Exception handling

Raising and catching

In the following an exception of type E is thrown an then catched.

class E: Exception = {}

try
   raise table E{text = "Some message"}
catch e if e: E
   print(e.text)
end

The catch-part is only reached if the exception is of type E. It can also be stated by means of an unconditional catch.

try
   raise table E{text = "Some message"}
catch e
   if e: E
      print(e.text)
   else
      raise e
   end
end

But in case of an unconditional catch, the traceback so far will be clipped off.

Destructors

Destructors are called automatically during stack unwinding. The following program demonstrates the behavior.

class E: Exception = {}

class Duck = {
   function drop()
      print("A duck flies away.")
   end
}

function subroutine()
   d = table Duck{}
   raise table E{text = "Error"}
end

try
   subroutine()
catch e if e: E
   print(e.text)
end

The output is produced always in this order:

A duck flies away.
Error

Assertions

Assertions check at runtime for an invariant that shall stay true each time the program execution passes the specific point. When an assertion is not fulfilled, an exception is thrown. The syntax allows for brief and verbose exception messages:

assert type(x) is Int and x>=0

assert type(x) is Int and x>=0, "x: Int, x>=0"

assert type(x) is Int and x>=0, begin
   "Expected x: Int, x>=0, but got x = {}" % [x]
end

assert type(x) is Int and x>=0, begin
   argv = [type(x),x]
   "Expected x: Int, x>=0, but got x: {}, x = {}" % argv
end