Hi Simon, I suppose this is possible, but it would change lots of error messages. (Currently, the uo_thing -- when reported -- is included inline with other parts of the message.) In my opinion, if we're going to reformat error messages, we could do better than this, anyway. I'm thinking of something like Type mismatch error: `uo_thing` has one type, but I expected a different type. type of `uo_thing`: `actual` expected type: `expected` where `expected` and `actual` are vertically aligned, as they are today. (This would require the ability to query the rendered length of an SDoc, which is not currently possible... but I don't think would be all that hard, as long as we were in a context that returned an SDoc.) What I like about this formatting is that it puts `uo_thing` right at the top of the error; this is the chunk of user-written syntax that causes the error. I've not tried this yet because I haven't the time to sift through the thousand error-message changes that this would cause. Putting uo_thing in the ErrCtxt would make something like this impossible, I think. Richard

On Jul 21, 2017, at 3:50 AM, Simon Peyton Jones wrote:

Richard

I’m on a train, so can’t look at the detail. But the following thought occurs to me.

uo_thing is a Maybe SDoc attached to a constraint. But a constraint also has a stack of [ErrCtxt], captured in the CtLoc. That’s what produces the “In the declaration for …” stuff.

Idea: instead of uo_thing, could we just push an extra ErrCtxt onto the stack, in the CtLoc for this constraint? (Or not, in the Nothing case.)

Simon   <> From: noreply@phabricator.haskell.org [mailto:noreply@phabricator.haskell.org] Sent: 18 July 2017 19:39 To: Simon Peyton Jones Subject: [Differential] [Request, 533 lines] D3754: Refactor uo_thing

goldfire created this revision. Herald added subscribers: thomie, rwbarton. Herald added a reviewer: hvr.

View Revision https://phabricator.haskell.org/D3754

REVISION SUMMARY

The uo_thing field of TypeEqOrigin is used to track the "thing" (either term or type) that has the type (kind) stored in the TypeEqOrigin fields. Previously, this was sometimes a proper Core Type, which needed zonking and tidying. Now, it is only HsSyn: much simpler, and the error messages now use the user-written syntax.

But this aspect of uo_thing didn't cause Trac #13819 https://ghc.haskell.org/trac/ghc/ticket/13819; it was the sibling field uo_arity that did. uo_arity stored the number of arguments of uo_thing, useful when reporting something like "should have written 2 fewer arguments". We wouldn't want to say that if the thing didn't have two arguments. However, in practice, GHC was getting this wrong, and this message didn't seem all that helpful. Furthermore, the calculation of the number of arguments is what caused Trac #13819 https://ghc.haskell.org/trac/ghc/ticket/13819 to fall over. This patch just removes uo_arity. In my opinion, the change to error messages is a nudge in the right direction.

Test case: typecheck/should_fail/T13819

TEST PLAN ./validate

REPOSITORY rGHC Glasgow Haskell Compiler

BRANCH wip/rae

REVISION DETAIL https://phabricator.haskell.org/D3754 https://phabricator.haskell.org/D3754

AFFECTED FILES compiler/ghci/RtClosureInspect.hs compiler/typecheck/Inst.hs compiler/typecheck/TcArrows.hs compiler/typecheck/TcErrors.hs compiler/typecheck/TcExpr.hs compiler/typecheck/TcHsType.hs compiler/typecheck/TcMType.hs compiler/typecheck/TcPat.hs compiler/typecheck/TcRnTypes.hs compiler/typecheck/TcSigs.hs compiler/typecheck/TcSplice.hs compiler/typecheck/TcSplice.hs-boot compiler/typecheck/TcTyClsDecls.hs compiler/typecheck/TcType.hs compiler/typecheck/TcUnify.hs compiler/typecheck/TcUnify.hs-boot compiler/types/Type.hs testsuite/tests/indexed-types/should_fail/T12867.stderr testsuite/tests/polykinds/T12593.stderr testsuite/tests/polykinds/T6039.stderr testsuite/tests/polykinds/T7278.stderr testsuite/tests/polykinds/T8616.stderr testsuite/tests/polykinds/T9200b.stderr testsuite/tests/rename/should_fail/rnfail026.stderr testsuite/tests/th/T3177a.stderr testsuite/tests/typecheck/should_fail/T11356.stderr testsuite/tests/typecheck/should_fail/T11672.stderr testsuite/tests/typecheck/should_fail/T12785b.stderr testsuite/tests/typecheck/should_fail/T13819.hs testsuite/tests/typecheck/should_fail/T13819.stderr testsuite/tests/typecheck/should_fail/T2994.stderr testsuite/tests/typecheck/should_fail/T3540.stderr testsuite/tests/typecheck/should_fail/T4875.stderr testsuite/tests/typecheck/should_fail/T7609.stderr testsuite/tests/typecheck/should_fail/T7778.stderr testsuite/tests/typecheck/should_fail/all.T testsuite/tests/typecheck/should_fail/tcfail070.stderr testsuite/tests/typecheck/should_fail/tcfail078.stderr testsuite/tests/typecheck/should_fail/tcfail113.stderr testsuite/tests/typecheck/should_fail/tcfail123.stderr testsuite/tests/typecheck/should_fail/tcfail132.stderr

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To: goldfire, simonpj, austin, bgamari, hvr Cc: rwbarton, thomie, Mikolaj, fryguybob, carter, mpickering, adamgundry