An interpreter for the WHILE programming language

The WHILE language is a simple imperative language. The instruction set is very limited:

• arithmetic operations on integers (addition, substraction, multiplication),
• boolean operations (AND, OR, NOT, ==, <=),
• variable assignments,
• if-then-else and while control flow statement.

interpreter.ml is an interpreter of this language.

Notes

• The interpreter has NOT been extensively tested.
• One should use the Reverse Polish Notation in arithmetic expressions.
• Negative number input is not supported. Please use 0 <integer> - instead.
• For usability concerns, some features has been added on top of the language:
  • Add of the print <arithmetic expression> command,
  • Support for indentation (ignored by the interpreter),
  • Support for comments (lines starting with #) and empty lines (both ignored by the interpreter).

Syntax

Inline syntax

• Variable: any uppercase letter

• Arithmetic expressions: any RPN expression with:

  • integers or variables as literals
  • +, - or * as operators

• Boolean operators and literals:

  • TRUE
  • FALSE
  • (<condition>) AND (<condition>)
  • (<condition>) OR (<condition>)
  • NOT (<condition>)
  • [<arithmetic expression>] == [<arithmetic expression>]
  • [<arithmetic expression>] <= [<arithmetic expression>]

Instruction syntax

Comment

# <comment>

Assignment

<variable> := [<arithmetic expression>];

Flow control

Indentation of code blocks is not needed (the interpreter ignores it), but is supported to write clearer scripts.

if <condition> then {
    <code block>
} else {
    <code block>
};   

while <condition> do {
    <code block>
};

Printing

print [<arithmetic expression>];

Additionnal details

Some .while scripts are included in this repository as examples.

For a more precise description of the syntax used, one can either:

• look at the BNF-like description in syntax.bnf,
• read the parsing section of the code,
• wait for a syntax diagram ;).

License

Please see LICENSE.

Use

To interpret any .while script, one can either:

• use the ocaml interpreter: ocaml interpreter.ml <file name>,
• compile the interpreter to get a standalone executable: ocamlopt -o while_interpreter interpreter.ml.