
Allegro CL 
ANSI Common Lisp 6 Iteration 6.1 The LOOP Facility 6.1.1 Overview of the Loop Facility
6.1.1.7 DestructuringThe dtypespec argument is used for destructuring. If the dtypespec argument consists solely of the type fixnum, float, t, or nil, the oftype keyword is optional. The oftype construct is optional in these cases to provide backwards compatibility; thus, the following two expressions are the same:
;;; This expression uses the old syntax for type specifiers. (loop for i fixnum upfrom 3 ...) ;;; This expression uses the new syntax for type specifiers. (loop for i oftype fixnum upfrom 3 ...) ;; Declare X and Y to be of type VECTOR and FIXNUM respectively. (loop for (x y) oftype (vector fixnum) in l do ...) A type specifier for a destructuring pattern is a tree of type specifiers with the same shape as the tree of variable names, with the following exceptions:
Destructuring allows binding of a set of variables to a corresponding set of values anywhere that a value can normally be bound to a single variable. During loop expansion, each variable in the variable list is matched with the values in the values list. If there are more variables in the variable list than there are values in the values list, the remaining variables are given a value of nil. If there are more values than variables listed, the extra values are discarded. To assign values from a list to the variables a, b, and c, the for clause could be used to bind the variable numlist to the car of the supplied form, and then another for clause could be used to bind the variables a, b, and c sequentially.
;; Collect values by using FOR constructs. (loop for numlist in '((1 2 4.0) (5 6 8.3) (8 9 10.4)) for a oftype integer = (first numlist) and b oftype integer = (second numlist) and c oftype float = (third numlist) collect (list c b a)) ((4.0 2 1) (8.3 6 5) (10.4 9 8)) Destructuring makes this process easier by allowing the variables to be bound in each loop iteration. Types can be declared by using a list of typespec arguments. If all the types are the same, a shorthand destructuring syntax can be used, as the second example illustrates.
;; Destructuring simplifies the process. (loop for (a b c) oftype (integer integer float) in '((1 2 4.0) (5 6 8.3) (8 9 10.4)) collect (list c b a)) ((4.0 2 1) (8.3 6 5) (10.4 9 8)) ;; If all the types are the same, this way is even simpler. (loop for (a b c) oftype float in '((1.0 2.0 4.0) (5.0 6.0 8.3) (8.0 9.0 10.4)) collect (list c b a)) ((4.0 2.0 1.0) (8.3 6.0 5.0) (10.4 9.0 8.0)) If destructuring is used to declare or initialize a number of groups of variables into types, the loop keyword and can be used to simplify the process further. ;; Initialize and declare variables in parallel by using the AND construct. (loop with (a b) oftype float = '(1.0 2.0) and (c d) oftype integer = '(3 4) and (e f) return (list a b c d e f)) (1.0 2.0 3 4 NIL NIL) If nil is used in a destructuring list, no variable is provided for its place.
(loop for (a nil b) = '(1 2 3) do (return (list a b))) (1 3) Note that dotted lists can specify destructuring.
(loop for (x . y) = '(1 . 2) do (return y)) 2 (loop for ((a . b) (c . d)) oftype ((float . float) (integer . integer)) in '(((1.2 . 2.4) (3 . 4)) ((3.4 . 4.6) (5 . 6))) collect (list a b c d)) ((1.2 2.4 3 4) (3.4 4.6 5 6)) An error of type programerror is signaled (at macro expansion time) if the same variable is bound twice in any variablebinding clause of a single loop expression. Such variables include local variables, iteration control variables, and variables found by destructuring. 