Functions that raise in Mojo

A def cannot raise unless its signature says so.

def factorial(n: Int) raises -> Int:
    if n < 0:
        raise Error("factorial needs a non-negative input")
    var result = 1
    for i in range(2, n + 1):
        result *= i
    return result

def main() raises:
    print(factorial(6))
    print(factorial(0))

How it works

factorial guards its input and raises an Error on a negative.
raises sits between the argument list and the return arrow.
Callers handle it or are marked raises themselves, as main is.

Keywords and builtins used here

Error
Int
def
factorial
for
if
main
print
raise
range
return
var

The run, in numbers

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Characters to type: 225
Tokens: 62
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Type this snippet

Step 6 of 6 in Variables & functions, step 6 of 26 in Language basics.

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