Merge pull request #354 from haskellfoundation/GHC-53633

Document GHC-53633, redundant patterns

Author: david-christiansen

message-index/messages/GHC-53633/index.md

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+---
+title: Redundant patterns
+summary: A pattern is impossible to reach due to earlier patterns
+introduced: 9.6.1
+severity: warning
+flag: -Woverlapping-patterns
+---
+
+In multi-equation definitions and case expressions, patterns are matched from top to bottom.
+This means that earlier patterns are tested before later patterns.
+If every value that a pattern could match has already been ruled out by earlier patterns, then it is unreachable, which typically indicates a bug or misunderstanding.
+

message-index/messages/GHC-53633/overlapping-catchall/after/Overlapping.hs

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+module Overlapping where
+
+f [] = True
+f (_:_:xs) = f xs
+f [_] = False

message-index/messages/GHC-53633/overlapping-catchall/before/Overlapping.hs

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+module Overlapping where
+
+f [] = True
+f (_:_:xs) = f xs
+f [_] = False
+f _ = False

message-index/messages/GHC-53633/overlapping-catchall/index.md

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+---
+title: Redundant catchall pattern
+---
+
+## Message
+```
+Overlapping.hs:6:1: warning: [GHC-53633] [-Woverlapping-patterns]
+    Pattern match is redundant
+    In an equation for ‘f’: f _ = ...
+  |
+6 | f _ = False
+  | ^^^^^^^^^^^
+```
+
+## Explanation
+
+In this case, all the patterns that can match a list were matched in the prior patterns, so the catch-all pattern in the last case will never be matched and is dead code.

message-index/messages/GHC-53633/overlapping-patterns/after/Overlapping.hs

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+module Overlapping where
+
+f :: Maybe a -> Bool
+f (Just _) = True
+f _ = False

message-index/messages/GHC-53633/overlapping-patterns/before/Overlapping.hs

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+module Overlapping where
+
+f :: Maybe a -> Bool
+f (Just _) = True
+f (Just _) = False
+f _ = False
+

message-index/messages/GHC-53633/overlapping-patterns/index.md

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+---
+title: Redundant constructor pattern
+---
+## Message
+
+```
+Overlapping.hs:5:1: warning: [GHC-53633] [-Woverlapping-patterns]
+    Pattern match is redundant
+    In an equation for ‘f’: f (Just _) = ...
+  |
+5 | f (Just _) = False
+  | ^^^^^^^^^^^^^^^^^^
+```
+
+## Explanation
+
+The definition of `f` contains two separate patterns that both match `(Just _)`. This usually indicates a bug, because the second one would be unreachable. This can be fixed by removing the redundant case, but it usually indicates that a change to the program was made quickly and demands more thought.