Re: Proposal: Make intersperse lazier

17 Sep 2010

      Am 17.09.2010 14:21, schrieb Bas van Dijk:
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On Fri, Sep 17, 2010 at 10:48 AM, Christian Maeder
 wrote:
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Am 16.09.2010 16:39, schrieb Daniel Fischer:
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The proposed implementation,
intersperse             :: a -> [a] -> [a]
intersperse _   []      = []
intersperse sep (x:xs)  = x : go xs
  where
    go []     = []
    go (y:ys) = sep : y : go ys
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Therefore I proposed the following implementation:
intersperse :: a -> [a] -> [a]
intersperse s l = case l of
 [] -> l
 x : r -> x : if null r then r else
   s : intersperse s r
One additional benefit about the original proposed implementation is
that it applies the static argument transformation: the 'sep' argument
is brought into scope of the worker function 'go' which then doesn't
need to pass it to each recursion as in the original implementation.
This is the point. Taking the simpler prepend function, do we did not write:

prepend :: a -> [a] -> [a]
prepend sep = go where
  go [] = []
  go (x : xs) = sep : x : go xs

for more efficiency (instead of the direct recursion or a concatMap or
folding solution)?

Cheers Christian
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It would be interesting to see if this provides a noticeable
performance benefit. Criterion anyone?
Bas