Re: Adding wc to Bournish

[Efraim Flashner <efraim@flashner.co.il>]

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On Thu, May 26, 2016 at 10:46:21AM +0200, Ludovic Courtès wrote:
> 
> As Ricardo suggests, you could use ‘port->stream’ and ‘stream-fold’ to
> iterate over the characters read from the port.  I suspect that’d be
> rather slow though, at least on 2.0, so another option is something
> like:
> 
>   (define (lines+chars port)
>     ;; Return the number of lines and number of chars read from PORT.
>     (let loop ((lines 1) (chars 0))
>       (match (read-char port)
>         ((? eof-object?)              ;done!
>          (values lines port))
>         (#\newline                    ;recurse
>          (loop (+ 1 lines) (+ 1 chars)))
>         (_                            ;recurse
>          (loop lines (+ 1 chars))))))
> 
>   (define (wc-command file)
>     (let-values (((lines chars)
>                   (call-with-input-file file lines+chars)))
>       (format #t "~a ~a ~a~%" lines chars file)))
> 

Are you suggesting just dropping the word count part of `wc'? I've been
thinking about it, and the simplest way I can think of to describe a
word is a space followed by a character, or, to use the char-sets from
the guile manual, a character from char-set:whitespace followed by a
character from char-set:graphic. I can compare (read-char port) and
(peek-char port) to get a word count (possibly).

> > +(define (wc-command file)
> > +  (if (and (file-exists? file) (access? file 4))
> 
> This check is not needed and is subject to a race condition (“TOCTTOU”);
> just let ‘call-with-input-file’ error out if the file cannot be read.
> 
> Bonus point: catch ‘system-error’ exceptions and report the inability to
> open the file in a nice user-friendly way (but really, don’t bother
> about it for now.)
> 

I'm still wrapping my head around the following part. My wife says when
I work I scowl at the computer a lot and mutter :)
> 
> Remember that Bournish is a compiler that compiles Bash to Scheme.
> So we must distinguish the support functions that are used at run time,
> such as ‘ls-command-implementation’, from what the Scheme code that the
> compiler emits (compile time).
> 
> In the case of ‘ls’, when the compiler encounters ‘ls’ in the input, it
> emits this code:
> 
>   ((@@ (guix build bournish) ls-command-implementation))
> 
> ‘ls-command-implementation’ is the implementation that is called when we
> run the compiled program.
> 
> Thus, you must similarly distinguish those two stages by providing:
> 
>   1. A ‘wc-command-implementation’ procedure that implements ‘wc’;
> 
>   2. A ‘wc-command’ procedure that emits the code that calls
>      ‘wc-command-implementation’; so something like:
> 
>         (define (wc-command args)
>           `((@@ (guix build bournish) wc-command-implementation)
>             ,@args))
> 
>      Better yet, ‘wc-command’ could check for the presence of “-l” or
>      “-c” at compile time and emit a call to the right thing.

I checked with coreutil's 'wc', and it emits in its particular order
whether you call 'wc -l -c' or 'wc -lc' or 'wc -cl'

> 
> HTH!
> 
> Ludo’.


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Efraim Flashner   <efraim@flashner.co.il>   אפרים פלשנר
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