Merge pull request #426 from TauOmicronMu/09646

Add docs for GHC-09646

Author: david-christiansen

message-index/messages/GHC-09646/index.md

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+---
+title: Tuple section in pattern
+summary: There has been an attempt to pattern match on a tuple section.
+severity: error
+introduced: 9.6.1
+---
+
+[Tuple sections](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/tuple_sections.html) are a concise syntax for functions that result in tuples. The missing expressions in the tuple become arguments to the function. Because tuple sections always result in functions, and functions cannot be matched in patterns, tuple sections are not allowed in patterns.
+
+## Example Text
+
+```
+Tuple section in pattern context
+``` 

message-index/messages/GHC-09646/snd/after/Snd.hs

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+module Snd where
+
+{-# LANGUAGE TupleSections #-}
+
+snd' :: (a, a) -> a 
+snd' (_,x) = x 

message-index/messages/GHC-09646/snd/before/Snd.hs

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+module Snd where
+
+{-# LANGUAGE TupleSections #-}
+
+snd' :: (a, a) -> a 
+snd' (,x) = x 

message-index/messages/GHC-09646/snd/index.md

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+---
+title: Using tuple section in pattern
+---
+
+When pattern matching, tuple sections are not allowed as patterns to be matched against. In the example below, one of the components of a tuple pattern was forgotten. We can fix this by adding the wildcard _ to match on the first element of the pair. 
+
+#  Error Message 
+
+```
+before.hs:4:6: error: [GHC-09646] Tuple section in pattern context
+  |
+4 | snd' (,x) = x 
+  |      ^^^^
+Failed, no modules loaded.
+```