Project 1: A simple interpreter

In this project, you will write your first interpreter in this class, for the very small language (MathEx) we saw in class. This will allow you to practice thinking about how the representation of a language is used in an interpreter, and also to practice using ML. In fact, the only hard part of this assignment is getting used to using ML for this purpose.

1. Setup

In a directory for this class and project, copy the files from

~tvandrun/Public/cs365/proj1-mathex

This includes a variety of files. The follownig files provide some base code for the interpreter, mainly implementing the parser. You won't be modifying these:

mathex.grm.sml
mathex.lex.sml
parse.sml
sources.cm

The following contains the datatypes for the representation of the syntax:

absyn.sml

The following contains the code for the pretty-printer, as we saw in class:

prettyPrint.sml

This is the one you will be modifying:

interpret.sml

These test the interpreter and pretty-printer, respectively:

mathex-main.sml
mathex-print.sml

These are sample "programs" in MathEx.

a.mex
b.mex
c.mex
d.mex
e.mex
z.mex

2. Writing the interpreter

The file interpreter.sml contains the code for the interpreter. Only the function interpret is seen by the outside world-- it takes in a string (the name of a file) and returns an int, the result of executing the Mathex program in the file.

The work is done by the function interpretExpr and its helper functions interpetTerm and interpretFactor. As the file is, they just return zero. Your task is to write these functions the way they are supposed to be.

You will want to use pattern-matching. The patterns that interpretExpr, for example, can take include things like

interpretExpr(Expr(t, r))
interpretExpr(Expr(t, []))
interpretExpr(Expr(t1, (a, t2)::rest))
interpretExpr(Expr(t1, (Plus, t2)::rest))

You may write additional functions inside the struct, but it is not necessary in as simple a program as this.

3. Testing

Remember that in the file mathex-print.sml, the line

runTests(["z.mathex", "a.mathex", "b.mathex", "c.mathex", "d.mathex", "e.mathex"]);

executes all the test files in the list. It works the same way in mathex-main.sml. Then to run the interpreter on the listed test files, just enter at the command line

sml < mathex-main.sml

ML will spit out a verbose list of processing information, but you'll also see the result-- or a list of your mistakes.

You should also write testfiles of your own and test them.

4. Turn-in

Copy your interpret.sml to

/cslab/class/cs365/turnin/proj1/xxxxxx

where "xxxxxx" is [lily|chet|christopher|tim|david|ben|caleb]. I will grade your project by running it against a collection of test files.

DUE: Tuesday, Jan 26, 5:00 pm.