Merge pull request #431 from TauOmicronMu/07626

Add docs for GHC-07626

Author: david-christiansen

message-index/messages/GHC-07626/example/after/Example.hs

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+module Example where
+
+merge [] bs rest = bs ++ rest
+merge as [] rest = as ++ rest
+merge (a:as) (b:bs) rest
+    | a > b     = merge as (b:bs) (a:rest)
+    | otherwise = merge (a:as) bs (b:rest)

message-index/messages/GHC-07626/example/before/Example.hs

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+module Example where
+
+merge [] bs rest = bs ++ rest
+merge as [] rest = as ++ rest
+merge a:as (b:bs) rest
+    | a > b     = merge as (b:bs) (a:rest)
+    | otherwise = merge (a:as) bs (b:rest)

message-index/messages/GHC-07626/example/index.md

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+---
+title: Parse error in pattern
+---
+
+This error occurs because we haven’t properly pattern matched on the list in our definition of the `merge` function. In particular, the parentheses are missing. The type here is a list, which means our options are to match on either the empty list, `[]`, or a list with head and tail, `(a:as)`. By omitting the parentheses around `a:as`, the compiler reads this as an argument, `a`, followed by a misplaced infix cons `(:)` operator, rather than considering the `:` to be part of the list being matched. 
+
+#  Error Message
+```
+Example.hs:5:1: error: [GHC-07626] Parse error in pattern: merge
+  |
+5 | merge a:as (b:bs) rest
+  | ^^^^^^^
+
+```

message-index/messages/GHC-07626/index.md

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+---
+title: Parse error in pattern
+summary: Compiler not able to parse pattern. 
+severity: error
+introduced: 9.6.1
+---
+
+This error occurs when the compiler is unable to understand a pattern that you're attempting to match on.
+
+## Example text 
+```
+Parse error in pattern: merge
+```
+