Call-by-need

Call-by-need

http://homepages.inf.ed.ac.uk/wadler/topics/call-by-need.html

How call-by-need fits into the picture...

Evaluation Strategies (Wikipedia)

• 1 Strict evaluation
  • 1.1 Applicative order
  • 1.2 Call by value
  • 1.3 Call by reference
  • 1.4 Call by sharing
  • 1.5 Call by copy-restore
  • 1.6 Partial evaluation
• 2 Non-strict evaluation
  • 2.1 Normal order
  • 2.2 Call by name
  • the arguments to a function are not evaluated before the function is called — rather, they are substituted directly into the function body (using capture-avoiding substitution) and then left to be evaluated whenever they appear in the function
  • 2.3 Call by need
  • a memoized version of call-by-name
  • Haskell is the most well-known language that uses call-by-need evaluation. R also uses a form of call-by-need. .NET languages can simulate call-by-need using the type Lazy<T>.
  • 2.4 Call by macro expansion
• 3 Nondeterministic strategies
  • 3.1 Full β-reduction
  • 3.2 Call by future
  • 3.3 Optimistic evaluation