Merge pull request #423 from TauOmicronMu/main

Add docs for GHC-04584

Author: BinderDavid

message-index/messages/GHC-04584/index.md

@@ -0,0 +1,15 @@
+---
+title: Expression syntax in pattern
+summary: There has been an attempt to pattern match on expression syntax. 
+severity: error
+introduced: 9.6.1
+---
+
+Pattern matching allows constructors and literals to be used to take apart values, exposing their contents. The syntax of patterns is very similar to that of the expressions that create values, but many constructs that are allowed in expression contexts are not meaningful patterns. 
+
+## Example Text
+
+```   
+Expression syntax in pattern: [1 .. 10]
+```
+

message-index/messages/GHC-04584/matchingdots/after/Matchingdots.hs

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+module Matchingdots where
+
+-- If the input is [1..10], return 0, otherwise return 1 
+f :: [Int] -> Int
+f xs 
+    | xs == [1..10] = 0 
+    | otherwise = 1      
+

message-index/messages/GHC-04584/matchingdots/before/Matchingdots.hs

@@ -0,0 +1,7 @@
+module Matchingdots where
+
+-- If the input is [1..10], return 0, otherwise return 1 
+f :: [Int] -> Int
+f [1..10] = 0 
+f _ = 1 
+

message-index/messages/GHC-04584/matchingdots/index.md

@@ -0,0 +1,16 @@
+---
+title: Using .. in pattern 
+---
+
+When pattern matching, expressions are not allowed as patterns to be matched against. Instead, consider matching on a list, and using a conditional. 
+
+
+# Error Message
+
+```
+before_dots.hs:3:3: error: [GHC-04584]
+    Expression syntax in pattern: [1 .. 10]
+  |
+3 | f [1..10] = 0 
+  |   ^^^^^^^
+```