Method: Kernel#catch

Defined in:

vm_eval.c

#catch([tag]) {|tag| ... } ⇒ Object

catch executes its block. If throw is not called, the block executes normally, and catch returns the value of the last expression evaluated.

catch(1) { 123 }            # => 123

If throw(tag2, val) is called, Ruby searches up its stack for a catch block whose tag has the same object_id as tag2. When found, the block stops executing and returns val (or nil if no second argument was given to throw).

catch(1) { throw(1, 456) }  # => 456
catch(1) { throw(1) }       # => nil

When tag is passed as the first argument, catch yields it as the parameter of the block.

catch(1) {|x| x + 2 }       # => 3

When no tag is given, catch yields a new unique object (as from Object.new) as the block parameter. This object can then be used as the argument to throw, and will match the correct catch block.

catch do |obj_A|
  catch do |obj_B|
    throw(obj_B, 123)
    puts "This puts is not reached"
  end

  puts "This puts is displayed"
  456
end

# => 456

catch do |obj_A|
  catch do |obj_B|
    throw(obj_A, 123)
    puts "This puts is still not reached"
  end

  puts "Now this puts is also not reached"
  456
end

# => 123

Yields:

• (tag)

Returns:

• (Object)

# File 'vm_eval.c', line 1970

static VALUE
rb_f_catch(int argc, VALUE *argv)
{
    VALUE tag;

    if (argc == 0) {
  tag = rb_obj_alloc(rb_cObject);
    }
    else {
  rb_scan_args(argc, argv, "01", &tag);
    }
    return rb_catch_obj(tag, catch_i, 0);
}