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<p class="MsoNormal" style="margin-left:36.0pt">Do TypeReps need to be cyclic?<o:p></o:p></p>
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<p class="MsoNormal">No – I think David is working on a non-cyclic version<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<span style="mso-fareast-language:EN-US"><o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Simon Marlow<br>
<b>Sent:</b> 02 November 2017 10:27<br>
<b>To:</b> David Feuer <david.feuer@gmail.com><br>
<b>Cc:</b> Joachim Breitner <mail@joachim-breitner.de>; ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: Bringing some observable sharing to GHCi<o:p></o:p></span></p>
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Not being able to :print an exception would be quite annoying. Do TypeReps need to be cyclic?<o:p></o:p></p>
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Observable sharing can be done with StableName, but that would be a bit pile of hassle in the printer. Maybe exceptions warrant a special case.<o:p></o:p></p>
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<o:p> </o:p></p>
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Cheers<o:p></o:p></p>
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Simon<o:p></o:p></p>
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<o:p> </o:p></p>
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On 29 October 2017 at 20:40, David Feuer <<a href="mailto:david.feuer@gmail.com" target="_blank">david.feuer@gmail.com</a>> wrote:<o:p></o:p></p>
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The :sprint, :print, and :force commands in GHCi fall into infinite<br>
loops when confronted by cyclical data. This bit me hard in<br>
<a href="https://phabricator.haskell.org/D4085" target="_blank">https://phabricator.haskell.org/D4085</a> because that produces cyclical<br>
TypeReps, which is trouble for (e.g.) the test break011 which tries to<br>
:force a SomeException (which wraps an Exception dictionary, which has<br>
a Typeable constraint). I could try coming up with a fix myself, but<br>
I'm rather curious whether some of the work you (or others) have<br>
already done on observing GHC data graphs could be yanked into GHCi<br>
itself for this purpose. We want<br>
<br>
1. To be able to display cyclical data in some sensible way.<br>
<br>
> x = "hi" : x<br>
> x `seq` ()<br>
> :print x<br>
<br>
should print some useful representation of x.<br>
<br>
2. To be able to force cyclical data without looping.<br>
<br>
> x = "hi" : x<br>
> :force x<br>
<br>
should print a useful representation of x.<br>
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<o:p> </o:p></p>
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