Typo: these potential instance exists

added Plow Tbug error messages newcomer labels and removed needs triage label

Question from a newcomer here. After some initial study, two ideas came to me. Does either of them point me in the right direction?

Follow practices in this similar case
Add a function thisOrThese in Outputable.hs

@CarrieMY that seems like a solid plan of attack.

@wz1000 Thanks for reply. Which one of them sounds better, or it does not matter? One more question, is any test case needed to be added?

You can refactor error messages from places like those linked in (1) using a common helper function in Outputable.hs, fixing this bug along the way.

A test case does need to be added. To do this, first come up with a small reproducible example that illustrates the bug in this ticket. Then add it to testsuite/tests/typecheck/should_fail/ along with a corresponding entry in testsuite/tests/typecheck/should_fail/all.T.

Hi @wz1000 , I am doing make test and found other than the test I added (T20122), several tests output also got affected. Do I need to modify their stderr files as well? Or other solutions? If additional information needed, please let me know.

Test cases affected:

   should_fail/T20122.run     T20122 [stderr mismatch] (normal) -- The test I aded
   should_fail/T4921.run      T4921 [stderr mismatch] (normal)
   should_fail/T5858.run      T5858 [stderr mismatch] (normal)
   should_fail/tcfail040.run  tcfail040 [stderr mismatch] (normal)
   should_fail/tcfail043.run  tcfail043 [stderr mismatch] (normal)
   should_fail/tcfail128.run  tcfail128 [stderr mismatch] (normal)

Take T4921 as an example:

--- should_fail/T4921.run/T4921.stderr.normalised       2021-07-24 22:39:33.261186400 +0800
+++ should_fail/T4921.run/T4921.comp.stderr.normalised  2021-07-24 22:39:33.262180600 +0800
@@ -3,8 +3,9 @@
      Ambiguous type variables ‘a0’, ‘b1’ arising from a use of ‘f’
       prevents the constraint ‘(C a0 b1)’ from being solved.
       Relevant bindings include x :: a0 (bound at T4921.hs:10:1)
-      Probable fix: use a type annotation to specify what ‘a0’, ‘b1’ should be.
-      These potential instance exist:
+      Probable fix: use a type annotation to specify what ‘a0’,
+                                                          ‘b1’ should be.
+      This potential instance exists:
         instance C Int Char -- Defined at T4921.hs:7:10
      In the first argument of ‘fst’, namely ‘f’
       In the expression: fst f
@@ -14,7 +15,7 @@
      Ambiguous type variable ‘b0’ arising from a use of ‘f’
       prevents the constraint ‘(C Int b0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘b0’ should be.
-      These potential instance exist:
+      This potential instance exists:
         instance C Int Char -- Defined at T4921.hs:7:10
      In the first argument of ‘fst’, namely ‘f’
       In the expression: fst f :: Int
*** unexpected failure for T4921(normal)

Yes, those tests need to be accepted. You can either manually modify them, or do so automatically using the instructions at https://gitlab.haskell.org/ghc/ghc/-/wikis/building/running-tests/updating

Trying out the instructions. Very helpful!

Now on the Travis validating process. Edit: cannot trigger travis as described in wiki. I am new to travis and is this wiki still up to date? Anyway, I will try to validate locally before submit the patch.

@wz1000 I am ready for MR. May I know which branch should I merge to?

Just submit an MR to the master branch.

@wz1000 I will correct the trailing spaces and rebase problem.

Thanks for your advice. I will work on this.

mentioned in merge request !6244 (closed)

mentioned in merge request !6246 (closed)

mentioned in commit f5da2454

closed with commit 7dc0dc99

mentioned in commit 7dc0dc99

Typo: these potential instance exists

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Expected behavior

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