How do I specify the return type for fn functions in Mojo

Question

I am running a piece of code that calculates the factorial in Mojo:

fn fact(x:Int):
    var mul:Int=1
    for i in range(1,x+1):
        mul = mul*i
    return mul
print(fact(7))

But when I run this, I get the error: cannot implicitly convert 'Int' value to 'None' in return value

return mul

I tried using fn:Int, but that's not the correct syntax

Answer 1

In Mojo, the return type of a function is specified after the colon (:) following the function's parameter list. To specify the return type for the fact function in your code, you can use the -> operator followed by the desired return type.

Check this :

fn fact(x: Int) -> Int:
    var mul: Int = 1
    for i in range(1, x + 1):
        mul = mul * i
    return mul

print(fact(7))

Answer 2

In Mojo, the return type of a function is specified after the -> symbol, not after the function name. The correct syntax to specify a return type for your fact function would be to place -> Int after the parameter list, like so:

fn fact(x: Int) -> Int:
    var mul: Int = 1
    for i in range(1, x + 1):
        mul = mul * i
    return mul

print(fact(7))

With this syntax, you're correctly specifying that the fact function should return an Int, which will resolve the error you were seeing.