[Haskell-cafe] unfoldr [ANN: HLint 1.2]
gale at sefer.org
Tue Jan 13 08:45:02 EST 2009
Andrew Coppin wrote:
>>> Does it suggest unfoldr too?
I think Neil's idea to have this customizable is a good one.
It's often a matter of taste.
I would rarely want to use unfoldr, and I wouldn't want HList
to bother me about it. Instead, I prefer to use iterate for both
of Andrew's examples:
> convert b 0 = 
> convert b n = n `mod` b : convert b (n `div` b)
> convert b = unfoldr (\n -> if n > 0 then Just (n `mod` b, n `div` b) else
convert b = map (`mod` b) . takeWhile (> 0) . iterate (`div` b)
> heap_to_list = unfoldr (\h -> if heap_empty h then Nothing else Just
> (heap_top h, heap_delete_top h))
heap_to_list = map heap_top . takeWhile (not . heap_empty) .
Here is one case where I actually do use unfoldr:
-- Mine free-form user input for occurrences of a data type
readMany = unfoldr $ listToMaybe . concatMap reads . tails
ghci> readMany "The numbers are 3, 7, and 42." :: [Int]
But I don't believe HLint should be expected to come up with
something like that. It's quite rare.
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