Chapter 3: Types and Typeclasses

Learn You a Haskell for Great Good!

Believe the type

  • Haskell has a static type system
  • type of every expression is known at compile time, leads to safer code
  • check type for both variable and functions
    • :t
  • x has type of t
    • x :: t
  • element in a list must have the same type
    • e.g. “HELLO!” :: [Char]
  • element in a tuple can have different type
    • e.g. (True, ‘a’) :: (Bool, Char) or (‘a’, ‘b’, ‘c’) :: (Char, Char, Char)
  • function declaration
    • e.g. removeNonUppercase :: [Char] -> [Char]
    • mapping string to string
  • String is equivalent to [Char]
  • function declaration with multiple parameters
    • e.g. addThree :: Int -> Int -> Int -> Int
    • mapping the first three Int as parameters and the last item as return type
  • Some common types and all types start with a Capital letter
    • Int stands for Integer but bounded with minimum and maximum values
    • Integer stands for Integer also but without bound.
    • Float is single precision real floating point number.
    • Double is double precision real floating point number.
    • Bool is a boolean type (True or False).
    • Char is a character.

Type variables

  • type variable is use to represent any type and are usually given names like a, b, c, …
    • e.g. :t head will result head :: [a] -> a
    • this mean head operate on a list of type a and return an element of type a.
    • e.g. :t fst will result fst :: (a, b) -> a
    • this mean fst operate on a tuple pair of type a, b and return an element of type a (the first element of the tuple)

Typeclasses 101

  • typeclasses is a sort of interface that defines some behavior.
    • e.g. :t (==) will gives (==) :: (Eq a) => a -> a -> Bool
    • Note: == is a function and comprise of only special characters, so it is consider to be infix function by default, use parentheses to call it as a prefix function (e.g. infix a==b vs prefix (==) a b).
    • equality function takes any two parameters that are of same type and return a Bool.
  • class constraint, =>
  • Some basic typeclasses
    • Eq typeclass provides an interface for testing for equality
      • e.g. == or /=
    • Ord  takes two values of same type and returns an ordering.
      • e.g. parameter >, <, >= and <=
      • To be a member of Ord, a type must first have membership in Eq.
    • Ordering is a type that can be GT, LT or EQ.
    • Show can be presented as strings. All types covered so far except function are a part of Show. show function deals with the Show typeclass.
      • e.g. show 3 gives “3”
    • Read is sort of the opposite of typeclass of Show.
      • e.g. read “6” – 1 gives 5
      • However, read “6” alone will give an error because GHCi cannot infer what type it suppose to be.
      • Using explicit type annotations to solve the issue
      • e.g. read “6” :: Float gives 6.0
    • Enum members are sequentially ordered types.
      • e.g. [LT .. GT] gives [LT,EQ,GT]
      • e.g. succ ‘B’ gives ‘C’
    • Bounded members has upper and lower bound
      • e.g. minBound :: Int gives the lower bound of integer (depend on architectural)
      • e.g. maxBound :: Bool gives True
      • All tuple are also part of Bounded if Bounded components are in it.
    • Num is a numeric typeclass. It includes all numbers.
      • e.g. :t 20 gives 20 :: (Num t) => t
      • To join Num, a type must already be friends with Show and Eq
    • Integral is also numeric typeclass that include only whole numbers
    • Floating includes only floating point numbers.
      • fromIntegral x
      • signature of fromIntegral has multiple constraint (Num b, Integral a) => a -> bturn Integral type to more general number
      • e.g. length function return Int type instead of Num type
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