Document GHC-18365

message-index/messages/GHC-18365/index.md

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+---
+title: Pattern synonym arity mismatch
+summary: The number of arguments in the signature of a pattern synonym is fewer than the number of arguments in its equation.
+severity: error
+introduced: 9.8.1
+---
+
+Just as a function definition cannot specify fewer arguments than its signature,
+a pattern synonym's definition cannot specify fewer arguments than its signature.
+Specifying more arguments in the signature than in the definition is OK though,
+because it would just mean that the last parameter to the synonym is a function.

message-index/messages/GHC-18365/insufficientArgumentsInSignature/after/InsufficientArgumentsInSignature.hs

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+{-# LANGUAGE PatternSynonyms #-}
+
+module InsufficientArgumentsInSignature where
+
+pattern P :: Int -> Int -> (Int, Int)
+pattern P a b = (a, b)

message-index/messages/GHC-18365/insufficientArgumentsInSignature/before/InsufficientArgumentsInSignature.hs

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+{-# LANGUAGE PatternSynonyms #-}
+
+module InsufficientArgumentsInSignature where
+
+pattern P :: Int -> (Int, Int)
+pattern P a b = (a, b)

message-index/messages/GHC-18365/insufficientArgumentsInSignature/index.md

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+---
+title: Insufficient arguments in type signature
+---
+
+## Error Message
+
+```
+InsufficientArgumentsInSignature:6:1: error: [GHC-18365]
+    • Pattern synonym ‘P’ has two arguments
+        but its type signature has 1 fewer arrows
+    • In the declaration for pattern synonym ‘P’
+  |
+6 | pattern P a b = (a, b)
+  | ^^^^^^^^^^^^^^^^^^^^^^
+```
+
+## Explanation
+
+In this case, the equation in the definition specified two arguments, but the
+signature specified only one.