Constructor as outermost match in RULE under GHC 8.2.2 & 8.4.3

[Ben Gamari]

On August 2, 2018 9:25:05 PM EDT, Conal Elliott <conal at conal.net> wrote:
>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

While we only started warning about them recently, previously the rules almost certainly weren't firing as you expected them to. The trouble is that constructors are often replaced with their wrappers rather early in simplification. This meant that matching on constructors in rules was quite unreliable. This is discussed in #13290.

Cheers,

- Ben