[Haskell-cafe] Restricted type classes
nccb2 at kent.ac.uk
Tue Sep 7 07:26:49 EDT 2010
On 07/09/10 05:24, wren ng thornton wrote:
>>> that other class would (most likely) be a subclass of pointed
>>> functors. In
>>> any case, it does mean there's something of a mismatch between
>>> singleton vs
>> Not quite sure what you mean by a "mis-match"
> Of course, I'd expect singleton to obey the pointed law as well, so
> Just that they're not the same thing. For example, ZipList supports
> pure but it has no meaningful instance of singleton since every
> ZipList is infinite.
I don't believe that every ZipList is infinite (if this should be the
case, the constructor shouldn't be exposed!), just that ZipLists created
by "pure" are infinite -- that's the obvious definition to meet the
Applicative laws. You can quite happily use:
(+) <$> ZipList [1,2,3] <*> ZipList [4,5] == ZipList [5,7]
So ZipList does have a meaningful definition of singleton (singleton x =
ZipList [x]; I'm sure there are other pointed functors that don't have a
good definition for singleton), and a meaningful definition of pure, but
they're not the same definition.
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