Recipe

Choose how a function reports failure

Use when: you're writing a function that can't always produce its answer, and need to pick its return type

Goal: a panic, a Result or an Option, chosen by who can respond to the failure

Taught in Panic or Result?

1. 1

   Write down, in words, each way the function can fail to produce its answer.

   Example

   average(scores): the list might be empty. load_settings(path): the file might be missing or unreadable, or its text might not parse.

   Why: you can only choose a response once you know what you're responding to.

2. 2

   For each failure, ask whether it comes from outside the code (a file, input, the network) or can only happen if the code itself is wrong.

   • only a code mistake can cause it — panic, with expect or panic! and a message saying what should have been true.
   • it comes from outside and the caller needs to know why — return Result<T, E>.
   • "no answer" is ordinary and has one obvious reason — return Option<T>.

   Why: a bug needs a person to change the code; an outside failure needs a response from whoever called.

3. 3

   In the callers, use ? until you reach a function that knows the response, and match there.

   Why: the function that noticed the failure is rarely the one that knows what the user should see.

4. 4

   At the top, turn what's left into a message on standard error and an exit status.

   Why: main is the only place that knows it's talking to a person or a script.