47 lines
1.1 KiB
Plaintext
47 lines
1.1 KiB
Plaintext
> import Data.List
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Experiment with foldl, foldr, and foldl'
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First, implement your own version of the foldl function,
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defined as myFoldl
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> myFoldl :: (a -> b -> a) -> a -> [b] -> a
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> myFoldl _ _ _ = error "TBD"
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Next, define a function to reverse a list using foldl.
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> myReverse :: [a] -> [a]
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> myReverse _ = error "TBD"
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Now define your own version of foldr, named myFoldr
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> myFoldr :: (a -> b -> b) -> b -> [a] -> b
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> myFoldr _ _ _ = error "TBD"
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Now try using foldl (the library version, not yours) to sum up the numbers of a large list.
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Why is it so slow?
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Instead of foldl, try using foldl'.
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Why is it faster?
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(Read http://www.haskell.org/haskellwiki/Foldr_Foldl_Foldl%27 for some hints)
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For an extra challenge, try to implement foldl in terms of foldr.
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See http://www.haskell.org/haskellwiki/Foldl_as_foldr for details.
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Next, using the map function, convert every item in a list to its absolute value
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> listAbs :: [Integer] -> [Integer]
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> listAbs _ = error "TBD"
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Finally, write a function that takes a list of Integers and returns the sum of
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their absolute values.
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> sumAbs :: [Integer] -> Integer
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> sumAbs _ = error "TBD"
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