Class 25: Haskell (3)

Held: Monday, 4 April 2011

Summary: We conclude our consideration of Haskell.

Related Pages:

• EBoard.
• Lab: Haskell (3).
• Reading: Tate 8.4-8.5.

Notes:

• I hope you had a good spring break. I once again succumbed to the planning falacy.
• I will distribute the reading for Wednesday later today. No Piazzza questions necesssary. (If I had time, I would have distributed Jean Sammet's 1972 history of programming languages,available at http://portal.acm.org/citation.cfm?doid=361454.361485).
• President Kington is talking to the faculty today about Choosing our Future. I'll read to you from some key points.
• Today will be a lecture-style class.
• I will reserve time at the start of class for questions on mid-semester examination.
• I will reserve time at the start of class for discussion of the presentations.

Overview:

• Haskell's type system.
• Monads.

Types in Haskell

• As you've seen, Haskell has a rich and robust type system.
• Haskell also takes advantage of an interface-style approach: You can indicate that a type implements an interface using the instance syntax.
• We'll look at a simple tree example.

Monads

• So, how do you do I/O and other side-effecting operations in a pure language?
• In essence, you need to pass the state around at all times. (That is, you make the implicit explicit.) For example, a random function, rather than returning just a random number, must also return the updated random number generator.
• Monads provide a uniform way of dealing with these issues.
• You are required to
  • indicate how to apply functions
  • make your functions return values in the same domain
  • indicate how to extract a value from the environment at the end of the monad.
• In some sense, Monads are just custom and syntax for dealing with these imperative issues.
• However, Monads also have a deep mathematical structure
  • I've tried to read the papers on that structure, and failed.