> . 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 <[email protected]> On
Behalf Of Brandon Allbery
Sent: 09 August 2021 16:32
To: Tom Smeding <[email protected]>
Cc: GHC users <[email protected]>; [email protected]
Subject: Re: Avoiding construction of dead dictionaries
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
<[email protected]<mailto:[email protected]>> 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 <
[email protected]<mailto:[email protected]>> wrote:
Thanks for thinking about this one!
On Fri, Aug 06 2021, Tom Smeding
<[email protected]<mailto:[email protected]>> 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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--
brandon s allbery kf8nh
[email protected]<mailto:[email protected]>
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