Difference between revisions of "99 questions/Solutions/2"

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m (Don't tell new people to write partial functions, come on.)
Line 14: Line 14:
   
 
myButLast'''' = head . tail . reverse
 
myButLast'''' = head . tail . reverse
  +
  +
lastbut1 :: Foldable f => f a -> a
  +
lastbut1 = fst . foldl (\(a,b) x -> (b,x)) (err1,err2)
  +
where
  +
err1 = error "lastbut1: Empty list"
  +
err2 = error "lastbut1: Singleton"
  +
  +
lastbut1safe :: Foldable f => f a -> Maybe a
  +
lastbut1safe = fst . foldl (\(a,b) x -> (b,Just x)) (Nothing,Nothing)
 
</haskell>
 
</haskell>
   

Revision as of 17:26, 28 December 2014

(*) Find the last but one element of a list.

myButLast :: [a] -> a
myButLast = last . init

myButLast' x = reverse x !! 1

myButLast'' [x,_]  = x
myButLast'' (_:xs) = myButLast'' xs

myButLast''' (x:(_:[])) = x
myButLast''' (_:xs) = myButLast''' xs

myButLast'''' = head . tail . reverse

lastbut1 :: Foldable f => f a -> a
lastbut1 = fst . foldl (\(a,b) x -> (b,x)) (err1,err2)
  where
    err1 = error "lastbut1: Empty list"
    err2 = error "lastbut1: Singleton"

lastbut1safe :: Foldable f => f a -> Maybe a
lastbut1safe = fst . foldl (\(a,b) x -> (b,Just x)) (Nothing,Nothing)