On Apr 2, 2012, at 3:25 AM, Pierpaolo Bernardi quoted: >> I thought that the position of the ufo can be find at (posn-y p) and >> (posn-x) because the postition is a struct of Posn.
and replied: > You must pay more attention to small details. What is the p in (posn-y > p)? Isn't an argument missing in (posn-x)? > What is the u in (ufo-loc u) (that's the variable that DrRacket says > it doesn't know). When I teach structs, I tell my students to write down the data definition (in a natural-language comment), then a define-struct, then contracts for all the "functions that come for free." For example, if I were defining "posn" from scratch, it would look like ; A posn has two numbers (x and y). (define-struct posn [x y]) ; make-posn : number(x) number(y) -> posn ; posn-x : posn -> number ; posn-y : posn -> number ; posn? : anything -> boolean That's in the HtDP student languages, of course; in #lang racket the constructor would be named posn rather than make-posn. Once you've written these contracts, make sure you're actually following them. "(posn-x)" doesn't follow its contract because posn-x is supposed to take a parameter. "p-x" doesn't follow the contract because it's not defined at all. "(posn-y u)" doesn't follow the contract because u is a ufo, and posn-y is supposed to take in a posn. And so on. Stephen Bloch sbl...@adelphi.edu ____________________ Racket Users list: http://lists.racket-lang.org/users
