In Scheme and Racket, a symbol is like an immutable string that happens to be interned so that symbols can be compared with eq? (fast, essentially pointer comparison). Symbols and strings are separate data types. One use for symbols is lightweight enumerations. For example, one might say a direction is either ‘north, ‘south, ‘east, or … Read more
You intend to execute two expressions inside the consequent part of the if, but if only allows one expression in the consequent and one in the alternative. Surrounding both expressions between parenthesis (as you did) won’t work: the resulting expression will be evaluated as a function application of the first expression with the second expression … Read more
This is an interesting way to represent data: as functions. Notice that this definition of cons returns a lambda which closes over the parameters x and y, capturing their values inside. Also notice that the returned lambda receives a function m as a parameter: ;creates a closure that “remembers’ 2 values (define (cons x y) … Read more
The following is an implementation of tail recursion modulo cons optimization, resulting in a fully tail recursive code. It copies the input structure and then appends the new element to it, by mutation, in the top-down manner. Since this mutation is done to its internal freshly-created data, it is still functional on the outside (does … Read more
What’s in a powerset? A set’s subsets! An empty set is any set’s subset, so powerset of empty set’s not empty. Its (only) element it is an empty set: (define (powerset aL) (cond [(empty? aL) (list empty)] [else As for non-empty sets, there is a choice, for each set’s element, whether to be or not … Read more
In this line, you’re calling multBins with (cons 0 x) and (rest y), and getting some result r, and then trying to call r: ((= (first y) 0) ((multBins (cons 0 x) (rest y)))) ; ^ ^ ; +— function application —–+ The same kind of thing is happening in the next line, where you’re … Read more
Why is it happening Scheme procedure/function calls look like this: (operator operand …) Both operator and operands can be variables like test, and + that evaluates to different values. For a procedure call to work it has to be a procedure. From the error message it seems likely that test is not a procedure but … Read more
TL;DR: They are different; use list when in doubt. A rule of thumb: use list whenever you want the arguments to be evaluated; quote “distributes” over its arguments, so ‘(+ 1 2) is like (list ‘+ ‘1 ‘2). You’ll end up with a symbol in your list, not a function. An in-depth look at list … Read more