Note for LI Functional Programming - Week 2

Handout

type classes and Instances

练习

Prelude> False == 'c'
<interactive>:1:10: error:
 • Couldn't match expected type 'Bool' with actual type 'Char'
 • In the second argument of '(==)', namely ''c''
 In the expression: False == 'c'
 In an equation for 'it': it = False == 'c'

Prelude> False == True
False

Prelude> False == not

<interactive>:3:10: error:
    • Couldn't match expected type 'Bool'
                  with actual type 'Bool -> Bool'
    • Probable cause: 'not' is applied to too few arguments
      In the second argument of '(==)', namely 'not'
      In the expression: False == not
      In an equation for 'it': it = False == not

Prelude> False == not True
True

Prelude> not == id

<interactive>:5:1: error:
    • No instance for (Eq (Bool -> Bool)) arising from a use of '=='
        (maybe you haven't applied a function to enough arguments?)
    • In the expression: not == id
      In an equation for 'it': it = not == id

Prelude> [not] == [ (id :: Bool -> Bool) ]

<interactive>:6:1: error:
    • No instance for (Eq (Bool -> Bool)) arising from a use of '=='
        (maybe you haven't applied a function to enough arguments?)
    • In the expression: [not] == [(id :: Bool -> Bool)]
      In an equation for 'it': it = [not] == [(id :: Bool -> Bool)]