Why is lazy evaluation useful?

Question

I have long been wondering why lazy evaluation is useful. I have yet to have anyone explain to me in a way that makes sense; mostly it ends up boiling down to \"trust me\".<

Answer 1

If you believe Simon Peyton Jones, lazy evaluation is not important per se but only as a 'hair shirt' that forced the designers to keep the language pure. I find myself sympathetic to this point of view.

Richard Bird, John Hughes, and to a lesser extend Ralf Hinze are able to do amazing things with lazy evaluation. Reading their work will help you appreciate it. To good starting points are Bird's magnificent Sudoku solver and Hughes's paper on Why Functional Programming Matters.