Handling error

[survuvi]

I would like to handle error with a message if the value passed to the function is not a number, but I get this error:

fn your_function(x: F32) -> StringRef:

    var result: StringRef = "value"
    
    if x == 0:
        return zero()
    
    elif x > 0 and x < 9 or x < 0:
        result = "small"
        return result

    else:
        result = "big"
        return result
    
fn zero() -> StringRef:
    
    let result: StringRef = "zero"

    return result

try:
    print(your_function(a))
except:
    print("Something went wrong")

error: Expression [16]:38:29: use of unknown declaration 'a'
print(your_function(a))

[mfranzon]

I think that the problem is because that this piece of code is part of a main (which is the process running under the notebook) so, the undefined variable error is raised by the master process not from the actual code you are running.
In fact, you can easily test this behavior with this simple piece of code:

def foo(a) -> object:
    return (a + 3.45) < [1, 2, 3]

a = 1 #  if commented Expression error

try:
    foo(a)
except:
    print("exception")

If you run this, the try/except works fine. If you run the function foo outside the try/except it raises a TypeError, if you do not assigned the variable, the master raises the Expression error

[survuvi]

I tried also raise but without success:

@raises
fn your_function(x: F32) -> StringRef:
    
    if not isinstance(x, (F32)):
        raise TypeError("Argument must be a number")

    var result: StringRef = "value"
    
    if x == 0:
        return zero()
    
    elif x > 0 and x < 9 or x < 0:
        result = "small"
        return result

    else:
        result = "big"
        return result
    
fn zero() -> StringRef:   
    let result: StringRef = "zero"
    return result

print(your_function(3)) #output should be: small

print(your_function("a")) #output should be: Argument must be a number

_error: Expression [6]:7:12: use of unknown declaration 'isinstance', 'fn' declarations require explicit variable declarations
if not isinstance(x, (F32)):

error: Expression [6]:41:24: invalid call to 'your_function': argument #0 cannot be converted from 'StringLiteral' to 'SIMD[f32, 1]'
print(your_function("a")) #output should be: Argument must be a number

Expression [6]:5:1: function declared here
fn your_function(x: F32) -> StringRef:_

[survuvi]

Now it works but with def, not works with fn

def foo(a) -> object:    
    if a == 0:
        print("zero")        
    elif a > 0 and a < 9 or a < 0:
        print("small")
    elif a >= 9:
        print("big")
try:
    a = "k"
    foo(a)
except:
    print("Something went wrong")

[mfranzon]

Yep, from my understanding def raise an exception error like in python. The fn functions has not the same behaviour but you can define an fn function that raises, as written in the changelog

Example:

fn raise_an_error() raises:
    raise Error("I'm an error!")
try:
   raise_an_error()
except:
    print("exception")

[maevadevs]

I ended up here after working on the HelloMojo notebook. Error Handling is not defined there except for one paragraph

Both [def and fn] support raising exceptions, but this must be explicitly declared on a fn with the raises function effect, placed after the function argument list.

I think it would be helpful to add @mfranzon 's example above on the notebook for reference. This makes more sense.

Making it more general:

fn raise_an_error(num: Int) raises -> Int:
    if num < 0:
        raise Error("I'm an error!")
    
    return num +1
    

try:
    print(raise_an_error(-1))
except:
    print("exception")