One solution would be to fold over a specific semigroup instead of a recursive function:

import Data.Semigroup
import Data.Foldable (foldMap)
import Data.Maybe (maybeToList)

data Darle a = Darle { getInit :: [a], getLast :: a }
  deriving Show
instance Semigroup (Darle a) where
    ~(Darle xs1 l1) <> ~(Darle xs2 l2) = Darle (xs1 ++ [l1] ++ xs2) l2

darle :: [a] -> Darle a
darle = foldr1 (<>) . map (Darle [])

It's somewhat more verbose, but the core idea is clearly expressed in the one line that defines <>, and IMHO it better shows what are we doing rather than how. It's sufficiently lazy so that you can do something like head . getInit $ darle [1..].

Best regards,
Petr

Dne 08/30/2013 08:18 PM, Lucas Paul napsal(a):

Suppose I need to get an element from a data structure, and also
modify the data structure. For example, I might need to get and delete
the last element of a list:

darle xs = ((last xs), (rmlast xs)) where
  rmlast [_] = []
  rmlast (y:ys) = y:(rmlast ys)

There are probably other and better ways to write rmlast, but I want
to focus on the fact that darle here, for lack of a better name off
the top of my head, appears to traverse the list twice. Once to get
the element, and once to remove it to produce a new list. This seems
bad. Especially for large data structures, I don't want to be
traversing twice to do what ought to be one operation. To fix it, I
might be tempted to write something like:

darle' [a] = (a, [])
darle' (x:xs) = let (a, ys) = darle' xs in (a, (x:ys))

But this version has lost its elegance. It was also kind of harder to
come up with, and for more complex data structures (like the binary
search tree) the simpler expression is really desirable. Can a really
smart compiler transform/optimize the first definition into something
that traverses the data structure only once? Can GHC?

- Lucas

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