Making Decisions with when

Programs often need to do different things in different situations: free delivery for big orders, a warning when stock runs low, a different greeting for new visitors. In this chapter you’ll learn how to ask questions and act on the answers.

Asking questions: comparisons

A comparison asks a yes-or-no question. Its answer is true or false:

total = 120
say total > 100
say total == 50

true
false

== and != work on any two values of the same kind, including text. <, <=, > and >= work on numbers.

Notice the difference between = and ==: = gives a value a name, while == asks whether two values are equal.

Combining questions: and, or, not

age = 25
has_ticket = true
sold_out = false

say age >= 18 and has_ticket
say not sold_out

true
true

To check that a number is between two others, use and: write 1 <= qty and qty <= 10. Quill doesn’t allow 1 <= qty <= 10, because it’s easy to misread.

and stops as soon as it knows the answer. In count != 0 and total / count > 10, if count is zero, the division never happens. That makes and a safe way to protect a calculation.

Acting on answers: when

when checks a list of cases from the top and runs the first one that’s true:

total = 45

when
  total > 100 -> say "Free delivery!"
  total > 30  -> say "Delivery: 5"
  else        -> say "Delivery: 10"

Delivery: 5

Here’s how to read it:

• The cases go on the lines below when, indented (moved in with spaces) further than when itself.
• Each case is a condition, then ->, then what to do.
• Read -> as “then”: “when total is more than 100, then say Free delivery”.
• Only the first true case runs. total is 45, so the first case is false and the second is true. Quill runs the second case and skips the rest, even though else would also match.
• else is the case for “none of the above”. It’s optional, and if you use it, it must come last.

If no case is true and there’s no else, nothing happens:

stock = 50

when
  stock < 10 -> say "Running low!"

say "Done"

Done

Why indentation matters

Indentation shows which lines belong to the when. The block ends when a line moves back to where when started:

temperature = 31

when
  temperature > 30 -> say "It's hot"
  else             -> say "It's pleasant"
say "Have a nice day"   -- not part of the when: this always runs

It's hot
Have a nice day

A block moves in by exactly 2 spaces. Not one space, not four, and not a tab. The cases of a when line up with each other, and a line that ends the block comes back to the same column as the when.

temperature = 31

when
    temperature > 30 -> say "It's hot"

Problem on line 4 of example.quill:

        temperature > 30 -> say "It's hot"
    ^^^^
This line is indented by 4 spaces.
A block moves in by exactly 2 spaces from the line above it, so this line should be indented by 2 spaces.

Changing values in a when

A case can change a var with set, but it can’t create a new value, because that value would only exist when that case runs. Create the value before the when, then set it inside:

total = 120
var delivery = 10

when
  total > 100 -> set delivery = 0
  total > 30  -> set delivery = 5

say "Delivery: {delivery}"

Delivery: 0

If you try to create a value inside a case, Quill explains what to do instead:

total = 120
when
  total > 100 -> delivery = 0

Problem on line 3 of example.quill:

      total > 100 -> delivery = 0
                     ^^^^^^^^
A case of a when can't create a new value.
delivery would only exist when this case runs. Create it before the when with var delivery = ..., then change it here with set.

Did you mean:  set delivery = 0

A when that gives a value

Often a when only exists to choose a value. Then you don’t need var and set at all. Put when where the value goes, and give a value after each ->:

total = 120

delivery = when
  total > 100 -> 0
  total > 30  -> 5
  else        -> 10

say "Delivery: {delivery}"

Delivery: 0

Read it out loud: “delivery is, when total is over 100, 0; when it’s over 30, 5; otherwise 10.” Quill picks the first case whose condition is true and uses its value. delivery never changes after that, so it doesn’t need var.

This works anywhere a value fits on its own: after =, after set, after say, and after return. A function that chooses between values can return the when directly:

fn grade(score: number) -> text
  return when
    score >= 70 -> "A"
    score >= 50 -> "Pass"
    else        -> "Fail"

say grade(85)
say grade(40)

A
Fail

Two rules keep the value certain to exist:

• The last case is else. Without it, no case might be true, and there would be no value:

total = 20
delivery = when
  total > 100 -> 0

Problem on line 2 of example.quill:

    delivery = when
               ^^^^
A when that gives a value needs else as its last case.
Without else, no case might be true, and there would be no value.

Did you mean:  else -> "..."

• Every case gives the same kind of value. delivery can’t be a number in one case and text in another:

total = 20
delivery = when
  total > 100 -> 0
  else        -> "ten"

Problem on line 4 of example.quill:

      else        -> "ten"
                     ^^^^^
Every case of this when has to give the same kind of value.
0 on line 3 is a Number, but "ten" is Text.

When each case should do something, like say, keep when on its own line as before. When each case should be something, put when after =.

A when inside a when

A case can hold another when, indented further:

logged_in = true
is_admin = false

when
  logged_in -> when
    is_admin -> say "Welcome, admin"
    else     -> say "Welcome back"
  else -> say "Please log in"

Welcome back

FizzBuzz, almost

FizzBuzz is a classic exercise: for a number, say “Fizz” if it divides by 3, “Buzz” if it divides by 5, “FizzBuzz” if both, and otherwise the number itself. % gives the remainder, so n % 3 == 0 asks “does n divide by 3?”:

n = 15

when
  n % 15 == 0 -> say "FizzBuzz"
  n % 3 == 0  -> say "Fizz"
  n % 5 == 0  -> say "Buzz"
  else        -> say n

FizzBuzz

The order of the cases matters: 15 also divides by 3, but the FizzBuzz case comes first. A for loop, in Repeating with for, runs this for every number from 1 to 100.

Conditions must be true or false

A case needs a yes-or-no question. A number on its own isn’t one:

count = 3
when
  count -> say "We have some"

Problem on line 3 of example.quill:

      count -> say "We have some"
      ^^^^^
A case of a when needs a condition that is true or false, but count is a Number.
Compare it to something, like count > 0.

Try it

1. Make a value age and use when to say “child” under 13, “teenager” under 18, and “adult” otherwise. Change age and run it again.
2. A shop gives 10% off orders over 200. Make a var price and use when to reduce it with set when it qualifies.
3. Rewrite FizzBuzz for a different number. What happens if you move the FizzBuzz case to the bottom?
4. Rewrite exercise 1 as stage = when ..., then say stage. Remove the else case and read what Quill says.