Re: Adding wc to Bournish

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

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On Wed, May 25, 2016 at 11:26:11AM +0200, Ricardo Wurmus wrote:
> 
> Efraim Flashner <efraim@flashner.co.il> writes:
> 
> > +(define (wc-l-command file)
> > +  (let* ((input-file (open-file file "r"))
> > +         (line       (read-line input-file))
> > +         (line-count 0))
> > +    (while (not (eof-object? line))
> > +           (set! line-count (1+ line-count))
> > +           (set! line (read-line input-file)))
> > +    line-count))
> 
> It’s unusual for me to see the use of “while” and “set!” in Scheme code.
> You could do this in a functional manner using a fold (see SRFI-1) or
> with file streams (see SRFI-41), which also provides a stream-fold.

I'm going to put set! in the GOTO box. GOTO worked well enough when I
learned QBasic, but just because its implemented doesn't mean I should
still use it. (ie: its there but try not to use it)

> 
> The idea with a fold is that you have a function that takes a value
> (e.g. from a list or a stream) and an intermediate result.  The function
> does something to the value and then returns a new intermediate result.

I already like this much better

> 
> Here’s a fold over a list of symbols implementing a count:
> 
>    (fold
>       (lambda (_ res) (+ res 1))  ; increase the result
>       0                           ; start at 0
>       '(hello world bye))         ; items to count
> 
> If you had a file stream, where each element represents one line, you
> can fold over all lines in much the same way to get a count.  You could
> use the same framework with a different stream element generator
> (reading one word or byte at a time instead of one line at a time) to
> implement the other features of “wc”.

I'll take another look at streams and rework that. I still think asking
the filesystem how big the file is works well for `wc -c', but `wc -l'
and `wc -w' should really be part of the same function.

> 
> There’s an example of how to define a file stream in the Guile manual in
> the documentation for SRFI-41.

That looks like a good place to start then :)

> 
> ~~ Ricardo

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