ANTLR expression interpreter - antlr

Question

I have created the following grammar: I would like some idea how to build an interpreter that returns a tree in java, which I can later use for printing in the screen, Im bit stack on how to start on it.
grammar myDSL;
options {
language = Java;
}
#header {
package DSL;
}
#lexer::header {
package DSL;
}
program
: IDENT '={' components* '}'
;
components
: IDENT '=('(shape)(shape|connectors)* ')'
;
shape
: 'Box' '(' (INTEGER ','?)* ')'
| 'Cylinder' '(' (INTEGER ','?)* ')'
| 'Sphere' '(' (INTEGER ','?)* ')'
;
connectors
: type '(' (INTEGER ','?)* ')'
;
type
: 'MG'
| 'EL'
;
IDENT: ('a'..'z' | 'A'..'Z')('a'..'z' | 'A'..'Z' | '0'..'0')*;
INTEGER: '0'..'9'+;
// This if for the empty spaces between tokens and avoids them in the parser
WS: (' ' | '\t' | '\n' | '\r' | '\f')+ {$channel=HIDDEN;};
COMMENT: '//' .* ('\n' | '\r') {$channel=HIDDEN;};

Answer 1

A couple of remarks:
There's no need to set the language for Java, which is the default target language. So you can remove this:
options {
language = Java;
}
Your IDENT contains an error:
IDENT: ('a'..'z' | 'A'..'Z')('a'..'z' | 'A'..'Z' | '0'..'0')*;
the '0'..'0') should most probably be '0'..'9').
The sub rule (INTEGER ','?)* also matches source like 1 2 3 4 (no comma's at all!). Perhaps you meant to do: (INTEGER (',' INTEGER)*)?
Now, as to your question: how to let ANTLR construct a proper AST? This can be done by adding output = AST; in your options block:
options {
//language = Java;
output = AST;
}
And then either adding the "tree operators" ^ and ! in your parser rules, or by using tree rewrite rules: rule: a b c -> ^(c b a).
The "tree operator" ^ is used to define the root of the (sub) tree and ! is used to exclude a token from the (sub) tree.
Rewrite rules have ^( /* tokens here */ ) where the first token (right after ^() is the root of the (sub) tree, and all following tokens are child nodes of the root.
An example might be in order. Let's take your first rule:
program
: IDENT '={' components* '}'
;
and you want to let IDENT be the root, components* the children and you want to exclude ={ and } from the tree. You can do that by doing:
program
: IDENT^ '={'! components* '}'!
;
or by doing:
program
: IDENT '={' components* '}' -> ^(IDENT components*)
;