In the equivalent C, Python, etc, the answer is clear: 3+4 gets evaluated. But, imagine we have a list that records all the results, Active 1 year, 1 month ago. The title text is a joke about Haskell's lazy evaluation. In Haskell, we can try giving an infinite list as the second argument and confirm that it does not get evaluated. Could you show me the pattern? The basic recursive definition is: f (0) <- 0 f (1) <- 1 f (n) <- f (n-1) + f (n-2) If evaluated directly, it will be very slow. However, until a particular element of the list is accessed, no work is actually done. Fibonacci, Using Lazy Evaluation. Haskell is a good example of such a functional programming language whose fundamentals are based on Lazy Evaluation. The key concept here is lazy evaluation which means that if the value is right there then take it without further computing say that i have got the value and the job is done, i don't need to compute future value temporary now. Lazy Evaluation. The evaluation mechanism in Haskell is by-need: when a value is needed, it is calculated, and kept ready in case it is asked for again. Using Haskell, we implement the Fibonacci sequence, Least Common Multiple (LCM), and the Greatest Common Divisor (GCD). Haskell infinite list of 1. As Dana Carvey would say “Well, isn’t that special!” For more info on lazy evaluation in Haskell… Ask Question Asked 10 years, 7 months ago. And, in this case, a lazy algorithm matched perfectly with Haskell’s lazy evaluation, and the problem was solved with a one line program! The basic concept is that a value is not computed until it is actually used. Lazy evaluation means Haskell will evaluate only list items whose values are needed. Lazy evaluation is an evaluation strategy which holds the evaluation of an expression until its value is needed. It avoids repeated evaluation. Note that the program does not actually attempt to multiply a infinite number of integers due to lazy evaluation. Lazy evaluation is commonly used in conjunction with list comprehensions in Haskell. time ./fibs 10000. real 0m0.010s. Another common example when demonstrating infinite lists is the Fibonacci sequence-- Wikipedia's page on Haskell gives two ways of implementing this sequence as an infinite list -- I'll add Prelude> fst (1+2, 3+4) 3 Prelude> fst (1+2, [1..]) 3 Lazy Evaluation. Thus, it is possible to have a name representing the entire infinite list of Fibonacci numbers. : is the list constructor that takes in an object and a list and returns a list with the object added to the head. If we define some list, ... Browse other questions tagged haskell lazy-evaluation fibonacci memoization pointfree or ask your own question. History. 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