Cap 3: Types and Typeclasses

Cap 3: Types and Typeclasses

1. The type

   • Haskell has a static type system
     • this is used for catch such errors at compile time instead of having your program crash
     • common types
       • Int and Integer
       • Float is a real floating point with single precision.
       • Double is a real floating point with double the precision
       • Bool is a boolean type. It can have only two values: True and False.
   • Haskell can infer the type
   • Functions also have types
     • For example
       • [Char] -> [Char]
         • meaning that it maps from a string to a string
       • Int -> Int -> Int -> Int
         • The parameters are separated with -> and there's no special distinction between the parameters and the return type

2. Type variables

   • if it's not in capital case
     • it's actually a type variable
       • that means that a can be of any type
       • like generics in other languages
       • example [a] -> a
         • it takes a list of any type and returns one element of that type.

3. Typeclasses

   • A typeclass is a sort of interface that defines some behavior
     • the => symbol
       • Everything before the symbol is called a class constraint
   • examples
     • The Eq typeclass provides an interface for testing for equality.
       • The functions its members implement are == and /=
     • Ord The compare function takes two Ord members of the same type and returns an ordering.
     • show: It takes a value whose type is a member of Show and presents it to us as a string.
     • The read function takes a string and returns a type which is a member of Read.
     • Enum are sequentially ordered types, they can be enumerated.
       • use its types in list ranges
       • can use the succ and pred functions.
     • Num is a numeric typeclass
       • include
         • Integral
         • Floating
         • (Num b) => length :: [a] -> b