control structure

catch catch-name catch-parameter-list


You can use catch to mark the point in your program where execution will resume following a throw. A catch has a name, so you can have multiple active catch clauses with different names. Each throw must name the catch it throws to. A catch clause introduces a block of code which is executed once the throw is caught.

In addition to the catches you declare yourself, you can also catch two predefined OmniMark throws: #program-error and #external-exception.

A catch is active only when it is in the current execution scope of your program. OmniMark will look for the catch named in a throw in the current scope. If the named catch does not exist in the current scope, OmniMark looks for it in the parent scope, and so on. Once the named catchis found, OmniMark closes the current scope and all intervening scopes as required to reach the catch. Execution is then transferred to the block of code following the catch clause.

If the named catch is not found, OmniMark raises an error which will be caught by the first available catchof #program-error.

It is a compile-time error to use a catch that has not been declared.

You can catch the same catch name at different points in your program, but there can be only one catch of a particular catch name in any one lexical scope. Only the catch in the innermost execution scope is active at any given point in program execution.

Many OmniMark constructs can result in a rule or function being called recursively. Therefore, a single catch clause may exist at many different levels of execution scope when a program is run.

catch clauses are always at the end of a scope. That is to say, a catch clause is outside the normal flow of that scope. Execution does not fall through into a catch clause. The code in a catch clause can only be executed as a result of a throw to that catch. A throw initiated inside a catch clause or an associated always clause cannot be caught in the same local scope. throws within a catch or always clause must be caught at a wider scope of execution.