CS 321: Programming Languages - Homework 2: Functions, Errors, Conditionals, Predicate and Multiplication
CS 321: Programming Languages — Homework 2
1 Functions with Multiple Arguments (with Deferred Substitutions)
Start with the f1WAE interpreter for deferred substitution, and extend the implementation to support any number of arguments to a function (including zero), and any number of arguments (including zero) in a function application:
<FunDef> = {deffun {<id> <id>*} <fnWAE>} <fnWAE> = <num>
| {+ <fnWAE> <fnWAE>} | {- <fnWAE> <fnWAE>} | {with {<id> <fnWAE>} <fnWAE>} | <id> | {<id> <fnWAE>*}
As with homework 1, you must change the f1WAE datatype, and you must thus provide a parse func- tion that produces values of your modified f1WAE datatype. It must accept a quoted expression and pro- duce an fnWAE value. Similarly, you must provide a parse-defn function to parse definitions.
See homework 1 for details on these functions.
You must also provide an interp-expr function with signature:
fnWAE (listof FunDef) -> Number
This function should be a very simple wrapper for your interp function, which our tests will call on the
results of your parsers. If yours is more than a few lines (one line is possible), you may be overthinking it.
2 Errors
Your interpreter and parser must obey the formats and precedence rules described in homework 1. Otherwise, assume that the input to your parser is a well-formed program.
3 Conditionals
Add if0, a conditional expression. It has three subexpressions: <fnWAE> = ...
| {if0 <fnWAE> <fnWAE> <fnWAE>}
1
Evaluating an if0 expression evaluates the first subexpression; if it produces 0, then the result of the entire expression is the result of the second subexpression. Otherwise, the result is the result of the third subexpression.
Examples:
(test (interp-expr (parse '{if0 0 1 2}) '()) 1) (test (interp-expr (parse '{if0 1 2 3}) '()) 3)
4 Negative predicate
Implement, in the fnWAE language (without any extensions, i.e., you cannot add new kinds of expressions to the language or to your interpreter), a predicate neg? that determines if an integer is negative. That is, write a defun like this one that can be passed to your parse function and then used with your interp function (with a suitable <fnWAE> expression that calls it):
{deffun {neg? x} ...}
It must return either 0 (if the input is negative), or 1 (if not). The number 0 is not itself considered negative.
5 Multiplication on integers
As you did in the previous problem, implement (in the fnWAE language without any extensions), a function mult that computes the product of two integers.
{deffun {mult x y} ...}
6 Handin instructions
Provide definitions for parse, parse-defn, and interp-expr, as above.
Provide a PLAI-level definition of mult-and-neg-deffuns that is bound to a list of (unparsed) def-
funs that contains both neg? and mult as well as any helper functions you need:
(define mult-and-neg-deffuns (list `{deffun {neg? x} ...}
`{deffun {mult x y} ...}
; other deffuns okay, too, for your helpers
))
Do not leave in any unused code (i.e., no subst). Tests for the above functions (or helpers you may use along the way) are fine, though.
Have the 8 rules from the Provost’s website (see the homework 0 for more details).
Submit your code via Canvas.
Your submission must include your test cases; submissions without test cases will not get any credit.
7 Grading
To receive a grade of on the way (1), your submission must correctly implement multiple argument functions and if0.
2
To receive a grade of got it (3), your submission must in addition correctly implement neg? and mult. Submissions which do not achieve on the way criteria will receive a grade of not yet (0).
3
