04-first-types.md

First Types

What is a Type?

• "Classifies" values
• Constrains what a value can be

Basic Types in Haskell

• Int: Integer numbers (-1, 0, 30)
• Bool: Truth values (True, False) (capitalized!)
• Char: Single characters ('a', 'c')
• String: Text ("Hello", "Cheese sandwich")
• a -> b: Function with
  • the input type (domain) a
  • the output type (codomain or range) b

Specifying a Type

x :: Int
x = 5

favoriteAnimal :: String
favoriteAnimal = "Cat, obviously"

firstLetter :: Char
firstLetter = 'A'

double :: Int -> Int
double x = x * 2

Recommendation: Always specify the type.

Lists

Ordered list with elements of type a: [a]

fruits :: [String]
fruits = ["Banana", "Apple", "Raspberry"]

numbers :: [Int]
numbers = [1,5,2]

empty :: [Int]
empty = []

Building Lists

Lists are a recursive data structure

A list is always one of two things:

• an empty list: []
• an element added in front of another list: element : list

These are all the same list:

• [1, 2, 3]
• 1 : [2, 3]
• 1 : (2 : (3 : []))
• 1 : 2 : 3 : []

moreFruits :: [String]
moreFruits = "Orange" : fruits