Useful Links

How I will run your code

Save your program in a file called proj.adb in folders called proj1 and proj2 respectively for phases 1 and 2 of your project.

I will test your code in the same environment as the lab machines in MI 302, using the commands

  /usr/bin/gnatmake proj.adb

Phase 2 Assignment

For phase 2 of your project, you will write a program to generate and then execute a bottom-up parser. You should submit your program in a folder called proj2, and be sure that is runs as described above.


Your program will take as input the output from a call to bison -v that we saw in Lab 5. That is, we will let bison generate and describe the CFSM for bottom-up parsing in some language, and then your program will actually create this CFSM and attempt to parse a string of tokens in the language.

Specifically, your program should take as input a file called spec.output. The format of this file is the same as any other .output file produced when you run bison -v. The file contains, in order:

Your program must read in this spec.output file and create the CFSM that is described there. This CFSM will simply be an ordered array of states. You should make a new object type for a CFSM state that will contain information about the transitions and actions for that state.

After creating the CFSM, you should read a series of strings from standard in (which you may safely assume are all token names), and parse that string of tokens using the CFSM. This will involve maintaining a stack of symbol-state pairs as described in class, an indicator of the current (or next) state, and the saved value of the next "peeked at" token from the input. Your parser doesn't need to do any interpreting, but it should print out the symbols of the stack at every step in the process. These should be printed on a single line, separated by spaces, from the bottom of the stack to the top. So for instance a partial stack in parsing the Scheme language might be printed as

LP exprseq LP expr

And of course your program should also identify and report any parse errors along the way. After reading the token $end, your program should halt. That is, you only have to parse a single stream of tokens.

The CFSM state object should contain code to actually perform the parser actions. I suggest you store a list of symbol-action pairs in each state, where each action is either a positive integer, meaning to shift and go to that numbered state, or a negative integer, meaning to reduce by that (negative) numbered rule in the grammar. After peeking at the next token, the CFSM transition function should either produce a parse error if there is nothing to do for that kind of token, or else perform the specified shift or reduce action. For a shift, this means adding the next token to the top of the stack and updating the current state. For a reduce, this means looking at the specified grammar rule, popping a certain number of symbols off the stack, adding a certain non-terminal to be the next "peeked at" symbol on the input stream, and finally updating the current state to be the saved state from the top of the stack.


I suggest you write your program in the following steps. Of course you are free to develop however you wish. As always, you are encouraged to submit every time you get some small step of the program working.

  1. For starters, don't worry about reading the spec.output file. Instead, start by making your object definitions and the global storage for the stack, next state, and peeked-at input symbol.
  2. Once you have the very basics set up from the first part, have your code manually create a very very simple CFSM with a single state that just shifts every token it sees and transitions back to itself. Run and test your program to make sure it just continually shifts tokens (strings) that you type in, and prints them out after each step on the stack.
  3. Still manually creating CFSMs within your program, start adding a few more states to make sure you have the transitions correct. Next, add a single reduce rule with a non-terminal. Again, keep testing your program as you go along to make sure it works properly.
  4. Once you are confident that your program works on a manually-created CFSM, go back and try to read in the CFSM description from the spec.output file. I would suggest creating a spec.output CFSM description that exactly matches a manually-created CFSM that you have working. Then, piece by piece, remove the parts of your program that specify some part of the parser, and instead read that input from the spec.output file. Stop often for testing!
  5. Once you think you have everything working, try more examples to make sure. Remember that any of the .ypp bison specification files that we have used in class and in labs can generate a .output file that your program should be able to read. Examine the stack contents at each step to make sure the bottom-up parse is executing correctly.

Style Requirements

You do not have to create your program in the exact way that I have suggested. However, for full credit your program should


The file spec.output is bison's output for a very simple grammar for adding and subtracting numbers. If this file is in the current directory, and I compile and run your program as above, then by typing the string


into standard in, your program should write

exp OPA
st $end

to standard out, and exit with status 0.

If I typed the string


instead, your program should write

exp OPA
Parse error!

and exit with nonzero exit code.