Errors

A Function that can fail says what it throws with Throws, and the compiler makes its callers deal with it: catch it, pass it on with Try, or turn it into a Null with Try?.

Import std.core

Class NotEnoughGold Extends Error
End Class

Function Buy(gold: Int, price: Int): Int Throws NotEnoughGold
    If gold < price Then Throw NotEnoughGold("costs {price}, you have {gold}")
    Return gold - price
End Function

Try
    Print("left: {Try Buy(100, 30)}")
    Print("left: {Try Buy(20, 30)}")
Catch e As NotEnoughGold
    Print("can't buy it: {e.Message}")
Finally
    Print("the shop closes")
End Try

Print(Try? Buy(5, 10) ?? -1)

left: 70
can't buy it: costs 30, you have 20
the shop closes
-1

• Try f() gives f’s value, or throws its error on to the caller, which has to say it Throws it too.
• Try? f() gives f’s value, or Null when it throws.
• Catch e As SomeError catches that error and the ones extending it.
• Top-level code can use Try too: an error nothing catches ends the program, with where it was thrown.

Panics are bugs

An error is something that can happen to a working program: a missing file, a bad save. A panic is a bug: an index out of range, overflow in a debug build, Null where it can’t be. It stops the program, with exit code 101 and a stack trace of FutureBlitz’s own names, files, and lines, in release builds too.

Import std.core

Function Third(items: Int[]): Int => items[2]

Print(Third([1, 2, 3]))
Print(Third([1, 2]))

3
panic: index 2 is out of range for 2 elements at main.fb:3:38
    at Third (main.fb:3)
    at top-level code (main.fb:6)

Assert(condition) panics when what it says isn’t so. Next: Interfaces and generics.

Next: Interfaces and generics