Merge pull request #449 from haskellfoundation/GHC-31574

Document GHC-31574

Author: BinderDavid

message-index/messages/GHC-31574/index.md

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+---
+title: Illegal use of linear functions
+summary: The linear function type is used, but LinearTypes are not enabled
+severity: error
+introduced: 9.6.1
+---
+
+Starting with version 9.0, GHC supports linear types.
+These types allow the programmer to express how often the argument to a function is used in the function body.
+For this reason, a new function type `a %1 -> b` has been introduced which stands for *linear* functions which use their argument of type `a` exactly once.
+In order to use these functions, however, the extension `LinearTypes` has to be enabled.

message-index/messages/GHC-31574/linearIdentity/after/LinearIdentity.hs

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+{-# LANGUAGE LinearTypes #-}
+module LinearIdentity where
+
+linearIdentity :: a %1 -> a
+linearIdentity x = x

message-index/messages/GHC-31574/linearIdentity/before/LinearIdentity.hs

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+
+module LinearIdentity where
+
+linearIdentity :: a %1 -> a
+linearIdentity x = x

message-index/messages/GHC-31574/linearIdentity/index.md

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+---
+title: Linear identity function needs LinearTypes extension
+---
+
+In this example, the programmer correctly implemented the identity function with a linear type signature.
+This type signature correctly states that the identity function uses its argument exactly once.
+In order to be able to use this type signature, the extension `LinearTypes` has to be enabled.
+
+```
+messages/GHC-31574/linearIdentity/before/LinearIdentity.hs:3:21: error: [GHC-31574]
+    Illegal use of linear functions
+    Suggested fix: Perhaps you intended to use LinearTypes
+  |
+3 | linearIdentity :: a %1 -> a
+  | 
+```