FW: 7.4.1-pre: Show & Integral
simonpj at microsoft.com
Fri Dec 23 18:41:23 CET 2011
I'm confused too. I'd welcome clarification from the Haskell Prime folk.
From: Serge D. Mechveliani [mailto:mechvel at botik.ru]
Sent: 23 December 2011 17:36
To: Simon Peyton-Jones
Subject: Re: 7.4.1-pre: Show & Integral
On Thu, Dec 22, 2011 at 08:14:54PM +0000, Simon Peyton-Jones wrote:
> | 2011/12/22 Edward Kmett <ekmett at gmail.com>:
> | > The change, however, was a deliberate _break_ with the standard that
> | > passed through the library review process a few months ago, and is now
> | > making its way out into the wild.
> | Is it reasonable to enquire how many standard-compliant implementations
> | of Haskell there are?
> Just to be clear, the change IS the standard. GHC has to change to be compliant.
> At least that's how I understand it.
I am confused.
I am looking now at the on-line specification of Haskell-2010,
6.3 Standard Haskell Classes.
It shows that Integral includes Show:
This is also visible in the further standard class declarations in this chapter.
Hence, for `x :: Integral a => a' it is correct to write (shows x "").
And ghc-18.104.22.16811219 does not allow this.
So, ghc-22.214.171.12411219 breaks the 2010 standard. Now, Edward Kmett writes that
this break is done deliberately.
Am I missing something?
I witness this for the first time: that GHC deliberately breaks the current
Probably, many people (as myself) dislike this point of the standard.
Well, they can write a dummy Show implementation for their type T:
showsPrec _ _ = showString "(<t> :: T)",
and wait for an improved standard, say, Haskell-II
mechvel at botik.ru
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