ITEC 380 2016fall ibarland

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R0
Interpreting R

The language R0

  Expr   ::= Num | Paren | BinOp | Parity
  Paren  ::= < Expr >
  BinOp  ::= | Expr Expr Op |
  Parity ::= / Expr Expr Expr \
  Op     ::= ;) | ;( | xD

Semantics:

• A Paren is a parenthesized expression (used for grouping only);
• A Binop is a binary operator expression, in postfix; ;) means addition; ;( means subtraction; xD means multiplication.
• A Parity evaluates its first expression; if even then the result is the second expression (evaluated), else the result is the third expression (evaluated).

Discussion

• Give five example programs (Exprs) written in R0.
• Give their parse trees.
• How should we represent these trees, internally? (In Racket, and equivalently in Java).

Where we're headed

• For interpreting this language, we will implement three methods:
  • parse, which turns source-code into an internal representation,
  • toString which turns internal representation back into source-code,
  and
  • eval, which actually interprets the program, returning a value (a number for R0-R2, and a number-or-function in R3).
  I recommend implementing and testing the methods in this order. Some test-cases will be provided. There might be other helper functions to implement as well.

R0 Implementations

• R0.rkt, with helper files scanner.rkt (and, scanner-demo.rkt); test cases expr-test-R0.rkt, and student-extras.rkt.

  Note:Download the racket files, and then choose Open… from DrRacket. Do not copy/paste into an empty DrRacket window, since that window is probably using a student-language, and some of the files below use full racket.

  TODO (once): If your DrRacket prints 1.25 as “5/4” … (This tip deleted, since it was not the actual problem.)

  Note: If you downloaded R0.rkt before Mon. 17:00, and (expr->string (parse! "1.2")) doesn't return "1.2" then you'll want to make sure the first line of expr->string is: (cond [(number? e) (number->string (if (integer? e) e (exact->inexact e)))] ….

• Java R0-java/ (browse the directory), or see the .jar.
  Note the composite pattern to implement union-types, in Java: The classes Expr and its four variants Num/BinOp/Parity/Paren are organized in the same way AncTree and its two variants Child/Unknown were.

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©2016, Ian Barland, Radford University Last modified 2016.Nov.14 (Mon)

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