Values, Names, and Procedures

Step 1 showed a complete program. This lesson explains the small set of shapes inside it. The key is to separate a value, a name that refers to a value, and a procedure call that computes a value.

Values can stand alone

Numbers, booleans, strings, characters, and quoted data are values:

42
#t
"Lispex"
#\L
'ready

A bare symbol such as ready is different: Lispex looks it up as a name. Writing 'ready preserves the symbol as data.

A list in call position asks for work

Read this from the innermost call outward:

(+ 10 (* 2 3))

First Lispex finds +, then evaluates 10, then evaluates (* 2 3) from left to right. The inner call produces 6, so the outer call becomes the equivalent of (+ 10 6).

16

Some parenthesized forms are not ordinary calls. define, lambda, let, and if control which parts are evaluated and where names are visible. The syntax map calls these out explicitly.

Give a value a name

define creates or updates a top-level binding:

(define fee 3)
(+ 20 fee)

23

The definition prints nothing; the final addition prints one value.

Use let when a name is needed only for one body:

(let ((base 7))
  (* base 2))

14

The binding base is visible inside the body and disappears afterward. That small scope makes a temporary decision easier to inspect.

Create a reusable procedure

Function-definition shorthand names both the procedure and its inputs:

(define (price-with-fee price fee)
  (+ price fee))

(price-with-fee 20 3)

23

The equivalent long form makes the procedure value visible:

(define price-with-fee
  (lambda (price fee)
    (+ price fee)))

A procedure checks its arity. Calling price-with-fee with one or three arguments is a runtime error rather than an implicit default.

Put the pieces together

(define (price-with-fee price fee)
  (+ price fee))

(list (price-with-fee 20 3)
      (let ((base 7))
        (* base 2)))

(23 14)

Observation order matters: the first list element is computed, then the second. list receives the two resulting values and constructs (23 14).

Exercise

Define a procedure named triple that multiplies its input by 3, then return (12 21) by calling it twice inside list.

(define (triple n)
  (* n 3))

(list (triple 4) (triple 7))

Ready for Step 3?

Continue when you can explain why a quoted symbol is data, why (* 2 3) runs before the outer addition uses it, and where a let name is visible. Next: Data, Conditions, and Decisions.