Bringing some observable sharing to GHCi

Simon Peyton Jones simonpj at
Thu Nov 2 10:37:52 UTC 2017

Do TypeReps need to be cyclic?

No – I think David is working on a non-cyclic version


From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Simon Marlow
Sent: 02 November 2017 10:27
To: David Feuer <david.feuer at>
Cc: Joachim Breitner <mail at>; ghc-devs <ghc-devs at>
Subject: Re: Bringing some observable sharing to GHCi

Not being able to :print an exception would be quite annoying. Do TypeReps need to be cyclic?

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.


On 29 October 2017 at 20:40, David Feuer <david.feuer at<mailto:david.feuer at>> wrote:
The :sprint, :print, and :force commands in GHCi fall into infinite
loops when confronted by cyclical data. This bit me hard in because that produces cyclical
TypeReps, which is trouble for (e.g.) the test break011 which tries to
:force a SomeException (which wraps an Exception dictionary, which has
a Typeable constraint). I could try coming up with a fix myself, but
I'm rather curious whether some of the work you (or others) have
already done on observing GHC data graphs could be yanked into GHCi
itself for this purpose. We want

1. To be able to display cyclical data in some sensible way.

> x = "hi" : x
> x `seq` ()
> :print x

should print some useful representation of x.

2. To be able to force cyclical data without looping.

> x = "hi" : x
> :force x

should print a useful representation of x.
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