Re: Avoiding construction of dead dictionaries

9 Aug 2021


      They didn't show code (this is sadly common), so we had only speculation. :(

On Mon, Aug 9, 2021 at 11:56 AM Simon Peyton Jones 
wrote:
...
...
. 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  *On
Behalf Of *Brandon Allbery
*Sent:* 09 August 2021 16:32
*To:* Tom Smeding 
*Cc:* GHC users ;
sperber@deinprogramm.de
*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  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 < sperber@deinprogramm.de> wrote:
Thanks for thinking about this one!
On Fri, Aug 06 2021, Tom Smeding  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
allbery.b@gmail.com
-- 
brandon s allbery kf8nh
allbery.b@gmail.com