ch04.adoc

4 Basic datatypes

4.2 Anatomy of a data declaration

given data type, answer

data Mood = Blah | Woot deriving Show

1. type constructor is Mood

2. values are Blah and Woot

3. :: Mood → Woot to :: Mood → Mood, as Woot is value, not type

4. change mood

   link:ch04_4.2_0.hs[role=include]

5. done :)

4.4 Comparing values

Intermission: Exercises

1. bug

   not True && true
   -- fix
   not True && True

2. bug

   not (x = 6)
   -- fix
   not (x == 6)

3. correct

   (1 * 2) > 5

4. bug

   [Merry] > [Happy]
   -- fix
   ["Merry"] > ["Happy"]

5. bug

   [1, 2, 3] ++ "look at me!"
   -- fix
   ['1', '2', '3'] ++ "look at me!"

4.7 Chapter Exercises

awesome = ["Papuchon", "curry", "Haskell"]
alsoAwesome = ["Quake", "The Simons"]
allAwesome = [awesome, alsoAwesome]

1. length

   1. type signature

      :: [a] -> Int

   2. takes one argument

   3. result is Int

2. results

   1. length [1, 2, 3, 4, 5] == 5

   2. length [(1, 2), (2, 3), (3, 4)] == 3

   3. length allAwesome == 2

   4. length (concat allAwesome) == 5

3. second breaks

   -- works
   6 / 3
   -- both values need to be Fractional, length returns Int
   6 / length [1, 2, 3]

4. fixed previous example

   6 `div` length [1, 2, 3]

5. 2 + 3 == 5 type is Bool and result is True as 5 == 5

6. type is Bool, result is False as 8 /= 5

   Prelude> let x = 5
   Prelude> x + 3 == 5

7. as follows

   Prelude> length allAwesome == 2
   -- works - True
   Prelude> length [1, 'a', 3, 'b']
   -- breaks on types, list cannot be heterogenic
   Prelude> length allAwesome + length awesome
   -- works - 5
   Prelude> (8 == 8) && ('b' < 'a')
   -- works - False
   Prelude> (8 == 8) && 9
   -- breaks - 9 is not a Bool

8. palidrome function

   link:ch04_4.7_0.hs[role=include]

9. return absolute value

   link:ch04_4.7_1.hs[role=include]

10. tuple function

    link:ch04_4.7_2.hs[role=include]

Reading Syntax

1. length + 1

   link:ch04_4.7_3.hs[role=include]

2. identity

   \x -> x

3. head

   \ (x:xs) -> x

4. fst

   f (a, b) = a

Match the function names to their types

1. c) Show a ⇒ a → String

2. b) Eq a ⇒ a → a → Bool

3. a) (a, b) → a

4. d) Num a ⇒ a → a → a