Merge pull request #344 from haskellfoundation/GHC-66228

Document GHC-66228

Author: david-christiansen

message-index/messages/GHC-66228/index.md

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+---
+title: View pattern in expression context
+summary: A view pattern was used in an expression, rather than a pattern.
+extension: ViewPatterns
+introduced: 9.6.1
+---
+
+A [view pattern](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/view_patterns.html) allows pattern matching on the result of a function.
+While most kinds of patterns look just like the expressions that construct the data that the pattern recognizes, view patterns don't correspond directly to an expression.
+This means that using them where an expression is expected is an error.

message-index/messages/GHC-66228/not-lambda/after/NotLambda.hs

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+module NotLambda where
+
+five = (\x -> x) 5

message-index/messages/GHC-66228/not-lambda/before/NotLambda.hs

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+module NotLambda where
+
+five = (x -> x) 5

message-index/messages/GHC-66228/not-lambda/index.md

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+---
+title: Missing lambda
+---
+## Message
+```
+NotLambda.hs:3:9: error: [GHC-66228]
+    View pattern in expression context: x -> x
+  |
+3 | five = (x -> x) 5
+  |         ^^^^^^
+```
+
+## Explanation
+
+In this example, the programmer forgot the `\` at the beginning of their anonymous function.
+This caused the parser to recognize the function as a view pattern instead of a lambda.

message-index/messages/GHC-66228/view-pattern-expr/after/ViewPatternExpr.hs

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+module ViewPatternExpr where
+
+h = sqrt

message-index/messages/GHC-66228/view-pattern-expr/before/ViewPatternExpr.hs

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+module ViewPatternExpr where
+
+h = (sqrt -> y)

message-index/messages/GHC-66228/view-pattern-expr/index.md

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+---
+title: View pattern in expression
+---
+## Message
+```
+ViewPatternExpr.hs:3:6: error: [GHC-66228]
+    View pattern in expression context: sqrt -> y
+  |
+3 | h = (sqrt -> y)
+  |      ^^^^^^^^^
+```
+
+## Explanation
+
+In this context, the view pattern is being used instead of a function.
+The issue can be corrected by not using a view pattern.