Headline

Evaluate an expression as soon as it gets assigned or passed

Description

This is a very much simplified description. In a language with eager evaluation, each function (or method) application is evaluated such that the arguments are evaluated prior to invoking the body of the function (the method).

Illustration

Eager evaluation in Java

The following code throws NullPointerException because Java uses eager evaluation.

import java.util.LinkedList;

public class Demo {
	public static void noOp(int x) {
		// I don't care about my argument.
	}
		public static void main(String[] args) {
		LinkedList<String> l = null;
		noOp(l.size());
	}
}

Arguably, the execution should not throw because the argument is not needed anyway, but that's simply not the semantics of eager evaluation.

Lazy evaluation in Haskell

The de-facto counterpart in Haskell will not throw an exception because the argument of noOp will simply not be evaluated, as it is never needed.

noOp :: Int -> IO ()
noOp _ = return ()
main = noOp (length (undefined::[String]))

This example should not be considered a good example of why laziness is interesting from a programming perspective. The example is merely meant to hint at the semantics of laziness.

Relationshhips

These are synonyms for eager evaluation (in a broad sense):

See lazy evaluation for another major evaluation strategy.

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