Statemachine Dsl

Statemachine DSL

A small textual language for describing finite state machines: a named machine, the events it recognizes, optional commands, an initial state, and a set of states connected by transitions.

File structure

A file is one machine, in this fixed order:

statemachine <Name>

events
    <eventName>
    <eventName>

commands
    <commandName>

initialState <StateName>

state <StateName>
    actions { <commandName> }
    <eventName> => <TargetStateName>
end

• statemachine <Name> — required header naming the machine. <Name> is an identifier ([_a-zA-Z][\w_]*).
• events — optional block listing one or more event names, one per line (whitespace separated).
• commands — optional block listing one or more command names.
• initialState <StateName> — required; must reference a state defined in the file.
• state ... end — zero or more state definitions.

Syntax (EBNF)

statemachine_file = "statemachine", identifier,
                    [ events_block ], [ commands_block ],
                    "initialState", state_ref,
                    { state } ;

events_block      = "events", { event_name } ;
commands_block    = "commands", { command_name } ;

state             = "state", identifier,
                    [ actions_clause ],
                    { transition },
                    "end" ;

actions_clause    = "actions", "{", { command_ref }, "}" ;
transition        = event_ref, "=>", state_ref ;

event_name        = identifier ;
command_name      = identifier ;
event_ref         = identifier ;   (* must match a declared event *)
command_ref       = identifier ;   (* must match a declared command *)
state_ref         = identifier ;   (* must match a defined state *)

identifier        = letter, { letter | digit | "_" } ;
letter            = "A" | "B" | ... | "Z" | "a" | "b" | ... | "z" ;

There is no closing end for the whole machine — only each state block ends with end.

States and transitions

Each state is a state <Name> ... end block containing:

• An optional actions { <command> ... } clause listing one or more command names to run while in the state. Every command named here must be declared in the commands block.
• Zero or more transitions, each written <event> => <targetState>. The event must be declared in the events block; the target must be a state defined in the file.

Semantics and rules

• Names are case-sensitive identifiers.
• Every cross-reference must resolve: initialState, transition events, transition targets, and actions commands all point to declarations elsewhere in the file. A reference with no matching declaration is an error.
• A state may have multiple transitions, typically one per relevant event. Two transitions on the same event from the same state are ambiguous — avoid them.
• Comments use // (line) and /* */ (block).

Example

statemachine TrafficLight

events
    switchCapacity
    next

initialState PowerOff

state PowerOff
    switchCapacity => RedLight
end

state RedLight
    switchCapacity => PowerOff
    next => GreenLight
end

state GreenLight
    switchCapacity => PowerOff
    next => YellowLight
end

state YellowLight
    switchCapacity => PowerOff
    next => RedLight
end

When generating or modifying

• Declare every event and command before referencing it.
• Make sure initialState and every transition target name an existing state.
• When adding a state, give it at least the transitions needed to keep the machine reachable and not trap the user in a dead end unless that is intended.