Pretty.lhs: bug found, feature missed

[Sigbjorn Finne]

(don't know if others are/have attended to this)

Hi,

thanks, I've committed the obvious fix for this one.

As for you other question, GHC's Outputable is one example
of a Pretty use which lets you prune documents at a certain
depth. The sources can be found in ghc/compiler/utils/Outputable.lhs -
maybe something in there that's of use to you..

hth
--sigbjorn

----- Original Message ----- 
From: "Johannes Waldmann" <joe@isun.informatik.uni-leipzig.de>
To: <glasgow-haskell-users@haskell.org>
Sent: Tuesday, November 06, 2001 13:40
Subject: Pretty.lhs: bug found, feature missed


> We were recently bitten by what seems to be 
> a bug in hslibs/text/Pretty.lhs.
> The following code prints an infinite string:
> 
> 
> import Pretty
> 
> ncat x y = nest 4 $ cat [ x, y ]
> d1 = foldl1 ncat $ take 50 $ repeat $ char 'a'
> d2 = parens $  sep [ d1, text "+" , d1 ]
> main = print d2
> 
> 
> I think this is due to the function indent/spaces
> called with a negative argument.  The following is in Pretty.lhs:
> 
> 
> indent n | n >= 8 = '\t' : indent (n - 8)
>          | otherwise      = spaces n
> 
> spaces 0 = ""
> spaces n       = ' ' : spaces (n - 1)
> 
> 
> This looks Really Dangerous. 
> 
> 
> 
> Another point: we would want to use a version of `render (d :: Doc)' 
> that always produces only a bounded amount of output,
> by cutting off sub-documents that are too deeply nested.
> (Some LISP pretty printers do have this feature -
> they cut long lists, and deeply nested lists).
> Before sitting down and trying to implement this, we would like to
> check whether someone already did it. If so, please tell us.
> 
> 
> Best regards,
> -- 
> -- Johannes Waldmann ---- http://www.informatik.uni-leipzig.de/~joe/ --
> -- joe@informatik.uni-leipzig.de -- phone/fax (+49) 341 9732 204/252 --
> 
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