[racket] Simple Scheme Interpreter written in Drracket

On Nov 9, 2010, at 12:32 PM, Peter Breitsprecher wrote:

> Ok, here is the code I have written so far for the function I am having trouble with...
> 
> (define (rep-loop)
>   (newline)
>   (display "repl>")
>   (read-token))

I'm going to ignore rep-loop.

> 
> (define (read-token)
>   (let ((expr (read-char)))        
>     ((cond [(char-whitespace? expr) (read-token1)]
>            [(eq? expr #\() (read-token2)]
>            [(eq? expr #\)) (read-token3)]
>            [(char-alphabetic? expr) (read-token4)]
>            [(char-numeric? expr) (read-token5)]
>            [else (display "Exiting Loop")]))))

Okay! we've got a name: read-token.

The way it's written, it's going to be very hard to test.  This is because it operates on typed-in input.

This program would be much easier to develop if you could write a function that accepts strings as input.  Can we break the problem up this way?

John Clements


> The cond calls other functions I have written just so I could see it if was reading the output letter by letter...  Now I need to make read-token read the whole word if it sees a character and the entire number if it sees a number.
> 
> Is that getting closer?
> 
> On 9 November 2010 15:28, John Clements <clements at brinckerhoff.org> wrote:
> 
> On Nov 9, 2010, at 12:27 PM, Peter Breitsprecher wrote:
> 
> > I'd lie but no.. I was just trying to clarify what I was looking for.  I gave a the code for one of the functions I had written, and then an example of the input the user is giving below, and then tried to describe in more detail what I needed help with...
> 
> Well, I'm glad you're not lying :).
> 
> Can you start by picking a name for the function that you're trying to write?
> 
> John Clements
> 
> 
> > On 9 November 2010 15:23, Shriram Krishnamurthi <sk at cs.brown.edu> wrote:
> > Hi,
> >
> > That would not be considered an "example" for at least two reasons. Do you see why?
> >
> >
> >> On Nov 9, 2010 12:17 PM, "Peter Breitsprecher" <pkbreits at lakeheadu.ca> wrote:
> >>
> >> As an example, I want have most of the functions for Plus, Minus already written.
> >>
> >> Here is the code for it.
> >>
> >> (define (plus num1 num2)
> >> (+ num1 num2))
> >>
> >> That is childs stuff...  Here is where I am having the problem, and please keep in mind I am new to DrRacket...
> >>
> >> If my program accepts input such as
> >> (plus 13 5)
> >>
> >> I need to parse it so that I know what it is asking for.  If It is a left parent, then I need to move on, if it is a right parent, move on to the next character, once i get to the next character I can see it is an alphanumeric character, but then I need to somehow read the whole word, so that I can store that value in a variable so that I can read the next number.  If the numbers are single digits, it is easy, because they are only one charater each and read-char works perfect, but I need something like a read-word, or read-number and I can't seem to figure out how to do it.
> >>
> >> So really I need to know how do I read the words when there are characters present, and read the whole numbers when there is a number present.
> >>
> >>
> >>
> >> On 9 November 2010 15:00, John Clements <clements at brinckerhoff.org> wrote:
> >> >
> >> >
> >> > On Nov 9, 2010, a...
> >>
> >> --
> >> Kurt Breitsprecher
> >> (807) 474-9601
> >> pkbreits at lakeheadu.ca
> >>
> >>
> >> _________________________________________________
> >>  For list-related administrative tasks:
> >>  http://lists.racket-lang.org/listinfo/users
> >
> >
> >
> > --
> > Kurt Breitsprecher
> > (807) 474-9601
> > pkbreits at lakeheadu.ca
> >
> 
> 
> 
> 
> -- 
> Kurt Breitsprecher
> (807) 474-9601
> pkbreits at lakeheadu.ca
> 

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