XYZ expressions

This is a simple example of how ocamllex and menhir can be used to build and process an abstract syntax tree (AST) for a language.

It was developed as part of (a previous version of) the undergraduate compiler course at Aarhus University.

The AST is defined in ast.ml. It is built recursively in the action statements of each grammar production in parser.mly, and it is traversed in both pprint.ml and eval.ml.

Supported syntax:

exp ::=  x  |  y  |  z
     |  exp + exp
     |  exp - exp
     |  exp * exp
     |  exp / exp
     |  ( exp )

To build:

$ make

To run:

Example 1:

$ ./xyz 3 4 5   (pass a command line value for variable x,y, and z, respectively)
x + y - z       (then press return and Ctrl-D on Linux or MacOSX)
((x+y)-z)
2$ 

(hence the string 'x + y - z' is accepted, it is parsed as ((x+y)-z), and it evaluates to 2 when x=3, y=4, and z=5)

Example 2:

$ ./xyz 3 4 5
x + y--
Parser.Error
$

(hence the string 'x + y--' is rejected by the parser)

Example 3:

$ ./xyz 3 4 5
2 + 2
Failure in lexing: empty token
$

(hence the string '2 + 2' is rejected by the scanner)