Chapter mini: calculator

Implement a small calculator function.

Your job is to write a function that takes two integers and an operator, then returns both a numeric result and whether the operation was valid.

Task

Implement this function:

func calculate(a, b int, op string) (int, bool)

The bool return should mean whether the operation succeeded.
Use these examples to define the behavior:

Call	Return
`calculate(8, 5, "+")`	`(13, true)`
`calculate(20, 4, "/")`	`(5, true)`
`calculate(10, 0, "/")`	`(0, false)`
`calculate(7, 3, "?")`	`(0, false)`

In main, call calculate with those same four inputs.
When a call succeeds, print result: <value>.
When a call fails, print error: invalid operation.

Expected output

result: 13
result: 5
error: invalid operation
error: invalid operation