Signatures

Each state defines a signature requiring zero or more parameters. And all events leading into a state must conform to that state's signature.

The state's signature is specified only when it defines at least one parameter as a parenthesized parameter list on the state header line.

state Request redirect( new dest: Level Name )

The grammar:

state_header = "state" SP state_name signature? (SP DELETION)? EOL*
signature = '()' / '(' SP? parameter_set SP? ')'
parameter_set = parameter (',' SP parameter)*
parameter = parameter_name SP? ':' SP? type_name
parameter_name = name
type_name = name

A signature is a pair of parentheses directly after the state name, with no space between the name and the opening paren. Inside is a comma separated list of parameters, and each parameter is a name, a colon and a type name.

The spacing rules are worth reading off the grammar carefully, because they are not symmetric:

• Inside the parens, a space after ( and before ) is optional — ( new dest: Level Name ) and (new dest: Level Name) both parse.
• Around the colon, spaces are optional on both sides.
• Between parameters, the comma must be followed by exactly one space. , alone or , will fail.

Here are several real signatures from the elevator models, all of which parse:

state Update location( floor: Level Name )
state Set emergency stop( nearest safe stop : Level Name)
state Changing destination before cabin moves( new dest: Level Name )

Types

The type_name is just a name as far as this grammar is concerned. It isn't resolved or checked here — the parser hands the string along and a downstream module decides whether Level Name is a real type in this domain. Types like Level Name and Direction are modeled types from the domain, not programming language primitives.

By convention, a type name has each initial character in uppercase.

Empty signatures

The grammar explicitly allows ():

state OPENING()

There's no reason to write it, since a state with no parameters simply omits the signature, but if it turns up in a file it will parse. Be aware of one quirk if you're consuming the parse result: for the () case the signature field comes back as the literal string '()' rather than an empty list.

Matching the event

A signature describes what arrives with the event that causes the transition into this state. So if Try redirect carries a new destination, then the state that Try redirect leads to is the state that declares ( new dest: Level Name ). The activity (see Activities) then refers to the parameter with a caret, as in ^new dest.