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Document GHC-56147 #462

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Sep 12, 2023
Merged

Document GHC-56147 #462

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Document GHC-56147
BinderDavid Sep 3, 2023
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6 changes: 6 additions & 0 deletions message-index/messages/GHC-56147/ignored-rule/after/IgnoredRule.hs
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{-# LANGUAGE Safe #-}
module IgnoredRule where


myId :: a -> a
myId x = x
6 changes: 6 additions & 0 deletions message-index/messages/GHC-56147/ignored-rule/before/IgnoredRule.hs
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{-# LANGUAGE Safe #-}
module IgnoredRule where

{-# RULES "myId" forall x. myId x = x #-}
myId :: a -> a
myId x = x
14 changes: 14 additions & 0 deletions message-index/messages/GHC-56147/ignored-rule/index.md
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---
title: 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 #-}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
9 changes: 9 additions & 0 deletions message-index/messages/GHC-56147/index.md
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---
title: Rewrite rules are ignored in SafeHaskell
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summary: Rewrite rules are not allowed in SafeHaskell and therefore ignored
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severity: warning
introduced: 9.6.1
---

GHC implements the SafeHaskell extension which allows programmers to restrict modules to a specific subset which is considered safe.
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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.