Raising errors in Mojo

raise Error(...) rejects input a function cannot honor.

def withdraw(balance: Int, amount: Int) raises -> Int:
    if amount <= 0:
        raise Error("amount must be positive")
    if amount > balance:
        raise Error("insufficient funds: " + String(balance))
    return balance - amount

def main() raises:
    print(withdraw(100, 30))
    print(withdraw(70, 70))

How it works

1. Each guard checks one precondition and raises with a message.
2. String(balance) folds a value into the error text.
3. The return is reached only when both guards let the call through.

Keywords and builtins used here

• Error
• Int
• String
• def
• if
• main
• print
• raise
• return
• withdraw

The run, in numbers

Lines

10

Characters to type

277

Tokens

70

Three-star pace

85 tpm

At the three-star pace of 85 tokens a minute, this run takes about 49 seconds.

Type this snippet

Step 1 of 3 in Errors, step 23 of 26 in Language basics.

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