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lab04b: impl

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Yura
2026-02-14 22:43:50 -08:00
parent 7911a8d160
commit 52e7e119b6
1 changed files with 15 additions and 5 deletions
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lab04/lab.lhs
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@@ -6,9 +6,8 @@ First, implement your own version of the foldl function,
defined as myFoldl defined as myFoldl
> myFoldl :: (a -> b -> a) -> a -> [b] -> a > myFoldl :: (a -> b -> a) -> a -> [b] -> a
> myFoldl _ _ _ = error "TBD"
> myFoldl f acc [] = acc > myFoldl f acc [] = acc
> myFoldl f acc (x:xs) = f acc $ myFoldl f x xs > myFoldl f acc (x:xs) = myFoldl f (f acc x) xs
Next, define a function to reverse a list using foldl. Next, define a function to reverse a list using foldl.
@@ -20,7 +19,6 @@ Next, define a function to reverse a list using foldl.
Now define your own version of foldr, named myFoldr Now define your own version of foldr, named myFoldr
> myFoldr :: (a -> b -> b) -> b -> [a] -> b > myFoldr :: (a -> b -> b) -> b -> [a] -> b
> myFoldr _ _ _ = error "TBD"
> myFoldr f acc [] = acc > myFoldr f acc [] = acc
> myFoldr f acc (x:xs) = f x $ myFoldr f acc xs > myFoldr f acc (x:xs) = f x $ myFoldr f acc xs
@@ -28,10 +26,13 @@ Now define your own version of foldr, named myFoldr
Now try using foldl (the library version, not yours) to sum up the numbers of a large list. Now try using foldl (the library version, not yours) to sum up the numbers of a large list.
Why is it so slow? Why is it so slow?
foldl is slow because it repeatedly pushes unevaluated expressions on the stack
Instead of foldl, try using foldl'. Instead of foldl, try using foldl'.
Why is it faster? Why is it faster?
(Read http://www.haskell.org/haskellwiki/Foldr_Foldl_Foldl%27 for some hints) (Read http://www.haskell.org/haskellwiki/Foldr_Foldl_Foldl%27 for some hints)
foldl' evaluates the accumulator at each step, preventing the build up of unevaluated expressions
For an extra challenge, try to implement foldl in terms of foldr. For an extra challenge, try to implement foldl in terms of foldr.
See http://www.haskell.org/haskellwiki/Foldl_as_foldr for details. See http://www.haskell.org/haskellwiki/Foldl_as_foldr for details.
@@ -40,11 +41,20 @@ See http://www.haskell.org/haskellwiki/Foldl_as_foldr for details.
Next, using the map function, convert every item in a list to its absolute value Next, using the map function, convert every item in a list to its absolute value
> listAbs :: [Integer] -> [Integer] > listAbs :: [Integer] -> [Integer]
> listAbs _ = error "TBD" > listAbs = map abs
Finally, write a function that takes a list of Integers and returns the sum of Finally, write a function that takes a list of Integers and returns the sum of
their absolute values. their absolute values.
> sumAbs :: [Integer] -> Integer > sumAbs :: [Integer] -> Integer
> sumAbs _ = error "TBD" > sumAbs = sum . listAbs
> main :: IO ()
> main = do
> putStrLn $ "myFoldl (+) 0 [1..10] = " ++ show (myFoldl (+) 0 [1..10])
> putStrLn $ "myReverse [1..5] = " ++ show (myReverse [1..5])
> putStrLn $ "myFoldr (+) 0 [1..10] = " ++ show (myFoldr (+) 0 [1..10])
> putStrLn $ "listAbs [-1, -2, 3, -4] = " ++ show (listAbs [-1, -2, 3, -4])
> putStrLn $ "sumAbs [-1, -2, 3, -4] = " ++ show (sumAbs [-1, -2, 3, -4])
> putStrLn $ "foldl (+) 0 [1..10000000] = " ++ show (foldl (+) 0 [1..10000000])
> putStrLn $ "foldl' (+) 0 [1..10000000] = " ++ show (foldl' (+) 0 [1..10000000])