2. Values

Goal

Show a sentence the program computes.

New in this chapter

• (apples times price) form paren spec/02 §2.3
• {"Total: ", …} form list spec/04 §4.4
• 3, 5.5 form value-number spec/01 §1.5
• set total to … form set-local spec/02 §2.3
• a plus b form plus spec/04 §4.5
• a minus b form minus spec/04 §4.5
• a times b form times spec/04 §4.5
• a divided by b form divided-by spec/04 §4.5
• 3 type number spec/04 §4.1
• list of text type list spec/04 §4.2
• text.Join package std/text std/README.md § std/text

Steps

We start from the last program of chapter 1. A local is a name for a value: set price to 4 creates price. Showing a number needs a conversion, because a caption is text; text.From turns any value into text. A phrase with a value inside another sentence goes in parentheses:

# package main
use std/lesson
use std/text

# Say hello, then work out the bill.
define main
  set price to 4
  lesson.Note "Hello, Synod"
  lesson.Note (text.From price)

Locals can be computed from other locals. times is one of four arithmetic words, written between its two values:

# package main
use std/lesson
use std/text

# Say hello, then work out the bill.
define main
  set apples to 3
  set price to 4
  set total to (apples times price)
  lesson.Note "Hello, Synod"
  lesson.Note (text.From total)

Showing "13" alone is not much of a sentence. text.Join glues a list of texts together. A list is written in braces, items separated by commas, and each item is one thing: a text, or a parenthesized sentence such as (text.From total):

# package main
use std/lesson
use std/text

# Say hello, then work out the bill.
define main
  set apples to 3
  set price to 4
  set total to (apples times price)
  lesson.Note "Hello, Synod"
  lesson.Note (text.Join {"Total: ", (text.From total)})

Now the whole bill. plus, minus and divided by work like times. An operator takes exactly two items, so a longer calculation nests parentheses, and there are no precedence rules to learn:

# package main
use std/lesson
use std/text

# Say hello, then work out the bill.
define main
  set apples to 3
  set price to 4
  set tip to 1
  set coupon to 2
  set people to 2
  set total to ((apples times price) plus tip)
  set share to ((total minus coupon) divided by people)
  lesson.Note "Hello, Synod"
  lesson.Note (text.Join {"Total: ", (text.From total)})
  lesson.Note (text.Join {"Each pays ", (text.From share)})

What to watch

Three captions in order: the greeting, Total: 13 and Each pays 5.5. Everything happened at the very first instant; the computation is not drawn, only its result.

Why it looks like this

A local that nobody reads is almost always a mistake, so Synod refuses it. Here coupon is set and never used:

hello/hello.synod:9:7: error[unused-name]: "coupon" is never used
  |
9 |   set coupon to 2
  |       ^^^^^^

The diagnostic is unused-name. Use the value, or delete the line (spec/02 §2.3).

Exercises

1. Change tip to 3. What does each person pay?

2. Write (apples times price plus tip) and run synod check hello. What happens?

3. Add lesson.Note (text.Join {"People: ", (text.From people)}).