Assignment 2: Parser
Due: class time (2:50 pm), Thursday, February 11.
Project 2 is to build a parser for Appel's Tiger language using
The Tiger language is defined in Appendix A of the course textbook.
Documentation on ML-Yacc is available on the class web page, and also
(in less detail) in chapters three and four of the textbook.
Your parser should produce an abstract syntax tree using the
datatypes defined in the Absyn structure defined in the file
Skeleton files to get you started are available in the
$TIGER/chap3/ and $TIGER/chap4/ directories, which you can find at
Appel's textbook homepage. In particular, these files will be useful:
Remember to update your sources.cm file. Be certain to add
ml-yacc-lib.cm to it, in addition to any other new
files you are using. You'll want to remove
tokens.sml from it,
because a tokens file will be automatically generated by ml-yacc now.
Also, you'll need to update some header information in your lex file—see
the end of chapter 3 for details.
You should submit:
- A tiger.grm file, the ML-Yacc source for your parser.
- Any other source files you wrote to support your parser.
- A text file describing:
- The members of your team
- How you handled each shift-reduce conflict, and why you
think it is correct
- Anything you think is of interest about your parser
Anything subtle about your grammar, such as ambiguity resolutions
of various kinds, should be clearly commented. You knew that, of
Your parser should use the same error-reporting machinery you
employ in your lexer.
Words to the wise:
- Make no attempt to write the ML code for the semantic actions that
construct the AST when you start. Your first task should be to simply
write the grammar to parse the concrete syntax; just have each rule
produce the unit value ().
- Once your grammar is able to parse all of the test files in
$TIGER/testcases, then add semantic actions to the grammar to
construct the AST while parsing tiger source code.
- I estimate writing the grammar was about 2/3 of the time I spent
writing the parser; adding the semantic actions was about 1/3 of the
- You may find it helpful to write tiny toy grammars to explore
ideas. I did.
- If you don't understand how shift/reduce parsers work, and can't
figure out the table information left in the tiger.grm.desc file by
ML-Yacc, you are going to be in a world of pain.
- Be warned that there are some grammatically tricky bits to the
grammar that will require some thinking. I tried four different
grammars before I settled on my final solution. Had I been smarter,
I'd have thought more and hacked less.
- Is your LR parser efficiently parsing lists of arbitrary length
(e.g., declarations, or expression sequences)?
- Did you report the locations of syntax errors properly?
See the course text for more information, in particular chapters
three and four and appendix A.
Good luck; have fun.