Can you construct an example [...]
Certainly. Here's the example that caught my attention (in my GHC 6.5 STABLE): Prelude Text.PrettyPrint> isEmpty empty True Prelude Text.PrettyPrint> isEmpty (empty<>empty) False Similarly, Prelude Text.PrettyPrint> isEmpty (empty$$empty) False Cheers, - Conal -----Original Message----- From: Christian Maeder [mailto:maeder@tzi.de] Sent: Thursday, November 24, 2005 1:12 AM To: Conal Elliott Cc: libraries@haskell.org Subject: Re: lawbreakers in Text.PrettyPrint.HughesPJ Conal Elliott wrote:
The comments in HughesPJ say that empty is an identity for <> and $$, 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. Christian