Rewrite rules are ignored in Safe Haskell [GHC-56147]

GHC implements the SafeHaskell extension which allows programmers to restrict modules to a specific subset which is considered safe and does not contain loopholes such as unsafePerformIO. More details are available in the GHC User’s Guide. One of the features which isn’t considered safe are user-specified rewrite rules. GHC therefore warns that all rewrite rules in a module which is declared safe will be ignored.

Examples

Cannot add a rewrite rule for the identity function

Wanting to rewrite occurrences of myId x to x in programs is sensible. But since GHC cannot check in general whether rewrite rules violate the guarantees of SafeHaskell, GHC chooses to ignore all such rules in modules which are annotated as Safe.These rewrite rules should therefore be removed.

messages/GHC-56147/ignored-rule/before/IgnoredRule.hs:4:11: warning: [GHC-56147]
    Rule "myId" ignored
    Defining user rules is disabled under Safe Haskell
  |
4 | {-# RULES "myId" forall x. myId x = x #-}
  |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^

IgnoredRule.hs

Before

{-# LANGUAGE Safe #-}
module IgnoredRule where

{-# RULES "myId" forall x. myId x = x #-}
myId :: a -> a
myId x = x

After

{-# LANGUAGE Safe #-}
module IgnoredRule where


myId :: a -> a
myId x = x