The MOOSE Parser object is responsible for interacting with the HIT input file parser and constructing the necessary Actions that ultimately execute to build up the objects which compose a complete MOOSE based simulation. It is important to note that +this+ object is not responsible for the raw file-base I/O. The underlying structure of the MOOSE input file is dictated by HIT and information on the format can be found [here](/application_usage/input_syntax.md optional=True). The parser abstraction expects information to be organized into a hierarchy of blocks with zero or more children at each level. All of the name/value pairs at each level are used to construct one or more complete objects.
The Parser works by using a list of registered syntax blocks with C++ types. Each time the Parser is handed a block, it will consult the registered syntax for a list of objects to build. MOOSE is delivered with a very low-level list of syntax where each type of object is exposed directly to the input file by system type (e.g. Kernels, BCs, Outputs, etc). Each application that builds upon MOOSE can augment or replace this syntax completely. The registered list of syntax can be viewed on the mooseframework.org website, or can be dumped on the command-line in a number of different formats. The registration itself occurs in Moose.C.
See [Brace Expressions](input_syntax.md optional=True)
The Parser supports the ability to selectively activate or deactivate individual blocks without the use of block comment characters through the use of the special "active" and "inactive" parameter blocks. Either of these parameters (but not both) can be added to any block (including the top level) to selectively turn individual blocks on/off.
When multiple inputs are supplied to a MOOSE application, the parser will read them in successively and add merge them into a single block hierarchy. Later inputs add to and override parameters to previous inputs. This permits the user to factor out common parts of a set of inputs and reuse them in multiple simulations.