Lazy sequences

Lazy Sequences

What is that?

• An expression´s evaluation is postponed until it is actually needed

• Can build complex sequences and pay only for the elements you actually need.

• Lazy techniques imply pure functions.

When to be lazy?

• When producing large, infinite or variable-sized sequences.

Fibonacci

Fibonacci Definiton

• Base : F0 = 0 ; F1 = 1• Induction : For n > 1 , Fn = Fn-1 + Fn-2

Non Lazy

Lazy

Lazy – Even better

Lazy

• There is no such thing as a free lunch. But with lazy sequences, you can have an infinite menu and pay only for the menu items you are eating at a given moment.

PseudoCode

• a = fibo[1000000]• if true return a• else return

PseudoCode

• So, if it is false, why do I need to calculate fibo[1000000] , if I won´t use it?

• Be Lazy

Some more code

• (def lots-o-fibs (take 1000000000 (fibo)))• This will take a LOT of time, won´t it?

• (nth lots-o-fibs 100)

Questions?

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