just had some cool ideas about a semantic rule specification language...i should write that up too... (also think about multi-threaded rewriting; no side-effects required
Some language translation problems can be described with a few rewrite rules that are predicated upon their context and potentially an arbitrary Boolean expression. For example, consider a few identity transformations such as
expr: // in context of expr, match left side, transformed to right side "x + 0" -> "x" "0 + x" -> "x"
You do not have to specify these rewrite rules inside a grammar, assuming you have a parser grammar that can parse the input (and has rule expr). The use of such concrete syntax notation is very easy for humans to look at. The only problem comes in when you have perhaps 100 of these rules. Their interaction can sometimes lead to bugs that are impossible to discover. Nonetheless, a number of academic systems exist that use a series of rewrite rules that live separately from the syntax grammar (e.g., Meta-Env (ASF+SFD) and Stratego). At least from the journal papers, it seems they are very powerful. Something must be preventing the average developer from using them, however.
Sometime I think I will spend a couple of weeks and try to build one myself ANTLR-style. The first thing I realize is that concrete syntax, while beautiful, is not always powerful enough to do what we want (I suppose it could be an option). For example, what if you want to match a series of identifiers and transform them to something else? You need some kind of repetition operator inside the strings of concrete syntax, but why invent new syntax. We have grammars to specify languages. Grammars both recognize and generate language as you see in ANTLR parser grammar to tree grammar rewrite rules. So, I propose something more along the lines of grammar to grammar transformations. Elements on the left are available as streams of data to the template on the right-hand side:
var:
type ID (',' ID)* -> (type ID ';')+
In this case, whatever was matched for the type rule would be replicated for each identifier match on the input stream. Input tokens int i,j would become output token stream int i; int j;. This token stream could then be repeatedly processed by the rules until nothing had changed, signifying the end of translation. Naturally, one could design rewrite rules that flip back and forth between two patterns causing an infinite loop. This is a well-known problem with rewrite systems. I'm satisfied to simply call that a bug in your rewrite rules, though I'm sure I could come up with something nice to let you know precisely which rules were the problem.
Rewrite rule precedence
Rewrite rules specified first would be given precedence over those specified later. There is therefore no notion of an ambiguous rewrite rule to match. The first one that matches, wins. So, be careful that you get the order right. In the following case, a cast expression will never be matched because it is mentioned after the parenthesized expression.
expr:
'(' expr ')' -> whatever
'(' ID ')' expr -> somethingelse // can't match
In general, you should put the longest matches first.
Generating text rather than tokens
You could also go to text instead of a token stream. In this case, the output grammar would actually be a StringTemplate (an "unparser"). For example, the following transformation would convert the token stream matched on the left hand push all of the elements into the attributes of the template on the right:
var:
type ID (',' ID)* -> "<type> <ID; separator=\", \">"
Grammatical context
A rewrite rule is applicable when the left-hand side matches the input in the context of the specified rule, such as expr above. The context means "what rule am I matching?" Context is important, because you might want to do something different depending upon context. For example, you might want to do do something different for variable declarations matched at the class level (fields) and for variables matched in methods (locals):
[classDef var]: // context is "field" type ID -> blort [... method var]: // context is "local" type ID -> blech
The "..." means anything on the stack of rule vacations. The entire context for the local variables then is a var rule match within a method rule.
One could also imagine wanting to combine contexts into a single translation if fields and locals are separated in the grammar:
[localVar | field]: type ID -> blort
In general, the stack context language is regular and rewrite rule candidates could be found for a given stack using a DFA looking from right to left on the stack. In other words, If the stack top has rule var, then the DFA would rule out any context do not end in var or "...".
Note that the rewrite matches only have to match starting at the left edge of the specified context rule. They do not have to match an exact alternative within that rule nor a complete alternative. You might only want to alter the first or second token:
classDef: 'class' ID -> "class _<ID>" // prefix with '_'
Arbitrary semantic content
Sometimes grammatical context is not enough. For example, you might want to change an identifier reference in an expression depending on symbol table information:
expr:
{is local or arg}? ID -> "locals.<ID>"
What if the right-hand side needs to reference something other than an element matched on the left-hand side? In this case, we need to allow actions to set attributes.
expr:
{is local or arg}? ID -> (fname={currentFunc.name}) "<fname>_locals.<ID>"
What is currentFunc, though? Normally that is a parser class instance variable set to the currently matched function by an action in the parser grammar. For constructs that can nest such as classes in some languages, the "current class" pointer needs to be treated like a stack. ANTLR has dynamically scoped very rules for this purpose; perhaps we could do something similar.
method:
scope { FunctionSymbol currentFunc; }
blah blah blah
The result of a pattern match could be an action or perhaps even just a context with no pattern:
method:
{System.out.println("heh, I just entered a method")} // always exec in method context
type ID {currentFunc = currentScope.resolve($ID.text);} // exec this but refer to ID
We might also need to be able to specify a predicate on token references:
expr:
ID<"null"> -> "nil" // test for null keyword as ID
^('*' INT<{is power of 2}> expr) -> (lg2={$INT.int}) "<expr> << <lg2>"
Rewrites for tree grammars
There is no reason we could not do rewrite patterns on trees. The left would always be a tree pattern but the right could be a tree pattern or template.
expr:
^('+' expr expr) -> add(blah blah blah) // rewrite with template ref
INT -> "0" // differentiate any integer to constant "0"
To rewrite to another tree:
expr:
^('+' INT<"0"> expr) -> expr
^('*' INT<"1"> expr) -> expr