Outline of Class 28: More Haskell Concepts

Held: Friday, April 10, 1998

Administrivia

• Any question on assignment five?
• Thanks very much to those of you who attended one of the sessions involving Mr. Earl. The department very much appreciates your comments.
• Did I leave you with an "end of class" question last class? If so, I've forgotten what it was, but I hope one of you will remind me.

More Haskell Concepts

Types

• Haskell is a typeful language. Types play a large role in the language.
  • Programmers are encouraged to define their own types.
  • Haskell has a rich type inference and checking system.
• A type declaration gives a type name and a set of type constuctors which tell how to build objects of that type.
  • Constructors may take arguments (e.g., Cons would take two arguments).
  • Constructcors may take no arguments (in which case they act like basic values).
• Often, there are multiple constructors for a type. These can be separated with vertical bars.
• For example, an enumerated type would be written something like

  data Days = Mon | Tue | Wed | Thu | Fri | Sat | Sun
  data Gender = Male | Female | Unspecified
  

• In a record type, we would also give arguments, as in

  data Person = Per String Int Gender
  

• In this instance, Per is the constructor, which takes three arguments:
  • A string, perhaps representing the person's name
  • An integer, perhaps representing the person's age
  • A gender
• Haskell also allows recursive types, such as

  data IntList = ILnil | ILcons Int IntList
  

• More importantly, Haskell allows you to define polymorphic types (what I've sometimes called metatypes) that take type variables as arguments. For example,

  data List a = Nil | Cons a (List a)