[plt-scheme] pretty-print not so pretty

On Jan 18, 2007, at 7:22 AM, Kyle Smith wrote:

> Hi folks,
>
> What I'm after is to display the application, whence a result came  
> from; this was my attempt:
>
> (require (lib "pretty.ss"))
>
> (define-syntax (phere stx)
>  (syntax-case stx ()
>    ((_ app ...) #'(begin (pretty-display 'app ...) app ...))))
> (phere (let ([x 0])
>         (let ([y 1])
>           (display (+ y x)))))
>
>
> Welcome to DrScheme, version 369.3-svn8jan2007.
> Language: Pretty Big (includes MrEd and Advanced Student).
> (let ((x 0)) (let ((y 1)) (display (+ y x))))
> 1
>>
> While it functionally did what I wanted, it didn't show the code as  
> I expected:
> (let ((x 0))
>  (let ((y 1))
>     (display (+ y x))))
>
> I've never used pretty-print before, but I thought it was supposed  
> to display the sexp's in some kind-of reasonably acceptable  
> standard way, not just a straight run of text.  I tried with both  
> pretty-print and pretty-display, as  shown above.  I tried it in  
> MzScheme with the same result.  Am I missing a parameter that needs  
> setting, or some larger concept?

Pretty-print has a broader task than the one you assign it; its job  
is to nicely format _any_ s-expression, not just code.  For this  
reason, pretty-print can't assume that its input should be formatted  
to look like scheme code.  Instead, pretty-print just tries to make  
sure that line breaks fall in nice places, and that lines after the  
first are appropriately indented.  This means that if the whole  
expression fits on one line, pretty-print will cheerfully put the  
whole thing on one line.

The thing that you're imagining sounds more complicated.  Parts of  
its functionality already live in DrScheme--e.g., how many spaces  
should DrScheme insert when you hit "Return"--but the difficult  
question of how many lines to break something into is (AFAIK) not  
addressed. It does sound useful, though, if you want to develop it...

As always, corrections cheerfully accepted.

John Clements

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