Merge pull request #516 from haskell/GHC-98980

Document GHC-98980

Author: david-christiansen

message-index/messages/GHC-98980/command-pattern/after/CommandPattern.hs

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+{-# LANGUAGE Arrows #-}
+module CommandPattern where
+
+f _ = 5

message-index/messages/GHC-98980/command-pattern/before/CommandPattern.hs

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+{-# LANGUAGE Arrows #-}
+module CommandPattern where
+
+f (y -< x) = 5

message-index/messages/GHC-98980/command-pattern/index.md

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+---
+title: A command in a pattern
+---
+## Message
+```
+CommandPattern.hs:4:4: error: [GHC-98980]
+    Command syntax in pattern: y -< x
+  |
+4 | f (y -< x) = 5
+  |    ^^^^^^
+```
+
+## Explanation
+
+In this example, the command `y -< x` is used where a pattern is expected.
+To fix the error, replace the command with a pattern.

message-index/messages/GHC-98980/index.md

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+---
+title: Command syntax in pattern
+summary: Arrow command syntax was used in a pattern.
+severity: error
+introduced: 9.6.1
+extension: Arrows
+---
+
+[Arrows](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/arrows.html) are an abstraction for computations that has special syntactic support in GHC.
+When the `Arrows` extension is enabled, some expressions that would normally be interpreted as containing the infix operators `-<` and `-<<` are instead interpreted as a new syntactic category of _commands_.
+These commands do not make sense as patterns, and using them in a pattern context is an error.