Personal tools

99 questions/Solutions/4

From HaskellWiki

< 99 questions | Solutions(Difference between revisions)
Jump to: navigation, search
m
(Their was a section for the length function in the prelude but it was missing so I took the function form the 2010 report and link to it.)
Line 32: Line 32:
   
 
This is <hask>length</hask> in <hask>Prelude</hask>.
 
This is <hask>length</hask> in <hask>Prelude</hask>.
  +
  +
-- length returns the length of a finite list as an Int.
  +
length :: [a] -> Int
  +
length [] = 0
  +
length (_:l) = 1 + length l
  +
  +
The prelude for haskell 2010 can be found [http://www.haskell.org/onlinereport/haskell2010/haskellch9.html#x16-1710009 here.]
   
 
A fancier one! :-)
 
A fancier one! :-)

Revision as of 19:21, 30 November 2012

(*) Find the number of elements of a list.

myLength           :: [a] -> Int
myLength []        =  0
myLength (_:xs)    =  1 + myLength xs
 
myLength' :: [a] -> Int
myLength' list = myLength_acc list 0 -- same, with accumulator
	where
		myLength_acc [] n = n
		myLength_acc (_:xs) n = myLength_acc xs (n + 1)
myLength'     =  foldl (\n _ -> n + 1) 0
myLength''    =  foldr (\_ n -> n + 1) 0
myLength'''   =  foldr (\_ -> (+1)) 0
myLength''''  =  foldr ((+) . (const 1)) 0
myLength''''' =  foldr (const (+1)) 0
myLength' xs = snd $ last $ zip xs [1..] -- Just for fun
myLength''   = snd . last . (flip zip [1..]) -- Because point-free is also fun
myLength'''  = fst . last . zip [1..] -- same, but easier
myLength = sum . map (\_->1)
This is
length
in
Prelude
.

-- length returns the length of a finite list as an Int. length  :: [a] -> Int length [] = 0 length (_:l) = 1 + length l

The prelude for haskell 2010 can be found here.

A fancier one! :-)

myLength = foldl (const . (+1)) 0