home—lectures—recipe—exams—hws—D2L—breeze (snow day)

P0
Interpreting P

For interpreting language, we will must implement three methods:
- parse, which turns source-code into an internal representation,
- toString which turns internal representation back into source-code,
- eval, which actually interprets the program, returning a value (a number for P0-P2, and a number-or-function in P3).
I recommend implementing and testing the methods in this order. See the provided test-cases, for examples. There might be other helper functions to implement as well.

The language P0

  Expr      ::= Num | ParenExpr | BinExpr | WhenZExpr
  ParenExpr ::= < Expr > 
  BinExpr   ::= (: Expr BinOp Expr :)
  WhenZExpr ::= whenz Expr then Expr otherwise Expr !
  BinOp     ::= +++ | --- | ***

where Num is any numeric literal (as written in either Java or Racket, your choice ¹). For the provided parsers to work, whitespace is required between all terminals with the exception of non-binary-operator punctuation.

Here is a Racket interpreter for P0 along with a scanner for racket (with an a scanner demo) and expr-test-P0.rkt (which requires the latest student-extras.rkt).
Here is a java interpreter for P0 (P0-java.jar or browse); it includes unit tests, and helpful Java parsing functions.

This is so we can just use our language's built-in number-parsing functions, without getting bogged down in tokening input. So racket implementations will allow exactly those strings recognized by number?, (including +nan.0, -inf.0, and 2+3i).

Similarly, if using Java, the semantics of P0's arithmetic will be similar to IEEE floating point arithmetic (rather than perfectly-correct arithmetic).

Don't confuse P0's class Num (which extends Expr) with the existing java.lang.Number, which doesn't extend Expr.

↩

home—lectures—recipe—exams—hws—D2L—breeze (snow day)

©2014, Ian Barland, Radford University
Last modified 2014.Nov.09 (Sun) Please mail any suggestions
(incl. typos, broken links)
to ibarlandradford.edu

P0 Interpreting P

The language P0

P0
Interpreting P