[plt-scheme] a tiny repl

For what do you really need it?

rac


On Jun 28, 2009, at 4:43 PM, Ryan Culpepper wrote:

> Hugh Myers wrote:
>> On the off chance that others who read this list are as needy as
>> myself, what follows is a tiny repl. I use an expanded version to  
>> test
>> and debug my current work--- single stepping through a chess game for
>> instance. Since I'm new to scheme, I've no knowledge of better ways  
>> to
>> do this, so feel free to criticize. It paid for itself by being
>> helpful to me, gravy if helpful to someone else...
>
> I try to stay away from 'eval' unless I really need it. You could  
> instead just keep a table of command names and the procedures that  
> implement them. Here's how to do it:
>
> ;; repl : string -> (loops)
> (define (repl prompt)
>   (display prompt)
>   (let* ([line (read-line)]
>          [parts (regexp-split #px"\\s" line)])
>     (when (pair? parts)
>       (let* ([command-part (car parts)]
>              [parameter-parts (cdr parts)]
>              [command (lookup-command command-part)]
>              [parameters (for/list ([part parameter-parts])
>                            (string->actual part))])
>         (when command
>           (apply command parameters))))
>     (repl prompt)))
>
> ;; lookup-command : string -> procedure/#f
> ;; Gets the procedure that implements the given command,
> ;; or #f if given an unknown command name.
> (define (lookup-command command-name)
>   ;; commands : (listof (cons string procedure))
>   ;; You can lift this to top-level, but you must put it *after*
>   ;; the definitions of 'repl-help', 'jmp', and 'print-board'.
>   (define commands
>     (list (cons "h" repl-help)
>           (cons "q" (lambda () (escape 'Bye!)))
>           (cons "j" jmp)
>           (cons "p" print-board)))
>   (let ([entry (assoc command-name commands)])
>     (and entry (cdr entry))))
>
> A few of your functions can now go away, and the rest are unchanged:
>
> (define (repl-help)
>   (printf "~a~%" "Commands are h,p, and q.")
>   (printf "~a~%" "Where:")
>   (printf "~a~%" "       h =s 'help'")
>   (printf "~a~%" "       j n =s 'jmp n plys'")
>   (printf "~a~%" "       p =s 'print-board'")
>   (printf "~a~%" "       q =s 'quit'"))
>
> (define (jmp n)
>   (printf "jump forward ~A ply in movelist~%" n))
>
> (define (print-board)
>   (printf "display board~%"))
>
> (define (escape v)
>   (abort-current-continuation
>    (default-continuation-prompt-tag)
>    (lambda () v)))
>
> (define (string->actual s)
>   (let ([value (if (string->number s)
>                    (string->number s)
>                    (string->symbol s))])
>     value))
>
> (define (test-repl)
>   (repl "test> "))
>
> Ryan
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