Hi Adam, Thakns for the pointer. Adding the type variable indeed seems to solve the problem. Greatly appreciated! Best, Sandy On Sat, 2023-02-04 at 12:49 +0000, Adam Gundry wrote:
Hi Sandy,
I don't think you can easily prevent GHC from caching solutions to specific constraints, and even if you could, the simplifier feels free to common-up dictionaries with the same type and hence would potentially introduce unintended semantic changes to the program.
You might be able to address at least the second issue by using implicit parameters instead. But I think a more robust approach would be to introduce an extra type parameter:
class KnownAnnotations a where rawAnnotationsVal :: [Annotation]
Now each rawAnnotationsVal call site will introduce a fresh unification variable α and the wanted constraint `KnownAnnotations α`. Your plugin can spot this constraint and solve it, while leaving α uninstantiated. Hence each call site will have a different constraint type, and the problems should go away.
Perhaps you could even make the parameter invisible/inferred, so it wouldn't change the user-visible API. Something like this might work, but I haven't tested it:
type KnownAnnotations :: forall {a} . Constraint class KnownAnnotations where rawAnnotationsVal :: [Annotation]
Hope this helps,
Adam
On 04/02/2023 09:29, Sandy Maguire via ghc-devs wrote:
Hi all,
I'm working on a little TC plugin that solves the following class:
```haskell class KnownAnnotations where rawAnnotationsVal :: [Annotation] ```
Like `HasCallStack`, the result of `KnownAnnotations` is unique to each callsite. Thus, I am trying to prevent my TC plugin from adding the solved constraint to the inert set.
As an example, the following program:
``` t2 :: ([Annotation], Int) t2 = (rawAnnotationsVal, test2)
t3 :: ([Annotation], Int) t3 = (rawAnnotationsVal, test3) ```
dumps this output with `-ddump-cs-trace`:
```
Step 1[l:0,d:0] Kept as inert: [WD] $dKnownAnnotations_a18UB {0}:: KnownAnnotations Step 2[l:0,d:0] Dict equal (keep inert): [WD] $dKnownAnnotations_a18UD {0}:: KnownAnnotations ```
Is there some trick to prevent the inert caching for these solved wanteds?
Cheers, Sandy