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List:       haskell-cafe
Subject:    RE: Haskell report typo (bad law for readsPrec)
From:       "Simon Peyton-Jones" <simonpj () microsoft ! com>
Date:       2001-06-25 7:38:01
Message-ID: 37DA476A2BC9F64C95379BF66BA26902D730C8 () red-msg-09 ! redmond ! corp ! microsoft ! com
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The Haskell report is far from a precise document, and now is
not the time to make it so.

Nevertheless, Wolfgang has a point, and I have slid a little way
into this tar pit by making a change at all.

OK, I think I buy this formulation:

       head (readsPrec d (showsPrec d x "")) == (x,"")

(I suppose I might say "provided x is a total value").

Simon "desperately trying to freeze the report" PJ

| -----Original Message-----
| From: Wolfgang Lux [mailto:lux@wi.uni-muenster.de] 
| Sent: 20 June 2001 10:30
| To: Patrik Jansson
| Cc: haskell-cafe@haskell.org
| Subject: Re: Haskell report typo (bad law for readsPrec) 
| 
| 
| Patrik Jansson wrote
| 
| > [Typo - you mean:]
| >       head (readsPrec d (showsPrec d x "")) == (x,"")
| 
| Yes.
| 
| > Both formulations look OK to me, but neither is an essential change 
| > and there are many other small variations possible with slight 
| > semantical differences. One example is
| > 
| >       (x,"") `elem` readsPrec d (showsPrec d x "")
| > 
| > And already the current (proposed) formulation is most likely 
| > impossible to satisfy if the element x contains bottoms somewhere 
| > inside. (Example: try to show and read the string (undefined : 
| > "hello").)
| 
| True, but in any case it seems sensible to me to require that 
| readsPrec should actually process the input string. With your proposal
| 
| > > |      fst (head (readsPrec d (showsPrec d x ""))) == x
| 
| it would be possible to return [(42,"42")] for the expression 
| readsPrec 0 (showsPrec 0 42 "").
| 
| Wolfgang
| 
| 
| 
| --
| Wolfgang Lux				  Phone: +49-251-83-38263
| Institut fuer Wirtschaftinformatik	    FAX: +49-251-83-38259
| Universitaet Muenster		      Email: wlux@uni-muenster.de
| 
| 
| 
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