. So apparently it is possible for a dictionary to be bottom somehow. That should not happen.
Except in the case of single-method dictionaries like
class C a where op :: a -> a
In these cases the "dictionary" is represented by a newtype, like this
newtype C a = MkC (a->a)
Then you could say
instance C Int where
op = bottom
and now a (C Int) dictionary is simply bottom.
It would be easy to change this decision, and use a data constructor even for single-method classes. Some programs would become slightly less efficient, but things would be a bit more uniform. If there was a real advantage to doing this, it'd definitely be worth measuring the perf cost (if any).
Simon
From: Glasgow-haskell-users
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 < sperber@deinprogramm.demailto:sperber@deinprogramm.de> wrote:
Thanks for thinking about this one!
On Fri, Aug 06 2021, Tom Smeding
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 Glasgow-haskell-users@haskell.orgmailto:Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-usershttps://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fglasgow-haskell-users&data=04%7C01%7Csimonpj%40microsoft.com%7C7bf3769704884ed6592e08d95b4aeb26%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637641199636853840%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=ix06XQPvpu%2B1PLzoc5rRQM6cMv8yPF6o87uVwD0sUwQ%3D&reserved=0 _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.orgmailto:Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-usershttps://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fglasgow-haskell-users&data=04%7C01%7Csimonpj%40microsoft.com%7C7bf3769704884ed6592e08d95b4aeb26%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637641199636863837%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=sQDTBnklNv7YLRvhiY5CEtbZgcZT8p7RR%2Bw57sCqFJk%3D&reserved=0 -- brandon s allbery kf8nh allbery.b@gmail.commailto:allbery.b@gmail.com