COSC 3410 Programming Languages

Fall 2011

Homework Assignment #6

Conditionals

Due: Wednesday, Oct 26, 11:00am CDT
Submit: Turn in Scheme source files called "hw6.scm" (your interpreter) and "hw6-test.scm" (a SchemeUnit testsuite for your interpreter) using the turnin command on the Systems Lab machines. Include the names of all authors at the top of the files in a comment block.
Work may be completed in pairs. Each team should have only one member turnin.

The Grammar

For this assignment, ammend the grammar from HW #5 to include the following productions:

<expr>	::=	( if <bool-expr> <expr> <expr> )
	|	( cond { ( <bool-expr> <expr> ) }* ( else <expr> ) )
<bool-expr>	::=	#t
	|	#f
	|	<compare-expr>
	|	<logical-expr>
<compare-expr>	::=	( equal <expr> <expr> )
	|	( greater <expr> <expr> )
	|	( lesser <expr> <expr> )
<logical-expr>	::=	( and <bool-expr> <bool-expr> )
	|	( or <bool-expr> <bool-expr> )
	|	( xor <bool-expr> <bool-expr> )

Parser

Use the SLLGEN parser generator system to specify your lexical and syntax rules, and automatically build your scan&parse function.

Modify your existing unparse function to work with the new abstract syntax and scan&parse function.

Your scanner specification must now accept Boolean literals "#t" and "#f"; note that these literals must scan into distinct tokens, or you will not be able to tell them apart later.

Interpreter

Modify your evaluate function to operate over the new syntax.

The cond construct should evaluate to only the first expression that corresponds to a true condition. If none of the clauses has a true condition, the else clause is always taken to be true.

Notes:

Although our input grammar includes lambda terms and applications, for now your evaluate function may throw an error if you encounter a lambda term or an application of anything other than a primitive operator. Note that this implies there will be no variable definitions.

Check rigorously for errors in the input, or for invalid expressions.

Appropriate errors should be thrown if your interpreter encounters any trouble. See the eopl:error construct used in the text. Think carefully about what kinds of errors the interpreter can encounter.