Bringing some observable sharing to GHCi
The :sprint, :print, and :force commands in GHCi fall into infinite loops when confronted by cyclical data. This bit me hard in https://phabricator.haskell.org/D4085 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.
Hi, Am Sonntag, den 29.10.2017, 16:40 -0400 schrieb David Feuer:
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.
the best that I know for this is the hack that calls itself ghc-heap- view: ~ $ cabal install ghc-heap-view ~ $ ghci GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Prelude> :script .cabal/share/x86_64-linux-ghc-8.0.2/ghc-heap-view-0.5.10/ghci Prelude> x = "hi" : x Prelude> take 2 x ["hi","hi"] Prelude> :printHeap x let x1 = _bh (C# 'h' : _bh (C# 'i' : _bh [])) : _bh x1 in x1 Prelude> System.Mem.performGC Prelude> :printHeap x let x1 = "hi" : x1 in x1 A graphical front-end is ghc-vis: http://felsin9.de/nnis/ghc-vis/ But it is mostly a hack and you should not expect a lot of reliability of it. I’d love to see it supported by GHC proper. Erik de Castro Lopo tried to merge it into GHC, but it seems to be very hard.
2. To be able to force cyclical data without looping.
x = "hi" : x :force x
That could be implemented on top of ghc-heap-view. Greetings, Joachim -- Joachim Breitner mail@joachim-breitner.de http://www.joachim-breitner.de/
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.
Cheers
Simon
On 29 October 2017 at 20:40, David Feuer
The :sprint, :print, and :force commands in GHCi fall into infinite loops when confronted by cyclical data. This bit me hard in https://phabricator.haskell.org/D4085 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. _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Do TypeReps need to be cyclic?
No – I think David is working on a non-cyclic version
Simon
From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Simon Marlow
Sent: 02 November 2017 10:27
To: David Feuer
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. _______________________________________________ ghc-devs mailing list ghc-devs@haskell.orgmailto:ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devshttps://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fghc-devs&data=02%7C01%7Csimonpj%40microsoft.com%7Ca85ca64bedd043ccfffc08d521dc59b5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636452152651212039&sdata=Ls32xDyWGP4oKhwrkbrxXf7nog1TtxbjGbjrODiwzB4%3D&reserved=0
participants (4)
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David Feuer -
Joachim Breitner -
Simon Marlow -
Simon Peyton Jones