Machine readable specifications

gaz Heyes gazheyes at
Fri Mar 22 04:51:43 PDT 2013

Hi all

I'd like to discuss a radical change on how JS specifications and others
are constructed. I suggest int based rules are used to define language
behaviour. I know this works with last state and next state tracking and
expected states too maybe other behaviour could be defined this way as
well. The first part of the specification should define a list of ints that
correspond to the state starting from 0 with 0 being a start state of
Nothing. The initial states should be specified as follows:

initalStates = {0:"Nothing",1:"FunctionExpression", 2:"FunctionStatement",
3:"FunctionStatementIdentifier" ...

Once every state is defined as an int, each state can be used in a lookup
table to determine the allowed last state and/or expected states. The
lookup table is used for error messages by the parser to convert the
corresponding int to human readable form. To define that FunctionStatement
can follow Nothing but FunctionExpression can't follow nothing and
FunctionStatementIdentifier is expected after FunctionStatement.

rules = {
        //FunctionStatement State
         2:    {
                   lastStates: {
                                          //FunctionStatement is allowed
after last state "Nothing"
                                          0: true
                   expectedStates: {
                                          //FunctionStatement expects the
next state to be FunctionStatementIdentifier

"lastStates" and "expectedStates" would also be ints but I added the text
for clarity, true could be shortened to 1 for compression. To use these
rules the parser can simply check the state machine which was the last
state and if the next state is valid. E.g.
lastState = 0;//Last state was Nothing
state = 2;//FunctionStatement state
if(rules[state].lastStates[lastState]) {
  // FunctionStatement is allowed to follow Nothing

lastState = 0;//Last state was Nothing
state = 1;//FunctionExpression state
if(!rules[state].lastStates[lastState]) {
   //FunctionExpression is not allowed to follow Nothing

The same technique could be used for expected states since you'd just need
to lookup the next state with the assigned expected state from the last
Once a machine readable specification has been done we could then generate
a human readable form of it based on the rules and it could be checked that
it conforms to what was intended. Both specifications (human readable and
machine readable) could be used by the implementer, when a specification
changes the parsers could automatically update based on the machine
readable form. Emulation for older parsers could be added within the
browser itself by using the new specification rules in a parser shim.



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