'lazy mindset' was: HUGS error: Unresolved overloading

Olaf Chitil olaf@cs.york.ac.uk
Tue, 22 May 2001 12:40:31 +0100

Laszlo Nemeth wrote:
> >   isSorted xs = and (zipWith (<=) xs (tail xs))
> > In other words: "When is a list xs sorted?  If each element in xs is
> > less than or equal to its successor in the list (i.e., the corresponding
> > element in tail xs)."
> That's right ... under cbn! At the same time David's version with
> explicit recursion is fine both in Hugs and in a strict language.
> I recently started using Caml for day to day work and I get bitten
> because of the 'lazy mindset' at least once a week. I am not in
> disagreement with you over the style, but explicit recursion in this
> case avoids the problem.

I find this remark very interesting. It reminds me of the many people
who say that they want a call-by-value language that allows call-by-need
annotations, because you rarely need call-by-need. That depends very
much on what you mean by `need call-by-need'! Mark has given a nice
example how a function can be defined concisely, taking advantage of the
call-by-need language. I very much disagree with Lazlo's comment, that
you should use explicit recursion to make the definition valid for
call-by-value languages as well. You should take full advantage of the
expressive power of your call-by-need language. Otherwise, why not avoid
higher-order functions because they are not available in other

As Lazlo says, you need a 'lazy mindset' to use this style. It takes
time to develop this 'lazy mindset' (just as it takes time to lose it
again ;-). To help people understanding and learning this 'lazy
mindset', I'd really like to see more examples such as Mark's. If there
are more examples, I could collect them on haskell.org. 

> PS. Why not go all the way
>     and . uncurry (zipWith (<=)) . id >< tail . dup
> with appropriate definitions for dup and >< (prod)?

It is longer, uses more functions and doesn't make the algorithm any
clearer. IMHO point-free programming and categorical combinators are not
the way to obtain readable programs.

Mark uses few standard functions. His definition is more readable than
the recursive one, because it is shorter and because it turns implicit
control into explicit data structures.


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