Exercise 2: Name a calculation

Aim: evaluate nested arithmetic and bind intermediate values. Save ex02.fib. Write the value of each binding before running it.

Type and predict

(defun main () -> i64
  (let ((price 7)
        (quantity 3)
        (subtotal (* price quantity))
        (delivery 4))
    (do
      (println subtotal)
      (println (+ subtotal delivery))
      (println (- subtotal delivery))
      0)))

(* price quantity) multiplies its arguments. let binds names in order; subtotal can use the earlier names. The binding list is separate from the body. Integer literals here are inferred as i64, a signed 64-bit integer type.

Check your prediction

21
25
17
0

Change it

1. Make quantity zero, then five. Predict all three printed values for each run.
2. Introduce a discount binding. Print subtotal + delivery - discount as a nested expression. For the original values and discount 2, require 23.
3. Compare (* (+ 2 3) 4) with (+ 2 (* 3 4)). Explain why they differ.

Break and repair

Move the subtotal binding before price. It now refers to a name before that local binding exists. Record the diagnostic and restore the order. In a second copy, replace quantity with "three"; explain why multiplying a number and a string is a type error rather than a request to repeat text.

Checkpoint

Calculate the output for price 9, quantity 2 and delivery 6 on paper. Check it by running. Explain the scope of a name introduced by let.

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