Lazy Evaluation

from Haskell Programming from First Principles

• Haskell is a programming language.
• Functions are only called when they are actually needed, so you can handle arrays with an infinite number of elements.
• For example, the two expressions below are equivalent because of lazy evaluation:
  • take [13,26,..24*13]
  • take 24 [13,26..] (an array with an infinite number of elements)
• Both expressions only process the first 24 elements of the infinite array.
• Since Haskell assumes arrays of infinite length, functions like cycle and repeat in Haskell return infinite arrays (with the assumption that you will use take to truncate them).
• This concept of lazy evaluation is similar to how humans do mathematics.
  • Even if you have an infinite sequence of even numbers, evaluation only occurs when you try specific numbers.
  • #infinite sets