[Haskell-cafe] ANN: unfoldable-0.4.0
wren ng thornton
wren at freegeek.org
Mon Apr 30 07:49:28 CEST 2012
On 4/28/12 12:10 PM, Sjoerd Visscher wrote:
> But I don't think an unfoldable class for * types is that interesting. Any type that would be an instance could also be in instance of Bounded and Enum:
In a technical sense, yes, but not necessarily in a semantic sense.
Usually Bounded and Enum are expected to respect the natural ordering on
the type; and, given Eq, they induce an ordering on the type (albeit an
inefficient one). But there are plenty of cases where you don't have a
natural ordering, or where it would be more efficient to enumerate
values in an unnatural order if the goal is just to get them all.
Unfortunately, we don't have a good way of distinguishing between
natural vs ad-hoc enumerations/orderings, so this sort of thing gets
handled poorly on case-by-case bases.
Though I agree that Biunfoldable is a lot more interesting than Unfoldable0.
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