GHC 8.2.2 and 8.4.3 dislike the following rules: ``` haskell {-# RULES "pair fst snd" forall p. (,) (exl p) (exr p) = p "swap" forall p. (,) (exr p) (exl p) = swap p #-} ``` Error messages: ``` haskell /Users/conal/Haskell/concat/plugin/src/ConCat/Rebox.hs:485:1: warning: A constructor, (,), appears as outermost match in RULE lhs. This rule will be ignored. | 485 | "pair fst snd" forall p. (,) (exl p) (exr p) = p | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ /Users/conal/Haskell/concat/plugin/src/ConCat/Rebox.hs:489:1: warning: A constructor, (,), appears as outermost match in RULE lhs. This rule will be ignored. | 489 | "swap" forall p. (,) (exr p) (exl p) = swap p | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ``` GHC 8.0.2 didn't complain about these rules, though I'm unsure whether they worked as desired. Why disallow such rules (with constructors at the head)? -- Conal