I am developing a compiler and have already implemented lexer, parser and semantic analyzer(using listener and visitor) using ANTLR4. For code generation I am planning to generate LLVM IR using StringTemplate(ST).
To do so I am thinking of first constructing an AST and then generating the code.
My question here is do I need to construct AST?? or can I use Parse Tree?
If I need to use AST, I am not able to find any examples of manually constructing AST using visitors or listeners. Even a small grammar example will be very helpful.
Thank you.
No, there is no fundamental need to construct an AST. In the simplest case, you can walk the parse-tree and output the IR, directly or using ST.
Where transformations are required for output as IR, the two basic approaches are to (1) analyze and annotate the parse-tree describing the necessary changes; or (2) walk the parse-tree, constructing a separate AST, and then walk and transform the AST.
For the annotation strategy, extend ParseTreeProperty to create context node type specific property classes. See the comment in that class for how to use.
The AST strategy is not discouraged -- it was the primary strategy used in Antlr3 -- but is essentially unsupported in Antlr4. As for why Antlr4 favors the annotation strategy, see the last few paragraphs of this answer.
Related
UML classdiagrams are a standard graphical notation to describe classes and their relationships.
Is there a standard textual notation (DSL) to describe the same? Don't say XMI or EMF;-)
I think you could do that with Corba IDL and use Interfaces for classes, but this is somehow too much on the Corba side. You could use Java Interfaces, but this is too Java.
Background of my question is writing generators. I think it is easier to write a generator based on the syntax tree of a DSL than to parse a graphical notation. A graphical notation first has to be translated into a syntax tree (that would be the same you'd get from the corresponding DSL). I think translating a graphical notation into the syntax tree is harder than to translate a DSL (where you can use ANTLR).
You've got the answer already, but I'd like to clarify. There is a standard notation, it's called HUTN, and nobody uses it.
Check this complete list of textual notations to describe UML models. Btw, the reasons to create one of these tools (in particular TextUML) can be find here.
It is no coincidence that UML separates abstract and concrete syntax.
Tying up code generation to a user-facing notation is a bad idea. Tools (code generators) and people (modelers) have totally distinct needs, so no single syntax can serve both audiences well. Not to mention you lose the ability of applying the same code generator to models created using different notations.
TextUML is a concrete syntax tailored to modelers. XMI is a much better notation for tools, and the UML2 object model makes it very easy to handle.
Rafael
http://abstratt.com/blog
No standard notation to my knowledge but a good summary of options here.
hth.
ANTRLR newbie question. Say for a given grammar ANTLR maven plugin has created all the necessary Java classes to traverse and parse a text. And it works just fine when used as prescribed in "The Definitive ANTLR4 Reference".
Now imagine I need to reuse the generated classes to parse an expression which is defined by a rule buried somewhere deep in the grammar file.
However the Reference doesn't seem to provide a clue as to how to select a specific rule as a starting one, the generated classes always expect the whole grammar tree being present in the source.
Using the generated classes as-is doesn't work either, cause the corresponding listener and parser methods expect a context parameter which can only be created when having a "parent context" and an "invoking state" which I don't know how to define.
The only (and rubbish) solution I came up with so far, is splitting the grammar into two files so that the low-level rule in question would become top-level one, and import the latter into the first.
Do I miss something obvious here? Any help would be appreciated.
This is very simple. Load your input stream with the text that you wanna match against one of the subrules, then call the function for that subrule in the parser as you did with the main rule. Each grammar rule is represented by a function, which you can simply call for your subtext and it will generate a stripped down parse tree then, which applies only to this subrule (and it's children).
I'm working on a project in which I have to generate Abstract Syntax Tree for a given program. Here program can be in any mainstream programming languages. What should be the standard way of generating AST in ANTLR4? I know only basics of ANTLR4 and I'm able to generate Parse tree for a given program.
ANTLR 4 automatically generates parse trees instead of relying on manually-structured ASTs. This decision was made after observing years of development with prior approaches encountering extreme maintainability challenges, especially when multiple tree parsers were involved.
If you need an abstract representation of your source code, you should create an object model that accurately represents the constructs in your language, rather than rely on weakly typed and generally unstructured AST nodes. You then walk the parse trees instead of ASTs to create your object model.
I would not advise going with ANTLR 3 for any new project.
I have just started using antlr3 and am trying to serialize the AST output of a .g grammar.
Thanks,
Lezan
As Vladimir pointed out, you can use a a custom AST node class that has serialize capabilities builtin. You could also use a tree adaptor to create the types of nodes you need.
If you only need serialization, and not de-serialization, you could probably just do:
ast.toStringTree()
The above will give you a LISP like tree structure. An easy way to do serialization would be to use that in combination with a custom AST node class with an overridden toString(). Since toStringTree() uses the node's toStringTree method, it'll essentially serialize whatever you put in toString. Make its output sufficient and useful and you should be set.
CommonTree nodes produced by Parser are not Serializable.
I'd suggest you to serialize Tokens and use a secondary grammar for parsing the (deserialized) stream of Tokens later. In the book (The Definitive ANTLR Reference), in the Quick Tour for Impatient chapter, Terence Parr gives exactly this scenario -- without serialization though, but serialization is trivial for tokens as they are just text.
My understanding also that you can replace the Tree class with your own:
options {
ASTLabelType = MyOwnTreeClass;
}
But I haven't tried it.
I'm currently pondering how best to take an AST generated using Antlr and convert it into useful objects which I can use in my program.
The purpose of my grammar (apart from learning) is to create an executable (runtime interpretted) language.
For example, how would I take an attribute sub-tree and have a specific Attribute class instanciated. E.g.
The following code in my language:
Print(message:"Hello stackoverflow")
would product the following AST:
My current line of thinking is that a factory class could read the tree, pull out the name (message), and type(STRING) value("Hello stackoverflow"). Now, knowing the type I could instanciate the correct class (e.g. A StringAttribute class) and pass in the required attribute data - the name and value.
The same approach could be used for a definition factory, pulling out the definition name (Print), instanciating the Print class, and then passing in the attributes generated from the attribute factory.
Things do get a bit more complicated with a more complicated program:
Program(args:[1,2,3,4,5])
{
If(isTrue:IsInArray(array:{Program.args} value:5))
{
Then {
Print(message:"5 is in the array")
} Else {
Print(message:"More complex " + "message")
}
}
}
ANY/ALL help or thoughts are very welcome. Many thanks.
Previous related questions by me (Could be useful):
How do I make a tree
parser
Solving LL recursion problem
Antrl3 conditional tree rewrites
I recommend reading chapter 9, Building High-Level Interpreters, from Language Implementation Patterns by Terence Parr.
EDIT
Okay, to get you through the time waiting for that book, here's what you're (at least) going to need:
a global memory space;
function spaces (each function space will also have a (local) memory space);
and classes that spring to mind (in UML-ish style):
class Interpreter
global : MemorySpace
functions : Stack<Function>
...
class MemorySpace
vars : Map<String, Object>
...
class Function
local: MemorySpace
execute(): void
...
Here's one with ANTLR -> LLVM:
Once you have the AST, all you need is an iterator to walk the tree and the template to emit the objects you want.
This Tutorial is based on Flex and Bison but at the end he details how he converts his AST to LLVM assembly code, it might be helpful.