lawbreakers in Text.PrettyPrint.HughesPJ

Christian Maeder maeder at
Thu Nov 24 04:12:19 EST 2005

Conal Elliott wrote:
> The comments in HughesPJ say that empty is an identity for <> and $$,
> but the implementation doesn't satisfy either of these laws.  Here are
> the implementations:

I think, the "observable" results of <> and $$ satisfy these laws. Can 
you construct an example were these laws can be observed to be not 
fulfilled (for a reduced doc)?

>> p $$ q = Above p False q
>> p <> q = Beside p False q

Above and Beside are not observable and (I think) your proposed fix is 
not necessary.


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