Types and Typeclasses

Types and Typeclasses

1. how to make our own and how to put them to work

2. use the data keyword to define a type

   • data Bool = False | True
     • The part before the = denotes the type
     • The parts after the = are value constructors
     • Bool can have a value of True or False
     • Both the type name and the value constructors have to be capital cased.
   • Using data type
     • data Shape = Circle Float Float Float | Rectangle Float Float Float Float
       • The Circle value constructor has three fields
       • The Rectangle value constructor has four fields
       • surface :: Shape -> Float surface (Circle _ _ r) = pi * r ^ 2 surface (Rectangle x1 y1 x2 y2) = (abs $ x2 - x1) * (abs $ y2 - y1)
         • the function takes a shape and returns a float
           • we can pattern match against constructors
     • You can export your data types in your modules. for it, separated by commas.
       • To do that, just write your type along with the functions you are exporting and then add some parentheses and in them specify the value constructors that you want to export
         • If you want to export all the value constructors for a given type, just write ...
       • module Shapes ( Point(..) , Shape(..) , surface , nudge , baseCircle , baseRect ) where
   • deriving (Show)
     • at the end of a data declaration, Haskell automagically makes that type part of the Show typeclass
       • in witch way we can print the object like toString in java
       • data Point = Point Float Float deriving (Show)

3. Record syntax

   • What if we want to create a function to get seperate info from a person
     • data Person = Person { firstName :: String , lastName :: String , age :: Int , height :: Float , phoneNumber :: String , flavor :: String } deriving (Show)
     • we use curly brackets
       • First we write the name of the field
         • then we write a double colon ::
           • and then we specify the type

4. Type parameters

   • A value constructor can take some values parameters and then produce a new value
     • data Maybe a = Nothing | Just a
       • The a here is the type parameter
         • Depending on what we want this data type to hold when it's not Nothing