The grammar language is the corner stone of Xtext and is defined in itself. Actually it is a DSL to create DSLs, so what would be more helpful than developing the Xtext language with Xtext?
The grammar language is a domain-specific language, carefully designed for the description of textual languages based on LL(*)-Parsing that is like ANTLR3's parsing strategy. The main idea is to describe the concrete syntax and how an EMF-based in-memory model is created during parsing.
To get an idea of how it works we’ll start by implementing a simple example introduced by Martin Fowler. It’s mainly about describing state machines used as the (un)lock mechanism of secret compartments. People who have secret compartments control their access in a very old-school way, e.g. by opening the door first and turning on the light afterwards. Then the secret compartment, for instance a panel, opens up. One of those state machines could look like this:
events
doorClosed D1CL
drawOpened D2OP
lightOn L1ON
doorOpened D1OP
panelClosed PNCL
end
resetEvents
doorOpened
end
commands
unlockPanel PNUL
lockPanel PNLK
lockDoor D1LK
unlockDoor D1UL
end
state idle
actions {unlockDoor lockPanel}
doorClosed => active
end
state active
drawOpened => waitingForLight
lightOn => waitingForDraw
end
state waitingForLight
lightOn => unlockedPanel
end
state waitingForDraw
drawOpened => unlockedPanel
end
state unlockedPanel
actions {unlockPanel lockDoor}
panelClosed => idle
end
So, we have a bunch of declared events, commands and states. Within states there are references to declared actions, which should be executed when entering such a state. Also there are transitions consisting of a reference to an event and a state. Please read Martin's description if it is not clear enough.
In order to get a complete IDE for this little language from Xtext, you need to write the following grammar:
grammar my.pack.SecretCompartments
with org.eclipse.xtext.common.Terminals
generate secretcompartment "http://www.eclipse.org/secretcompartment"
Statemachine :
'events'
(events+=Event)+
'end'
('resetEvents'
(resetEvents+=[Event])+
'end')?
'commands'
(commands+=Command)+
'end'
(states+=State)+;
Event :
name=ID code=ID;
Command :
name=ID code=ID;
State :
'state' name=ID
('actions' '{' (actions+=[Command])+ '}')?
(transitions+=Transition)*
'end';
Transition :
event=[Event] '=>' state=[State];