Merge pull request #452 from haskellfoundation/GHC-57396

Document GHC-57396

Author: david-christiansen

message-index/messages/GHC-57396/index.md

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+---
+title: Linear types are not supported in FFI
+summary: Linear types are not supported when using GHC's FFI functionality
+severity: error
+introduced: 9.6.1
+---
+
+If the extension `LinearTypes` is enabled, then programmers can write expressions which have the linear
+function type `a %1 -> b` for functions which use their argument exactly once.
+When calling external functions via GHC's FFI functionality, however, then it is not allowed to use a linear function type.
+
+More information about this specific restriction can be found in the discussion in the [GHC issue tracker](https://gitlab.haskell.org/ghc/ghc/-/issues/18472).

message-index/messages/GHC-57396/linear-ffi/after/LinearFFI.hs

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+{-# LANGUAGE LinearTypes #-}
+module LinearFFI where
+
+import Foreign.Ptr
+
+foreign import ccall "exp" c_exp :: Double -> Double

message-index/messages/GHC-57396/linear-ffi/before/LinearFFI.hs

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+{-# LANGUAGE LinearTypes #-}
+module LinearFFI where
+
+import Foreign.Ptr
+
+foreign import ccall "exp" c_exp :: Double %1 -> Double

message-index/messages/GHC-57396/linear-ffi/index.md

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+---
+title: Linear function type was used in foreign function call
+---
+
+In this example the programmer tries to use a C function with the help of GHC's FFI system.
+The type given to the imported function, however, is not allowed.
+Only non-linear function types can be given to foreign functions.
+
+```
+messages/GHC-57396/linear-ffi/before/LinearFFI.hs:6:1: error: [GHC-57396]
+    • Unacceptable argument type in foreign declaration:
+        Linear types are not supported in FFI declarations, see #18472
+    • When checking declaration:
+        foreign import ccall safe "exp" c_exp :: Double %1 -> Double
+  |
+6 | foreign import ccall "exp" c_exp :: Double %1 -> Double
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+```