libraries/base/Foreign/Ptr.hs¶
Note [Exporting constructors of marshallable foreign types]¶
One might expect that IntPtr, WordPtr, and the other newtypes in the Foreign.C.Types and System.Posix.Types modules to be abstract, but this is not the case in GHC (see #5229 and #11983). In fact, we deliberately export the constructors for these datatypes in order to satisfy a requirement of the Haskell 2010 Report (§ 8.4.2) that if a newtype is used in a foreign declaration, then its constructor must be visible.
This requirement was motivated by the fact that using a type in a foreign declaration necessarily exposes some information about the type to the user, so being able to use abstract types in a foreign declaration breaks their abstraction (see #3008). As a result, the constructors of all FFI-related newtypes in base must be exported in order to be useful for FFI programming, even at the cost of exposing their underlying, architecture-dependent types.
