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I did some searching in the user manual and haddocks (generated from master branch) -- as far as I can see the only documentation we have for the foreign import ccall "dynamic" syntax is this one sentence in Foreign.Ptr haddocs:

-- To convert 'FunPtr' values to corresponding Haskell functions, one
-- can define a /dynamic/ stub for the specific foreign type, e.g.
--
-- > type IntFunction = CInt -> IO ()
-- > foreign import ccall "dynamic"
-- >   mkFun :: FunPtr IntFunction -> IntFunction

Main question I had is whether a function called via a "dynamic" wrapper would have properties of a "safe" call (GC may happen) or an "unsafe" call (no GC during the call).

Looking at the code, I can see that we call dsFImport with safety == PlaySafe, but maybe that's the safety of the mkFun (the wrapper) and not the actual FFI call, I'm not sure.

In any case this should be documented somewhere.