I am planning on supporting global state in my peg generator. The way Rats!
does it with transactions looks pretty good to me but my question is how to
actually implement the state operations in a manner that work with
transactions. Specifically the state has to be pure in some sense so that it
can be "undone" by an abort operation.
My first naive idea is to have some State class (maybe defined by the user)
with a pointer to a child State class. The start() method of a transaction
would create a new State object and set parent->next to it. Then the user can
modify the new State object. commit() would be a no-op and abort() would reset
the parent->next pointer to null.
In psuedo-code
start(){
state = new State();
current->next = state;
return state;
}
abort(){
current->next = null;
}
commit(){}
# blah = a b c
# | d e f
rule_blah(){
# For each production, do a start/commit/abort sequence
state = start();
rule_a(); rule_b(); rule_c();
commit();
return
fail: # if any of the rules failed somehow
abort();
state = start();
rule_d(); rule_e(); rule_f();
commit();
return
fail:
abort();
}
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