Hi devs, Currently, type-checker plugins get to tell the solver its progress using a [TcPluginResult]( http://hackage.haskell.org/package/ghc-8.4.1/docs/TcRnTypes.html#t:TcPluginR... ): ``` data TcPluginResult = TcPluginContradiction [Ct] -- ^ The plugin found a contradiction. -- The returned constraints are removed from the inert set, -- and recorded as insoluble. | TcPluginOk [(EvTerm,Ct)] [Ct] -- ^ The first field is for constraints that were solved. -- These are removed from the inert set, -- and the evidence for them is recorded. -- The second field contains new work, that should be processed by -- the constraint solver. ``` So when asked to solve a _set_ of constraints, a tc plugin basically gets to say: A) This _one_ constraint out of the entire set is wrong, or B) The following _subset_ of constraints is solved (plus some new wanted constraints) Supposedly, picking A leads to better error messages when a constraint is obviously bad (i.e. Int ~ Char). However, the issue is that when a tc plugin picks A, then it will not be called again for the whole set of constraint it originally got; which in my use-case basically leads to a complete set of (actually solvable) unsolved constraints. This is "bad" because it leads to very confusing error messages, when 1. Start with a correct program: All constraints solvable => no errors reported 2. Add one line of code that doesn't type check 3. All constraints unsolvable => errors reported in parts of the program that used to type-check. I thought of multiple possible solutions, but changing TcPluginResult to: ``` data TcPluginResult = TcPluginResult { contradictions :: [Ct] -- ^ All of the contradictions the plugin found , solved :: [(EvTerm,Ct)] -- ^ Constraints that were solved , new :: [Ct] -- ^ New work to be processed by the rest of the constraint solver } ``` seems the best one as it allows a tc plugin to report _all_ the constraints that are bad, and _all_ the constraints that are bad. Thoughts? Perhaps a better solution? Thanks, Christiaan