ch10.adoc

10 Folding lists

10.5 Fold left

Intermission: Exercises

1. b) and c) have the same result, a) has error

   foldr (*) 1 [1..5]
   -- b)
   foldl (flip (*)) 1 [1..5]
   -- c)
   foldl (*) 1 [1..5]

2. evaluation steps of foldl (flip (*)) 1 [1..3]

   foldl (flip (*)) 1 [1..3]
    = foldl (flip (*)) (1 * 1) [2,3]
    = foldl (flip (*)) (2 * (1 * 1)) [3]
    = foldl (flip (*)) (3 * (2 * (1 * 1))) []
    = (3 * (2 * (1 * 1)))

3. c) one difference between foldr and foldl is that foldr, but not foldl, associates to the right

4. a) folds are catamorphisms, they are used to reduce structure

5. fixed functions

   1. foldr (++) "" ["woot", "WOOT", "woot"]

   2. foldr max "" ["fear","is","the","little","death"]

   3. foldr (&&) True [False, True]

   4. foldr (||) True [False, True] always return True

   5. foldl (\x y → x ++ show y) "" [1..5]

   6. foldr (flip const) 'a' [1..5]

   7. foldr (flip const) 0 "tacos"

   8. foldl const 0 "burritos"

   9. foldl const 'z' [1..5]

10.6 How to write fold functions

Intermission: Exercises

link:ch10_10.6_0.hs[role=include]

10.9 Scans

The fibs using scanl is a little bit of a mind bender at the beginning. I wrote out the evaluation https://gist.github.com/lukleh/67cf0a78205d3f6bd2d9

link:ch10_10.9_0.hs[role=include]

10.10 Chapter Exercises

Warm-up and review

1. tuples

   link:ch10_10.10_0.hs[role=include]

2. following function return average size (round to integer) of word in a sentece (string)

   link:ch10_10.10_1.hs[role=include]

3. returing Fractional values

   link:ch10_10.10_2.hs[role=include]

Rewriting functions using folds

1. myOr

   myOr :: [Bool] -> Bool
   myOr = foldr (||) False

2. myAny

   link:ch10_10.10_3.hs[role=include]

3. myElem

   link:ch10_10.10_4.hs[role=include]

4. myReverse

   link:ch10_10.10_5.hs[role=include]

5. myMap

   link:ch10_10.10_6.hs[role=include]

6. myFilter

   link:ch10_10.10_7.hs[role=include]

7. squish

   link:ch10_10.10_8.hs[role=include]

8. squishMap

   link:ch10_10.10_9.hs[role=include]

9. squishAgain using squishMap

   link:ch10_10.10_10.hs[role=include]

10. myMaximumBy

    link:ch10_10.10_11.hs[role=include]

11. myMinimumBy

    link:ch10_10.10_12.hs[role=include]