Spot on Alexis. Should not be hard to fix this.
I think the best thing would be in mkCallUDs
* not to use SpecDict for implicit parameters; instead use UnspecArg
* don’t require length theta = length dicts. Need to think about what else instead. Isn’t it implied by the arity test?
Make a ticket! Happy to help if you or Sandy need anything from me.
Simon
From: ghc-devs On Behalf Of Alexis King
Sent: 15 March 2020 02:47
To: Sandy Maguire
Cc: ghc-devs
Subject: Re: Specializing functions with implicit parameters
On Mar 14, 2020, at 20:03, Sandy Maguire mailto:sandy@sandymaguire.me> wrote:
What GHC are you testing against? I suspect https://gitlab.haskell.org/ghc/ghc/merge_requests/668https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fmerge_requests%2F668&data=02%7C01%7Csimonpj%40microsoft.com%7Cfc70cfc46c7146c59bd008d7c88b2c76%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637198372382352399&sdata=C%2BzhSCJ2EREtrQD%2FZedu03w0NxQKOnszWKXiKY5cSc4%3D&reserved=0 will fix this.
I’ve tested against HEAD. I think the change you link is helpful, but it doesn’t quite get there: the usage gets dumped before specHeader even gets a chance to look at the call. The relevant bit of code is here:
https://gitlab.haskell.org/ghc/ghc/blob/1de3ab4a147eeb0b34b24a3c0e91f174e6e5...https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fblob%2F1de3ab4a147eeb0b34b24a3c0e91f174e6e5cb79%2Fcompiler%2Fspecialise%2FSpecialise.hs%23L2274-2302&data=02%7C01%7Csimonpj%40microsoft.com%7Cfc70cfc46c7146c59bd008d7c88b2c76%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637198372382352399&sdata=qfC7hSxoWNABKLpgJe6kqzTeDG1jmLwyane9W8UCOh4%3D&reserved=0
Specifically, this line seals the deal:
ClassPred cls _ -> not (isIPClass cls) -- Superclasses can't be IPs
So maybe the right fix is just to change the role of type_determines_value so that it turns SpecDicts into UnspecArgs, and then with your change everything would just happily work out.
