Containers and folds
wren ng thornton
wren at community.haskell.org
Sat Sep 25 18:19:09 EDT 2010
On 9/24/10 1:45 PM, Thomas Schilling wrote:
> Regarding the questions: I prefer the foldr/l names, because
> otherwise I may need to look up the documentation to figure out which
> fold it is. It's much better to use the standard naming convention
> from lists. As an example, there is foldM which is actually a left
> fold. Data.Foldable fixes this and only uses the generic name 'fold'
> for the Monoid case where order does indeed not matter due to
> associativity of mappend.
I agree with both parts of this. That is, foldr/l are the best names for
when the order is specified. However, sometimes we do have an
associative operator and don't care about the order. In these latter
cases I think it's best to offer a "fold" which doesn't specify order in
order to prevent client code from over-specifying what they mean.
Also, having an order-independent fold allows the library to choose
whatever kind of fold would be most efficient (possibly changing it over
time), and allows for things like a parallel implementation for
overcoming the "fold considered harmful" problem.
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