brom...@lavabit.com writes: > Hello. > > Why does it return -1? Could anyone explain? > (Comments are mine.) The general principle is to try and give sensible results when +,*,etc are called with a number of arguments other than 2. By thinking them through at this level you can, in theory, write more general code and have it "just work". Not only does it apply to various functions, but it is generally considered a sensible macro design principle too, as in the case of, among others, 'and' and 'or'.
> > (call-with-values (lambda () (values 4 5)) > (lambda (a b) b)) ; a is 4 and b is 5; return 5 > ⇒ 5 > > (call-with-values * -) > ⇒ -1 If you call * with 0 arguments, it returns one value, the number 1. This makes some sense since 1 is the identity for multiplication. i.e. (* 1 x) = x = (* x 1) for all x If you call - with one argument, it negates that argument, as if you had done (- 0 x). Putting both of these together, (call-with-values * -) is equivalent to (- (*)) scheme@(guile−user)> (*) $2 = 1 scheme@(guile−user)> (- 1) $3 = −1 scheme@(guile−user)> (- (*)) $4 = −1 > (call-with-values + +) > 0 Similarly to multiplication, if you call (+) with one argument, you get the identity for +, which is 0. This expression then, is equivalent to (+ (+)) = (+ 0) = 0 > (call-with-values + -) > 0 Similar to the above situation, you can think of this as being equivalent to (- (+)). Negating 0, naturally enough, gives you 0. scheme@(guile−user)> (+) $5 = 0 scheme@(guile−user)> (- (+)) $6 = 0 > (call-with-values - -) > ERROR: Wrong number of arguments to - While we extend / and - to have sensible results for the one argument case (reciprocal and negation), and we extend it for more than two arguments, there is not an identity for / or - [0], so we mark the zero argument case as errors for both. scheme@(guile−user)> (/) ERROR: In procedure /: ERROR: Wrong number of arguments to / scheme@(guile−user)> (-) ERROR: In procedure −: ERROR: Wrong number of arguments to − [0] Well, more correctly, they have right identities, of 1 and 0 respectively, but no left ones -- Ian Price -- shift-reset.com "Programming is like pinball. The reward for doing it well is the opportunity to do it again" - from "The Wizardy Compiled"
