We haven't figured out what they did, but the other day we had someone in #haskell with an infinite loop evaluating a dictionary. So apparently it is possible for a dictionary to be bottom somehow.
On Mon, Aug 9, 2021 at 11:27 AM Tom Smeding <x...@tomsmeding.com> wrote: > Hi Mike, > > > > But wouldn't that imply that ghc can build dictionary-construction code > > > that evaluates to bottom? Can that happen? > > > I assume no, but here the dictionary is embedded as a field in the GADT, > right? So if the data value is bottom, there is not even a dictionary to be > found, let alone not-bottom. > > > This assumes that the Dict in `Entail (Sub Dict)` is a GADT like > > > Dict :: Con b => Dict something > > > where the Con dictionary is contained in the GADT. Remember that in Core, > dictionaries are values, and there is no difference between => and ->. > > > - Tom > > > > -------- Original Message -------- > > On 9 Aug 2021, 15:24, Michael Sperber < sper...@deinprogramm.de> wrote: > > > Thanks for thinking about this one! > > On Fri, Aug 06 2021, Tom Smeding <x...@tomsmeding.com> wrote: > > > Would it not be unsound for ghc to elide dictionary construction here? > > > After all, the right-hand side might actually be a bottom > > > (e.g. undefined) at run-time, in which case the pattern match cannot > > > succeed according to the semantics of Haskell. > > But wouldn't that imply that ghc can build dictionary-construction code > > that evaluates to bottom? Can that happen? > > > I suspect that if you make the pattern match lazy (i.e. ~(Entail (Sub > > > Dict))) or ignore the argument altogether (i.e. _), dictionary > > > construction will be elided. > > Thanks for the hint! ghc gives me this unfortunately, implying that it > > agreed with your first comment: > > src/ConCat/Category.hs:190:29: error: > > • Could not deduce: Con b arising from a use of ‘r’ > > from the context: Con a > > bound by the type signature for: > > (<+) :: forall a b r. Con a => (Con b => r) -> (a |- b) -> r > > at src/ConCat/Category.hs:189:1-46 > > • In the expression: r > > In an equation for ‘<+’: r <+ ~(Entail (Sub Dict)) = r > > • Relevant bindings include > > r :: Con b => r (bound at src/ConCat/Category.hs:190:1) > > (<+) :: (Con b => r) -> (a |- b) -> r > > (bound at src/ConCat/Category.hs:190:3) > > | > > 190 | r <+ ~(Entail (Sub Dict)) = r > > | ^ > > Other ideas welcome! > > -- > > Regards, > > Mike > > _______________________________________________ > > Glasgow-haskell-users mailing list > > Glasgowemail@example.com > > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgowfirstname.lastname@example.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > -- brandon s allbery kf8nh allber...@gmail.com
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