Maybe

Some questions have no answer. A price list might not contain the name you asked for. maybe says the answer might be missing, and Quill will not run until you handle both cases.

fn find_price(name: text) -> maybe number
  when
    name == "bread" -> return found 2.50
    else -> return missing

when find_price("bread")
  found price -> say price
  missing -> say "none"

2.50

-> maybe number means the function gives back a number, or it gives back nothing. found 2.50 is the number. missing is the gap. return missing is allowed only inside a function that gives a maybe back.

when followed by the call opens that maybe. found price holds the number for that one line. missing is the other case. Both cases are required:

fn find_price(name: text) -> maybe number
  when
    name == "bread" -> return found 2.50
    else -> return missing

when find_price("bread")
  found price -> say price

Problem on line 6 of example.quill:

    when find_price("bread")
    ^^^^
This when doesn't handle missing.
A maybe can be missing, and that case has to be handled too.

Did you mean:  missing -> say "none"

say find_price("bread") is a mistake too. say would hide the gap. Open the maybe with when first.

The kind after maybe can be text, number, true/false, a currency such as USD, or a list of one of those.

Try it

1. Call find_price("cake") and handle both cases. The cake is missing.
2. Change the function so "cake" is found at 4.
3. Remove the missing case and read what Quill says.