Reading the Parentheses

You will find the procedure and its inputs in a call, then work out a nested calculation. You will also distinguish a call from a definition.

No programming experience is needed. Try each runnable example in the Playground, which shows its result.

Read a call

(+ 2 3)

5

The first item, +, names the procedure to call. A procedure performs an operation on the values you pass to it. Here those values, called arguments, are 2 and 3.

An ordinary call has this shape.

(operation input1 input2 ...)

Lispex evaluates the first item to get a procedure. It then evaluates the input expressions from left to right and calls the procedure with their values.

Read a nested call

(+ 10 (* 2 3))

16

Lispex looks up +, evaluates 10, and then evaluates (* 2 3). The multiplication returns 6. The outer call then adds 10 and 6.

The parentheses group the inputs of each call. Here they put the multiplication inside the addition, so its result becomes an argument to +.

Read a definition

The procedure below adds trim width and the total bleed across both edges. Both inputs use the same unit.

(define (width-with-bleed trim-width bleed)
  (+ trim-width bleed))

This defines width-with-bleed and prints nothing. The procedure takes two arguments, named trim-width and bleed. Its body, (+ trim-width bleed), runs when you call the procedure.

define is a special form. A special form has its own rules for which parts to evaluate. Here (width-with-bleed trim-width bleed) declares the procedure and its input names. It is not a call.

Other special forms include if, quote, and lambda. Each uses parentheses, but you must follow that form's rules to know what runs.

Try this

Predict the value before running the expression.

(+ 1 (* 2 (+ 3 4)))

In the definition above, identify the procedure name, its two input names, and the call in its body.

Continue to First Program to run a complete program.