I am working on a simple Xquery processor and using Antlr4 to parse the grammar. I use the visitor pattern to walk through the parse tree. Now I want to rewrite a query if the query meet the some condition. The processor now can process a query if the query directly use the keyword like "join" and meet the "join" grammar.
I want to first rewrite the parse tree if the query can be changed to a join query or do nothing if not. Is there a way to manually manipulate the parse tree? Like adding a rule context or construct a new parse tree?
For Antlr4, the idiomatic approach is to decorate tree nodes with analysis products, rather than mutating the tree structure. That is, one or more tree walks can be uses to identify and mark the nodes that could be merged into a join and a final walk to output the results.
Of course, the parse-tree could be walked to generate a separate AST that, in turn, could be walked and further structurally modified. Antlr4 does not provide support for the building and walking of such an AST.
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So I have written my grammar in antlr4 syntax. Then I setup codegeneration, and now I can parse source files in my own defined language. This works great!
The next step I took is to create an object model from the expression tree. This is also working well.
However, now I want to generate an expression from my object model.
Can I generate code using the generated language parser objects API? Obviously, I can write methods that hand-generates strings. But I want to use a geenrated API based on the grammar to achieve some level of type safety and to detect errors when I make a grammar change.
I'm using the latest antlr4: antlr 4.7.1.
There's no generated solution. You have to wire this all up manually.
I am currently trying to implement a ruby compiler.To create the parser and lexer I used Antlr4. Now i am unable to figure out how to implement semantic analysis into the parser.Can someone explain how to do semantic analysis using the generated parser?It would be better if you can explain with a simple example, say how to check if a variable is initialized before use.
Well I can't describe everything you can and have to do but I will try to show you the principle behind it...
ANTLR generates a ParseTree for you which you can then process with a ParseTreeWalker. That walker will go through the parse tree node by node starting at the topmost, then processes through all children (Though that behaviour can be specified as far as I know). If you have registered a ParseTreeListener to the walker it will get notified about each step of it. There are two methods for each parser rule in your grammar: One that gets notfied whenever the parser enters this rule (before the children of that node are visited) and one when the parser exits the rule (after all children of the respective node have been visited).
This ParseTreeListener is where you can do your semantic analysis. You mentioned the check for undefined variables: For that you have to hook up your declaration rule, read out the variable name and store it in a List. Now you can hook up each rule that can contain a variable, read the name of it out and check whether it is in your list of declared variables. If not then the variable is undefined.
As an example on how something like that can be done you can have a look at a ParseTreeListener of mine here. The corresponding grammar can be found here.
Say I'd like to find instances of the expression while using the Java7 grammar:
FoobarClass.getInstanceOfType("Bazz");
Using a ParseTreeWalker and listening to exitExpression() calls sounded like a good first place to start. What surprised me was the level of manual traversal of the Java7Parser.ExpressionContext required to find expressions of this type.
What's the appropriate method to find matches to the above expression? At this point using a Regex in place of ANTLR4 yields simpler code, but this won't scale.
ANTLR 4 does not currently include feature allowing you to write concrete or abstract syntax queries. We hope to add something in the future to help with this type of application.
I've needed to write a few pattern recognition features for ANTLR 4 parse trees. I implemented the predicate itself with relative success by extending BaseMyParserVisitor<Boolean> (the parser in this example is called MyParser).
How do you define a directed acyclic graph (DAG) (of strings) (with one root) best in Haskell?
I especially need to apply the following two functions on this data structure as fast as possible:
Find all (direct and indirect) ancestors of one element (including the parents of the parents etc.).
Find all (direct) children of one element.
I thought of [(String,[String])] where each pair is one element of the graph consisting of its name (String) and a list of strings ([String]) containing the names of (direct) parents of this element. The problem with this implementation is that it's hard to do the second task.
You could also use [(String,[String])] again while the list of strings ([String]) contain the names of the (direct) children. But here again, it's hard to do the first task.
What can I do? What alternatives are there? Which is the most efficient way?
EDIT: One more remark: I'd also like it to be defined easily. I have to define the instance of this data type myself "by hand", so i'd like to avoid unnecessary repetitions.
Have you looked at the tree implemention in Martin Erwig's Functional Graph Library? Each node is represented as a context containing both its children and its parents. See the graph type class for how to access this. It might not be as easy as you requested, but it is already there, well-tested and easy-to-use. I have used it for more than a decade in a large project.
I want to implement an AST in Haskell. I need a parent reference so it seems impossible to use a functional data structure. I've seen the following in an article. We define a node as:
type Tree = Node -> Node
Node allows us to get attribute by key of type Key a.
Is there anything to read about such a pattern? Could you give me some further links?
If you want a pure data structure with cyclic self-references, then as delnan says in the comments the usual term for that is "tying the knot". Searching for that term should give you more information.
Do note that data structures built by tying the knot are difficult (or impossible) to "update" in the usual manner--with a non-cyclic structure you can keep pieces of the original when building a new structure based on it, but changing any piece of a cycle requires you to rebuild the entire cycle as well. Depending on what you're doing, this may or may not be a problem, of course.