To give the short answer, macros are used for defining language syntax extensions to Common Lisp or Domain Specific Languages (DSLs). These languages are embedded right into the existing Lisp code. Now, the DSLs can have syntax similar to Lisp (like Peter Norvig’s Prolog Interpreter for Common Lisp) or completely different (e.g. Infix Notation Math … Read more
A list in Scheme is either the empty list () (also known as nil in some Lisps), or a cons cell whose car (also known as first) is an element of the list and whose cdr (also known as rest) is either the rest of the list (i.e., another list), or an atom that terminates … 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
I think @Vijay’s answer is the best one so far and I just intend to Erlangify it. Pairs (cons cells) in Erlang are written as [Head|Tail] and nil is written as []. There is no restriction as to what the head and tail are but if you use the tail to chain more cons cells … Read more
Here’s a way (following the code structure by @coredump from their answer; runs about 4x faster on tio.run): (defun permutations (list callback) (if (null list) (funcall callback #()) (let* ((all (cons ‘head (copy-list list))) ; head sentinel FTW! (perm (make-array (length list)))) (labels ((g (p i &aux (q (cdr p))) ; pick all items in … Read more
Multiple Values in CL The language Common lisp is described in the ANSI standard INCITS 226-1994 (R2004) and has many implementations. Each can implement multiple values as it sees fit, and they are allowed, of course, to cons up a list for them (in fact, the Emacs Lisp compatibility layer for CL does just that … Read more
What you are missing is that in Lisp parentheses are meaningful. In C/Java/Python &c, the following expressions are the same: a+b (a+b) (a)+(b) (((a)+(b))) (((((a)+(b))))) In Lisp, the following expressions are very different: a — a symbol (a) — a list with a single element, which is the symbol a (1 (2)) — a list … Read more
The Bad test-a is self-modifying code. This is extremely dangerous. While the variable x disappears at the end of the let form, its initial value persists in the function object, and that is the value you are modifying. Remember that in Lisp a function is a first class object, which can be passed around (just … Read more
‘(0 0 0) is a literal object, which is assumed to be a constant (albeit not protected from modification). So you’re effectively modifying the same object every time. To create different objects at each function call use (list 0 0 0). So unless you know, what you’re doing, you should always use literal lists (like … Read more
Are you doing something like this: CL-USER> (defun foo () (let ((value ‘(1))) ; ‘(1) is literal data (incf (car value)))) FOO CL-USER> (foo) 2 CL-USER> (foo) 3 CL-USER> (foo) 4 CL-USER> (foo) 5 Quoted data is literal data; there’s only one copy of it, and the consequences of modifying it are undefined. The behavior … Read more