Scheme - LALR parser generation - line input

Question

We are trying to generate (in guile) a parser and lexer that read characters from a string instead of stdin.

We started modifying the example calculator included in the code at http://code.google.com/p/lalr-scm/source/browse/trunk/calc.scm?r=52

The problem is the following line:

(let* ((location (make-source-location "*stdin*" (port-line (current-input-port)) (port-column (current-input-port)) -1 -1)) 

We tried to define a new input port:

 (let* ((location (make-source-location "*stdin*" (port-line (open-input-string program)) (port-column (open-input-string program)) -1 -1)) 

and the program variable was defined as follows:

 (define program "int x = 2; int y = 0; y= x*(2+3);" ) 

but it does not work, it is still waiting for standard input characters.

There are no details in the documentation, so we cannot understand how we can solve this.

thanks

Answer 1

You are very, very close to a solution! Well, sort of. But here is the beginning. Look at the source code where you modified it:

 (let* ((location (make-source-location "*stdin*" (port-line (current-input-port)) (port-column (current-input-port)) -1 -1)) (c (read-char))) ...) 

Here you changed all of your (current-input-port) to your string port (BTW, do not call open-input-string more than once, because each time each new port is created each with independent cursors), but this is not only a place that actually uses (current-input-port) .

Do you see it? It really is in a call (read-char) ! This function actually takes a port argument, the default is (current-input-port) .

In fact, if you look above and find instances of (read-char) and (peek-char) , you will notice that using (current-input-port) is pretty much baked into the whole make-lexer function. Therefore, you will need to change this.

I would suggest specifying an input port for the make-lexer function:

 (define* (make-lexer errorp #:optional (input (current-input-port))) ... 

then change all instances of (read-char) and (peek-char) to use the input port. Also remember to also change your make-source-location call:

 (let* ((location (make-source-location "*stdin*" (port-line input) (port-column input) -1 -1)) (c (read-char input))) ...) 

Now you can use (make-lexer errorp (open-input-string program)) and it should work. (I have not tested it.)