The input type and output type of a function are fixed. So you can't write something like:
This is because the output type of f can be either always a Bool, as the first line implies, or always a number, as the second line implies.
See this FAQ for the correct approach.
  
    
      Last update:
      January 12, 2023
      
        
Created: January 7, 2023
  
  
  Created: January 7, 2023