Add GHC-22705 - NonCanonicalMonad

Author: 414owen

message-index/messages/GHC-22705/index.md

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+---
+title: Noncanonical ‘return’ definition detected
+summary: "'return', or '>>' not defined in terms of 'pure' or '*>'."
+severity: warning
+flag: -Wnoncanonical-monad-instances
+introduced: 9.6.1
+---
+
+Since Applicative became a superclass of Monad, the 'return', and '>>' functions of law-abiding
+Monad instances are assumed to be identical to the 'pure' and '*>' functions.
+
+This warning is caused by a definition of 'return' that isn't equal to 'pure', or a definition of
+'(>>)' that isn't equal to '(*>)'.

message-index/messages/GHC-22705/return/after/Return.hs

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+module Return where
+
+data P a = P
+
+instance Functor P where
+  fmap _ _ = P
+
+instance Applicative P where
+  pure _ = P
+  _ <*> _ = P
+
+instance Monad P where
+  _ >>= _ = P

message-index/messages/GHC-22705/return/before/Return.hs

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+module Return where
+
+data P a = P
+
+instance Functor P where
+  fmap _ _ = P
+
+instance Applicative P where
+  pure _ = P
+  _ <*> _ = P
+
+instance Monad P where
+  _ >>= _ = P
+  return _ = P

message-index/messages/GHC-22705/return/index.md

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+---
+title: Non-canonical 'return' defined
+---
+
+In this example, 'return' is defined in terms of 'P', which doesn't directly use Applicative's 'pure'.
+
+To fix this, you can define 'return' as 'pure', or leave the definition empty, to use the default value.

message-index/messages/GHC-22705/shortfish/after/Shortfish.hs

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+module Shortfish where
+
+data P a = P
+
+instance Functor P where
+  fmap _ _ = P
+
+instance Applicative P where
+  pure _ = P
+  _ <*> _ = P
+
+instance Monad P where
+  _ >>= _ = P

message-index/messages/GHC-22705/shortfish/before/Shortfish.hs

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+module Shortfish where
+
+data P a = P
+
+instance Functor P where
+  fmap _ _ = P
+
+instance Applicative P where
+  pure _ = P
+  _ <*> _ = P
+
+instance Monad P where
+  _ >>= _ = P
+  _ >> _ = P

message-index/messages/GHC-22705/shortfish/index.md

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+---
+title: Non-canonical '>>' defined
+---
+
+In this example, '>>' is defined in terms of 'P', which doesn't directly use Applicative's '*>'.
+
+To fix this, you can define '>>' as '*>', or leave the definition empty, to use the default value.