Making Our Own Types and Typeclasses

Making Our Own Types and Typeclasses

1. Algebraic Data Types

   • Data keyword define a type
     • data Bool = False | True
       • Bool define a type
       • False | True define a value constructors
   • Value constructors are functions
     • deriving (Show)
       • Get the string representation of our value
     • We can map them and partially apply them and everything
   • Export the functions and types
     • Modules

2. Record Syntax

   • Alternative way to write data types
     • Use curly brackets
     • Instance :: type
     • deriving (Show)
       • Constructor has several fields and it’s not obvious which field is which

3. Type Parameters

   • Take types as parameters to produce new types
   • Allow us to make data types that can hold different things
     • Type is polymorphic
     • We use them when our data type would work regardless of the type of the value it then holds
   • Never add type class constraints in data declarations.
     • We end up writing more class constraints, even when we don’t need them

4. Derived Instances

   • Haskell will see if the value constructors match
     • Data contained inside matches by testing each pair of fields with ==
   • The Show and Read type classes are for things that can be converted to or from strings
     • Read converting strings to values of our type
     • Show converting valyes of our type to strings

5. Type Synonyms

   • Give types different names so that they make more sense to someone reading our code and documentation
     • Types are equivalent and interchangeable
   • Word type defines a synonym for an existing type
     • Can be parametryzed
   • Describe what some existing type represents in our functions
     • Represents something more specific in the context of our functions
   • Either: Tell us about fails

6. Recursive Data Structures

   • Make types that have themselves as types in their fields
   • One value of some type contains values of that type
     • Contain more values of the same type
     • Cons is another word for :