor method 'fmap'
is instantiated by 'map' for lists. The monad method (>>=) for lists is
implemented by (flip concatMap). Shall we hide concatMap because (>>=) is
already there? I say no, because in cases where lists are the only
possible choice, 'map' and 'concatMap' are more informative to the reader
and let automatic type inference work better than 'fmap' and (>>=).
 Imagine the Monad concept is still not invented. Then we could only
define 'concatMap' but not '(>>=)'. Later the Monad class is introduced.  
Shall we discourage the use of 'concatMap' because there is now also
(>>=)? Now imagine we encounter an even more general concept, where monads
are a special case of. Shall we hide the more restrictive Monad method
(>>=) and the plain implementation 'concatMap' in favour of the more
general method?

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