The error control structure

It permits to execute a statement or a bloc of statement
- when the open method fails or
- after any conditional assignement.
As any scriptol structure, it may has the form of a tagged bloc or a single instruction.

 instruction 
 error
    ...instructions...
  /error 
 or: error ? instruction
 or: error let instruction 
 or: error instruction

The ? sign or let keyword are optional.

The statement inside the body of the control structure is processed only if an error occurs, and thus if open returns nil.

Example: file myfile
myfile.open("tutor.txt", "r")
error die("file not found")

If the file "tutor.txt" is not found, the "die" function is called and the program exited, otherwise, it is ignored and it continues.


 Exercises

 

1) A a variable is assigned the value of another b variable.
If the assignment is impossible, "a unchanged" is displayed, otherwise the content is assigned.
Write it in 5 instructions.

Answer