A mostly-readymade solution to INDENT/DEDENT tokens in ANTLR v4. Just plug in the DenterHelper
and you'll be good to go! See this blog post for some of the motivations behind this project.
antlr-helper is released under the MIT license, which basically means you can do whatever you want with it. That said, I'd really appreciate hearing from you if you find this project useful! Maybe star the project?
<dependency>
<groupId>com.yuvalshavit</groupId>
<artifactId>antlr-denter</artifactId>
<version>1.1</version>
</dependency>
- Define INDENT and DEDENT tokens in your grammar
- In your
@lexer::members
section, instantiate aDenterHelper
whosepullToken
method delegates to your lexer'ssuper.nextToken()
- Override your lexer's
super.nextToken
method to useDenterHelper::nextToken
instead. - Modify your
NL
token to also grab any whitespace that follows (in other words, have it end in' '*
,'\t'*
or similar).
DenterHelper
is an abstract class, and it also takes three arguments for its constructor: the token types for newline, INDENT and DEDENT. It's probably easiest to instantiate it as an anonymous class. The whole thing should look something like this:
tokens { INDENT, DEDENT }
@lexer::header {
import com.yuvalshavit.antlr4.DenterHelper;
}
@lexer::members {
private final DenterHelper denter = new DenterHelper(NL,
MyCoolParser.INDENT,
MyCoolParser.DEDENT)
{
@Override
public Token pullToken() {
return MyCoolLexer.super.nextToken();
}
};
@Override
public Token nextToken() {
return denter.nextToken();
}
}
NL: ('\r'? '\n' ' '*);
There is also a builder available, which is especially useful for Java 8:
tokens { INDENT, DEDENT }
@lexer::header {
import com.yuvalshavit.antlr4.DenterHelper;
}
@lexer::members {
private final DenterHelper denter = DenterHelper.builder()
.nl(NL)
.indent(MyCoolParser.INDENT)
.dedent(MyCoolParser.DEDENT)
.pullToken(MyCoolLexer.super::nextToken);
@Override
public Token nextToken() {
return denter.nextToken();
}
}
NL: ('\r'? '\n' ' '*);
Big thanks to @Bluepuff71 for porting this to Python!
pip install antlr-denter
Next, in Antlr put:
tokens { INDENT, DEDENT }
@lexer::header{
from antlr_denter.DenterHelper import DenterHelper
from MyCoolParser import MyCoolParser
}
@lexer::members {
class MyCoolDenter(DenterHelper):
def __init__(self, lexer, nl_token, indent_token, dedent_token, ignore_eof):
super().__init__(nl_token, indent_token, dedent_token, ignore_eof)
self.lexer: MyCoolLexer = lexer
def pull_token(self):
return super(MyCoolLexer, self.lexer).nextToken()
denter = None
def nextToken(self):
if not self.denter:
self.denter = self.MyCoolDenter(self, self.NL, MyCoolParser.INDENT, MyCoolParser.DEDENT, ***Should Ignore EOF***)
return self.denter.next_token()
}
NL: ('\r'? '\n' ' '*); #For tabs just switch out ' '* with '\t'*
Credit to @K2017 for this port.
TODO
In Antlr:
tokens { INDENT, DEDENT }
@lexer::header {
using AntlrDenter.DenterHelper;
}
@lexer::members {
private DenterHelper denter;
public override IToken NextToken()
{
if (denter == null)
{
denter = DenterHelper.Builder()
.Nl(NL)
.Indent(MyCoolParser.INDENT)
.Dedent(MyCoolParser.DEDENT)
.PullToken(base.NextToken);
}
return denter.NextToken();
}
}
NL: ('\r'? '\n' ' '*); #For tabs just switch out ' '* with '\t'*
Note the injected code is dedented with respect to the @lexer::members
block. This is so that it has the proper formatting in the resulting C# Lexer file.
Basically, just use them. One bit worth noting is that when the denter injects DEDENT tokens, it'll prefix any string of them with a single NL
. A single NL
is also inserted before the EOF token if there are no DEDENTs to insert. For instance, given this input:
hello
world
universe
dolly
... the tokens would be (roughly):
"hello"
INDENT
"world"
INDENT
"universe"
NL
DEDENT
DEDENT
"dolly"
NL
<eof>
This is done so that simple expressions can be terminated by the NL
token without worrying about surrounding context (an impending dedent or EOF). In this case, universe
and dolly
represent simple expressions, and you can imagine that the grammar would contain something like statement: expr NL | helloBlock;
. Easy peasy!
What happens when you dedent to an indentation level that was never established?
someStatement()
if foo():
if bar():
fooAndBar()
bogusLine()
Notice that bogusLine()
doesn't match with any indentation level: it's more indented than if foo()
but less than its first statement, if bar()
.
This is a buggy program in python. If you to run such a program, you'll get:
IndentationError: unindent does not match any outer indentation level
The DenterHelper
processor handles this by inserting two tokens: a DEDENT
followed immediately by an INDENT
(the total sequence here would actually be two DEDENT
s followed by an INDENT
, since bogusLine()
is twice-dedented from fooAndBar()
). The rationale is that the line has dedened to its parent, and then indented. It's consistent with the indentation tokens for something like:
someStatement()
bogusLine()
If your indentation scheme is anything like python's, chances are you want this to be a compilation error. The good news is that it will be, as long as your parser doesn't allow "spontaneous" indents. That is, if the example just before this paragraph fails, then so will the half-dedent example above. In both cases, the parser rules will bork on an unexpected INDENT
token.
- Java/core: The real thing. This is what you're interested in. Maven artifact
antlr-denter
. - Java/examples: Contains a real-life example of a language that uses
DenterHelper
, so you can see a full solution, including the pom, how to set up the parser (which is nothing extra relative to usual antlr stuff) and how to define a language that uses these INDENT/DEDENT tokens. The language itself is pretty basic, but it should get the point across. Maven artifactantlr-denter-example-examples
. - Python3: The python3 implementation
- CSharp: The C# implementation; to be used as a class library in your projects.
The maven run is as simple as mvn install
(or your favorite goal).
Don't be shy about opening an issue!