Merge pull request #429 from TauOmicronMu/78892

Add docs for GHC-78892

Author: david-christiansen

message-index/messages/GHC-78892/example/after/Example.hs

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+module Example where
+
+-- Is the second argument the square of the first?
+sq :: Int -> Int 
+sq x = x^2
+
+f :: Int -> Int -> Bool 
+f n m 
+    | m == sq n = True
+    | otherwise = False 

message-index/messages/GHC-78892/example/before/Example.hs

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+module Example where
+
+-- Is the second argument the square of the first? 
+f :: Int -> Int -> Bool 
+f n (let sq = x^2 in sq n) = True 
+f n _ = False

message-index/messages/GHC-78892/example/index.md

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+---
+title: Using let ... in ... syntax in pattern.  
+---
+
+When pattern matching, let expressions are not allowed as patterns to be matched against. The logic encapsulated here should either be lifted to outside of the function, or moved inside of the function itself. 
+
+#  Error Message 
+
+```
+Example.hs:5:6: error: [GHC-78892]
+    (let ... in ...)-syntax in pattern
+  |
+5 | f n (let exp = x^2 in exp n) = True 
+  |      ^^^^^^^^^^^^^^^^^^^^^^
+```
+
+#  Intermediate fix step 
+In the below example, it is worth noting that you may first have considered refactoring to something along the lines of 
+```
+sq :: Int -> Int 
+sq x = x^2 
+
+f :: Int -> Int -> Bool 
+f n (sq n) = True
+f n _ = False 
+
+``` 
+
+The above code would throw a parse error in the pattern [GHC-07626](https://errors.haskell.org/messages/GHC-07626/).  

message-index/messages/GHC-78892/index.md

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+---
+title: let-syntax in pattern
+summary: An attempt has been made to use a let expression whilst pattern matching. 
+severity: error
+introduced: 9.6.1
+---
+
+Pattern matching allows constructors and literals to be used to take apart values, exposing their contents. Even though patterns resemble expressions, they are not evaluated. The `let ... in ...` syntax is used to introduce declarations that are scoped to an expression, which doesn't make sense as a pattern without evaluation. See [GHC-04584](https://errors.haskell.org/messages/GHC-78892/) for more information on why expressions aren't permitted in pattern matching. 
+
+## Example Text 
+``` 
+(let ... in ...)-syntax in pattern
+```