[Haskell-cafe] Is this haskelly enough?

[Dan Weston]

 > Btw, if you don't want the empty lists, you can use
 >
 >     concatMap (init . tails) . tail . inits

Would it not be more efficient and perspicuous to keep the sublists 
definition as is, just interchanging inits and tails?

   where sublists = filter (not . null) . concatMap tails . inits

Or am I missing some argument about sublist sharing?

Dan

Bertram Felgenhauer wrote:
 > J. Garrett Morris wrote:
 >>    -- the tails function returns each tail of the given list; the
 >> inits function
 >>    -- is similar.  By mapping inits over tails, we get all the sublists.
 >>    where sublists = filter (not . null) . concatMap inits . tails
 >
 > Nice, but
 >
 >     concatMap tails . inits
 >
 > is much better in my opinion, for several reasons:
 >
 > - inits is expensive (O(n^2)) while tails is cheap (O(n)), so it's
 >   better to use inits only once.
 > - the result lists of inits can't be shared (which is essentially the
 >   reason why it's so expensive); tails shares the common part of the
 >   result lists.
 > - finally,  concatMap tails . inits  works nicely with infinite lists,
 >   with every substring occuring in the result eventually
 >
 > Btw, if you don't want the empty lists, you can use
 >
 >     concatMap (init . tails) . tail . inits
 >
 > Bertram