ediffGolden fails with ToExpr instances that don't preserve Eq, such as HashMap

[PiDelport]

Problem

I have some tests using tasty-golden + ediffGolden with data structures containing HashMap values. These tests work locally, but fail on CI due to the CI server generating Expr values for the HashMap values with the items in a different order than was captured in the golden snapshot.

Analysis

ediffGolden assumes (?) that a == b implies toExpr a == toExpr b.

However, the ToExpr instance for HashMap (source) captures the raw ordering returned by HM.toList, which isn't reliable across platforms (or in the presence of collisions and insert reordering), so the above property doesn't hold.

(This also affects the ToExpr instance of HashSet.)

Workaround

I'm already using a Expr construction in the affected code, so for the time being, I'm just using this variation of toExpr for HashMap sorts the items:

toExpr_HM :: (ToExpr k, ToExpr v, Ord k, Ord v) => HM.HashMap k v -> Expr
toExpr_HM x = App "HM.fromList" [ toExpr $ sort $ HM.toList x ]