values.md

Values

(princ (concatenate 'string "hello, " "world"))
;; => hello, world
;;
;;    "hello, world"

• concatenate can be used with different types, we need to specify the output type with the 'string symbol.

(princ (format nil "1 + 1 = ~a" (+ 1 1)))
;; => 1 + 1 = 2
;;
;;    "1 + 1 = 2"

• format is a function for creating formatted strings
• nil tells format to evaluate to the string it creates
• ~a is a format directive. It prints the next argument to format _a_esthetically.
• All format directives start with ~

(format t "~a" (/ 4.0 5.0))
;; => 0.8
;;
;;    NIL

• floating point numbers have a . in them
• when the second argument to format is t it prints the string to standard out
• in this case format evaluates to nil

(+ 1/2 1/4)
;; => 3/4

• Lisp has rational numbers as a separate type - they are written with a / between the numerator and the denomenator.

(and t nil)
;; => NIL

(and "ada" 5 "alonzo")
;; => "alonzo"

(and)
;; => T

(or t nil)
;; => T

(or nil "lucy" nil)
;; => "barry"

(or)
;; => NIL

(not nil)
;; => T

(not "amy")
;; => NIL

• There is no true or false - only t and nil
• t and nil are special symbols; they always evaluate to themselves
• anything that's not nil is treated as true!
• and and or can take multiple values
• and returns the first nil or the last non-nil argument
• or returns the first non-nil argument
• not will return nil for all non-nil values, and t for nil

See also

Common Lisp Hyperspec: FORMAT Common Lisp Hyperspec: Formatted output Practical Common Lisp: A Few Format Recipes