Add documentation for GHC-43085 (OverlappingInstances) (#546)

Author: Qbxs

message-index/messages/GHC-43085/index.md

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+---
+title: Overlapping Instances
+summary: Multiple type class instances are resolved for a single type class method call.
+severity: error
+introduced: 9.6.1
+---
+
+Type class methods that make use of
+[overlapping instances](https://downloads.haskell.org/ghc/9.12.1/docs/users_guide/exts/instances.html#overlapping-instances)
+lead may lead to an error because the resolved candidates would lead to indeterministic program behavior.
+
+We can solve this by annotating the instances with the `{-# OVERLAPS #-}`, `{-# OVERLAPPING #-}`, `{-# OVERLAPPABLE #-}` and `{-# INCOHERENT #-}` pragma.
+The former three can only be used if instances are *strictly more specific* than one another, i.e. the type of an instance can be substituted for the type of another instance.
+
+Note that `{-# INCOHERENT #-}` still leads to indeterministic behavior and thus should be used with caution.
+
+~~~
+before/Overlapping.hs:18:19: error: [GHC-43085]
+    • Overlapping instances for Formatter (Maybe Int)
+        arising from a use of ‘format’
+      Matching instances:
+        instance Formatter (Maybe Int)
+          -- Defined at before/Overlapping.hs:13:10
+        instance Formatter a => Formatter (Maybe a)
+          -- Defined at before/Overlapping.hs:9:10
+    • In the second argument of ‘($)’, namely
+        ‘format (Nothing :: Maybe Int)’
+      In the expression: putStrLn $ format (Nothing :: Maybe Int)
+      In an equation for ‘main’:
+          main = putStrLn $ format (Nothing :: Maybe Int)
+   |
+18 | main = putStrLn $ format (Nothing :: Maybe Int)
+   |                   ^^^^^^
+~~~

message-index/messages/GHC-43085/overlapping/after/Overlapping.hs

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+module Overlapping where
+
+class Formatter a where
+  format :: a -> String
+
+instance Formatter Int where
+  format = show
+
+instance {-# OVERLAPPABLE #-} (Formatter a) => Formatter (Maybe a) where
+  format (Just x) = format x
+  format Nothing = ""
+
+instance {-# OVERLAPS #-} Formatter (Maybe Int) where
+  format (Just x) = format x
+  format Nothing = "NaN"
+
+main :: IO ()
+main = putStrLn $ format (Nothing :: Maybe Int)

message-index/messages/GHC-43085/overlapping/before/Overlapping.hs

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+module Overlapping where
+
+class Formatter a where
+  format :: a -> String
+
+instance Formatter Int where
+  format = show
+
+instance (Formatter a) => Formatter (Maybe a) where
+  format (Just x) = format x
+  format Nothing = ""
+
+instance Formatter (Maybe Int) where
+  format (Just x) = format x
+  format Nothing = "NaN"
+
+main :: IO ()
+main = putStrLn $ format (Nothing :: Maybe Int)

message-index/messages/GHC-43085/overlapping/index.md

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+---
+title: Overlapping instances for Maybe
+---
+
+Here, the two instances `Formatter (Maybe a)` and `Formatter (Maybe Int)` are overlapping.
+Since the latter is more specific, we can solve this by annotating the instances with the `{-# OVERLAPPABLE #-}` and `{-# OVERLAPS #-}`.