Description
In this assignment, you will implement a parser for a small language similar to SML using a parser
generator. The parser will generate a tree that represents the code. Typically, the tree will be passed onto
a semantic analyzer, but for the purpose of this assignment, the tree will simply be printed to a file.
The grammar you will implement as the following production rules:
program ::= binding_list
| ε
binding_list ::= binding_list binding
| binding
binding ::= val id = expr
| fun id id = expr
expr ::= if expr then expr else expr
| op_expr
op_expr ::= op_expr + op_expr
| op_expr – op_expr
| op_expr * op_expr
| op_expr / op_expr
| op_expr > op_expr
| op_expr < op_expr
| op_expr andalso op_expr
| op_expr orelse op_expr
| call_expr
call_expr ::= call_expr basic_expr
| basic_expr
basic_expr ::= ( expr )
| id
| int_const
| true
| false
All operators are left associative. The order of operator precedence from lowest to highest is:
1. orelse
2. andalso
3. > , <
4. +, –
5. *, /
The syntax rules for terminals are:
Keywords (val, fun, andalso, orelse, if, then, else, true, false): are as written
Identifiers (id): may start with any alphabetical character or underscore, and may contain any
number of alphabetical characters, numeric characters, or underscore.
Integer literals (int_const): may contain an arbitrary number of numeric characters
Operators and separators (=, +, –, *, /, >, <, (, )) are as written
Whitespace: space, tab (\t), new line (\n), and carriage return (\r) should be ignored
To implement your parser, follow the following steps:
1. Download and install flex, a free and open source lexer generator, and bison, a free and open
source parser generator. On some Linux systems, you can install them using your package
manager (e.g., sudo apt-get install flex , sudo apt-get install bison). On Windows
systems, you can obtain them at these links:
https://gnuwin32.sourceforge.net/packages/flex.htm,
https://gnuwin32.sourceforge.net/packages/bison.htm.
2. Edit the file parser.y to implement your parser. In the actions for most production rules, you
should call the corresponding functions declared in tree.h to build the tree. Some examples are
provided for you.
3. When you are done editing parser.y, use bison to generate the parser code by executing the
following command in the command line:
bison –d parser.y
This command will generate a C file called parser.tab.c that contains the implementation of
your parser. It will also generate a header file called parser.tab.h that is included in the
corresponding flex file to allow the lexer to pass tokens to the parser.
4. Edit the file lexer.flex to implement your lexer. In the actions for most regular expressions, you
should pass a token to the parser, and possibly a value associated with the token. Some examples
are provided for you.
5. When you are done editing lexer.flex, use flex to generate the lexer code by executing the
following command in the command line:
flex lexer.flex
This command will generate a C file called lex.yy.c that contains the implementation of your
lexer.
6. Compile parser.tab.c and lex.yy.c and tree.c using any C compiler of your choice. The
generated binary is your parser.
7. Test your program. The generated binary takes the input file name as the first argument
(test.sml by default) and the output file name as the second argument (test.dot by default).
The output file can then be passed to the graph visualization software dot to draw the tree (you
can also find online dot to PDF converters). You are provided with the following reference files to
help test your tool: test.sml, test-reference.dot, test-reference.pdf. Executing your tool
with test.sml as input should generate a file identical to test-reference.dot which, when
drawn, should generate a drawing similar to test-reference.pdf.
Submission Instructions
Submit your modified parser.y and lexer.flex files via Moodle. Do not submit any other files or
compressed folders. Make sure to include a comment in each file with your name and AUBnet ID (e.g.,
abc01). If you did the assignment in a group of two, only one group member should submit. In this case,
make sure to include both group members’ names and AUBnet IDs in each file.