Merge pull request #5297 from sellout/impossible-in-lexer

Turn a possible “impossible” into a parse failure
unisonweb · Aug 23, 2024 · 1207653 · 1207653
2 parents 7178ec2 + a025454
commit 1207653
Showing 1 changed file with 3 additions and 1 deletion.
diff --git a/unison-syntax/src/Unison/Syntax/Lexer/Unison.hs b/unison-syntax/src/Unison/Syntax/Lexer/Unison.hs
@@ -203,7 +203,9 @@ token'' tok p = do
             else
               if column p < top l
                 then S.put (env {layout = pop l}) >> ((Token Close p p :) <$> pops p)
-                else error "impossible"
+                else -- we hit this branch exactly when `token''` is given the state
+                -- `{layout = [], opening = Nothing, inLayout = True}`
+                  fail "internal error: token''"
 
     -- don't emit virtual semis in (, {, or [ blocks
     topContainsVirtualSemis :: Layout -> Bool