LISP - every function

Lisp - The every Function

The every function is a powerful higher-order function in Common Lisp used to test if a given predicate holds true for every element of one or more sequences. It’s a concise and expressive tool for data validation and property testing.

Syntax

(every predicate sequence &rest more-sequences)

Arguments

• predicate: A function (or a symbol naming a function) that accepts as many arguments as there are sequences provided. It should return a non-nil value for true, and nil for false.
• sequence: The first sequence (list, vector, or string) to be tested.
• more-sequences: Optional additional sequences.

Return Value

Returns T if the predicate is true for all elements. If the predicate returns nil for any element, every immediately stops processing and returns nil.

Example 1: Basic Predicate Checks

Let’s test if all numbers in a sequence are even.

main.lisp

;; Using a built-in predicate 'evenp' on a list
(print (every #'evenp '(2 4 6 8 10)))
; => T

;; Using 'evenp' on a vector
(print (every #'evenp #(2 4 6 8 10)))
; => T

;; A failing case with an odd number
(print (every #'evenp '(2 4 5 6 8)))
; => NIL

Output

T
T
NIL

Example 2: Using a Custom lambda Predicate

You can define a custom test on the fly using a lambda function.

main.lisp

;; Check if all numbers are greater than 0 (positive)
(let ((numbers '(1 5 12 99)))
  (print (every #'(lambda (x) (> x 0)) numbers)))
; => T

;; Check if all strings have a length of 3
(let ((words '("cat" "dog" "ant")))
  (print (every #'(lambda (word) (= (length word) 3)) words)))
; => T

;; A failing case
(let ((mixed-words '("cat" "fish" "ant")))
  (print (every #'(lambda (word) (= (length word) 3)) mixed-words)))
; => NIL

Output

T
T
NIL

Example 3: Working with Multiple Sequences

When you provide multiple sequences, the predicate receives one element from each sequence at each step. The iteration stops when the shortest sequence is exhausted.

main.lisp

;; Check if corresponding elements in two lists are equal
(print (every #'= '(1 2 3) '(1 2 3)))
; => T

;; A failing case for equality
(print (every #'= '(1 2 3) '(1 5 3)))
; => NIL

;; Check if each element in the first list is less than the corresponding element in the second
(print (every #'< '(10 20 30) '(11 22 33)))
; => T

Output

T
NIL
T

Practical Applications and Best Practices

• Data Validation: Ensure all items in a dataset adhere to a specific format or constraint, e.g., (every #'valid-email-p user-emails).
• State Checking: In games or simulations, check if all objects share a certain state, e.g., (every #'is-enemy-defeated? active-enemies).
• Algorithm Preconditions: Verify that input data meets the requirements before running a complex algorithm.
• Pro Tip: The every function has siblings! Use some to check if the predicate is true for at least one element, notany to check if it’s false for all elements, and notevery to check if it’s false for at least one element.
• Debugging Tip: If every returns nil unexpectedly, you can debug by wrapping your predicate in a lambda that prints the values it’s testing, like (every #'(lambda (x) (format t "Testing: ~a~%" x) (evenp x)) my-list).