-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 Rein explains effects well enough. Now for side-effects: a side-effect is what is allowed to happen in the absence of a weak equivalence between call-by-name (including memoised versions) and call-by-value[0]. Let's play "spot the side-effect!" - -- Haskell function for adding two numbers. f x y = x + y - -- function in a language that happens to be syntactically like Haskell. f x y = print "Hallo!" >> x + y - -- program to evaluate the function. - -- it just happens to be equal in Haskell and the Haskell-like language. main = let a = f 4 2 in print a >> print a evaluation-strategy | call-by-need | call-by-value - ------------------------------------------------------------- x + y | 6\n6 | 6\n6 print "Hallo" >> x + y | Hallo!\n6\n6 | Hallo!\n6\nHallo!\n6 See how the latter function differs in call-by-need and call-by-value? This is because it has a *side-effect* of printing stuff. This is not allowed in Haskell. And in fact, the latter function is impossible in Haskell. As Rein was getting at by abstraction leaks -- side-effects have an observable interaction with the outside world. [0] https://www.cs.indiana.edu/~sabry/papers/purelyFunctional.ps - -- Alexander alexander@plaimi.net https://secure.plaimi.net/~alexander -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQIcBAEBCgAGBQJWgmfhAAoJENQqWdRUGk8BZfkQAOft2e7RmRheu0ZGAXXj3P4m 4zPnJ5SQ//+TTM2rzH1b6ifoidGgEhKiyDr2IajXbj0wIbPLXqORByXUFx6E6I1q rz9rslEb2XGwxTOm/Wi90IGO6niEPO1+/WCFCLE2G+Fh2M3uo381djlQfaS1MVla 1UtzElnX2bnpLfjeSjMFFB2joEuCCl8Gz+QXVGIX1H3BE+xSyM+vllDtkwob/6Bn OyPckgGyiQlt5RpPoBsdEBU5qzrT5yJWaRWeiRIg293XrOPls3kM0GvhvaAFXmWr 1A/L391BQtw65xeGTlPArCi+xemwVsgwK2hdQWPmTrpJATveV0vN1OhQMkiI5b+Q 5SfKo0MKKWezRu7avIfaJ0IUB5Pl/FM+IhMFoHWM4oE5ixh7Vr91nslhOMjvAWgR GzKyMuY63CLoy/1He5nNoCJ2Pjwdf65lUOD/sNJhjLbd2qw220UFE4L2SOLXLX5U uBY1GF3C+biH2ai7utKU9RBXyV7p5dVcV4vft+Eb118QJmp3fFP26HS6IwuD3V8q JNWAo7ZTK5vpujUHtee2J1ltHrleSlVaaJE0ONDzKDzi7QrwUAUbae3/Wnzy5zBF ZXSPqABh8MNjkItax5zQwHyvgcJTRNtt7xj41+d7WKscgy5XvDQC3/RSQPXYZ9Ib upR6CDDSP2bns7KfRDU0 =AGvd -----END PGP SIGNATURE-----