<div dir="ltr">Hello,<div><br></div><div>I haven't worked on that stuff in Haskell for a long time, but here are some thoughts:</div><div> - I think plugins should generally be agnostic to the form of the constraints they are given, thus flattening or not should not affect them---after all, the user might have written the constraints in the "flattened" form in the first place. So I think a plugin needs to convert the constraints to whatever form it assumes anyways.</div><div> - I always thought that derived constraints were a pretty clever way for disseminating information in the constraint solver, is there a note somewhere on what's going to be the new mechanism?</div><div><br></div><div>-Iavor</div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Nov 19, 2020 at 2:21 AM Christiaan Baaij <<a href="mailto:christiaan.baaij@gmail.com">christiaan.baaij@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>I always forget what flattening did/does. Is it the thing where it turns a "complex" type family application:<br><br></div><div>F (G y) (H z) ~ a<br></div><div><br></div><div>into:<br><br></div><div>G y ~ p</div><div>H z ~ q</div><div>F p q ~ a</div><div><br></div><div>?</div><div><br></div><div>If so, then I'm all for removing that. Since I actually wrote/hacked a function that "reverts" that process (for [G]ivens only): <a href="https://hackage.haskell.org/package/ghc-tcplugins-extra-0.4/docs/src/GHC.TcPluginM.Extra.html#flattenGivens" target="_blank">https://hackage.haskell.org/package/ghc-tcplugins-extra-0.4/docs/src/GHC.TcPluginM.Extra.html#flattenGivens</a><br></div><div>Which I use in all of my plugins. (PS it should perhaps be called "unflattenGiven"? like I said, I always get confused about flatten vs unflatten).</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, 19 Nov 2020 at 05:20, Richard Eisenberg <<a href="mailto:rae@richarde.dev" target="_blank">rae@richarde.dev</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi all,<br>
<br>
I'm hard at work on two significant refactorings within GHC's constraint solver. The first is at <a href="https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4149" rel="noreferrer" target="_blank">https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4149</a>. It removes flattening meta-variables and flattening skolems. This is a very nice simplification. Instead, it just reduces type families directly. My other patch (held up by the first) is at <a href="https://gitlab.haskell.org/ghc/ghc/-/tree/wip/derived-refactor" rel="noreferrer" target="_blank">https://gitlab.haskell.org/ghc/ghc/-/tree/wip/derived-refactor</a> and will remove Derived constraints, to be replaced by a little bit of cleverness in suppressing certain confusing error messages. My guess is that either or both of these will invalidate the current behavior of type-checker plugins. Sadly, this is not just a case of finding a new function that replicates the old behavior -- enough is changing under the hood that you might actually have to rewrite chunks of your code.<br>
<br>
I have never written a type-checker plugin, and so I don't currently have advice for you. But if you are a plugin author affected by this change and want help, please reach out -- I would be happy to walk you through the changes, and then hopefully make a little video explaining the process to other plugin authors.<br>
<br>
Neither patch will make it for 9.0, but I expect both to be in 9.2. There may be more where this came from (<a href="https://gitlab.haskell.org/ghc/ghc/-/issues/18965" rel="noreferrer" target="_blank">https://gitlab.haskell.org/ghc/ghc/-/issues/18965</a>) in the future, but it's all for a good cause.<br>
<br>
(I have bcc'd plugin authors that I'm aware of. Just adding this in case you're surprised at receiving this email.)<br>
<br>
Thanks,<br>
Richard</blockquote></div>
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