Errors¶

stone finds most mistakes before your program runs, and stops with a clear message on the rest.

Errors before running¶

stone run, stone build, and stone check all parse and type-check the whole file first. If anything is wrong, they print every error with its position and run nothing. For example, this program multiplies a float by an int:

price = 2.5
count = 3
print(price * count)

stone check prices.st reports

prices.st:3:15: error: expected float, found int
  |
3 | print(price * count)
  |               ^^^^^

and exits with status 1. The fix is price * float(count). Syntax errors look the same: forgetting the ; after if x > 1 reports expected ';', found end of line.

Among the things the checker reports:

operands of the wrong type, like 1 + "a" or -"a"
mixing int and float without int() or float()
assigning a value of a different type to a variable
calling a function with the wrong number of arguments
reading a variable that might not be assigned yet, such as one only assigned inside an if
break or cont outside a loop, and range outside a for loop

Errors while running¶

Some mistakes depend on values only known while running:

def average(values);
    total = 0
    for v in values;
        total = total + v
    ret total / values.len()

print(average([4, 8, 9]))
print(average([]))
print("never printed")

prints the first average, then stops:

7
runtime_error.st: error: division by zero

A runtime error prints error: and a message to stderr and exits with status 1, with the same message under stone run and a compiled executable. See Runtime errors for every one.

That is the whole tutorial. The Syntax and Builtins pages cover the details.