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
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