Fix diffs for assert and ≡ (#1266)

... as caught by @sjakobi in https://github.com/dhall-lang/dhall-haskell/issues/1256#issuecomment-526829515

dhall/src/Dhall/Diff.hs

@@ -698,6 +698,15 @@ diff l@(Pi {}) r =
     mismatch l r
 diff l r@(Pi {}) =
     mismatch l r
+diff (Assert aL) (Assert aR) =
+    align
+        (  "  " <> keyword "assert"
+        <> hardline <> colon <> " " <> diff aL aR
+        )
+diff l@(Assert {}) r =
+    mismatch l r
+diff l r@(Assert {}) =
+    mismatch l r
 diff l r =
     diffAnnotatedExpression l r
 
@@ -905,14 +914,30 @@ diffNotEqualExpression l@(BoolNE {}) r@(BoolNE {}) =
     enclosed' "    " (operator "!=" <> "  ") (docs l r)
   where
     docs (BoolNE aL bL) (BoolNE aR bR) =
-        Data.List.NonEmpty.cons (diffApplicationExpression aL aR) (docs bL bR)
+        Data.List.NonEmpty.cons (diffEquivalentExpression aL aR) (docs bL bR)
     docs aL aR =
-        pure (diffApplicationExpression aL aR)
+        pure (diffEquivalentExpression aL aR)
 diffNotEqualExpression l@(BoolNE {}) r =
     mismatch l r
 diffNotEqualExpression l r@(BoolNE {}) =
     mismatch l r
 diffNotEqualExpression l r =
+    diffEquivalentExpression l r
+
+diffEquivalentExpression
+    :: (Eq a, Pretty a) => Expr Void a -> Expr Void a -> Diff
+diffEquivalentExpression l@(Equivalent {}) r@(Equivalent {}) =
+    enclosed' "  " (operator "≡" <> " ") (docs l r)
+  where
+    docs (Equivalent aL bL) (Equivalent aR bR) =
+        Data.List.NonEmpty.cons (diffApplicationExpression aL aR) (docs bL bR)
+    docs aL aR =
+        pure (diffApplicationExpression aL aR)
+diffEquivalentExpression l@(Equivalent {}) r =
+    mismatch l r
+diffEquivalentExpression l r@(Equivalent {}) =
+    mismatch l r
+diffEquivalentExpression l r =
     diffApplicationExpression l r
 
 diffApplicationExpression :: (Eq a, Pretty a) => Expr Void a -> Expr Void a -> Diff

dhall/tests/Dhall/Test/QuickCheck.hs

@@ -242,6 +242,8 @@ instance (Arbitrary s, Arbitrary a) => Arbitrary (Expr s a) where
                 , ( 1, lift2 ToMap)
                 , ( 7, lift2 Field)
                 , ( 7, lift2 Project)
+                , ( 1, lift1 Assert)
+                , ( 1, lift2 Equivalent)
                 , ( 7, lift1 Embed)
                 ]
             )