Side Effect

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Reading or writing in a stream makes a mark move along to show where the most recent read or write occurred. (That’s a side effect!) Writing to a screen leaves an irreparable mark on it—a long lasting side effect—and writing to paper is even more indelible—yet another long term side effect. Requiring a user to type on a keyboard is likewise irreversible. In short, side effects are unfortunately omnipresent in computing… Of course, it is possible to imagine living in an ideal world with no side effects, but we would suffer from a kind of computer-autism there since we would not be able to communicate the results of computations — Christian Queinnec, Lisp in Small Pieces