control structure
catch catch-name catch-parameter*
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.