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m (Fix formating)
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<haskell>
 
<haskell>
myLength' = foldl (\n _ -> n + 1) 0
+
myLength' = foldl (\n _ -> n + 1) 0
myLength'' = foldr (\_ n -> n + 1) 0
+
myLength'' = foldr (\_ n -> n + 1) 0
myLength''' = foldr (\_ -> (+1)) 0
+
myLength''' = foldr (\_ -> (+1)) 0
myLength'''' = foldr ((+) . (const 1)) 0
+
myLength'''' = foldr ((+) . (const 1)) 0
myLength''''' = foldr (const (+1)) 0
+
myLength''''' = foldr (const (+1)) 0
  +
myLength'''''' = foldl (const . (+1)) 0
 
</haskell>
 
</haskell>
   
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This is <hask>length</hask> in <hask>Prelude</hask>.
 
This is <hask>length</hask> in <hask>Prelude</hask>.
   
A fancier one! :-)
+
-- length returns the length of a finite list as an Int.
<haskell>
+
length :: [a] -> Int
myLength = foldl (const . (+1)) 0
+
length [] = 0
</haskell>
+
length (_:l) = 1 + length l
  +
  +
The prelude for haskell 2010 can be found [http://www.haskell.org/onlinereport/haskell2010/haskellch9.html#x16-1710009 here.]

Revision as of 19:22, 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'''''' =  foldl (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.