Safe Haskell | Safe-Inferred |
---|---|
Language | Haskell2010 |
Data types for describing an array paired with an index
into it. This is intended to be used in wrappers for unsafe
FFI calls. For example, the POSIX function recvfrom
takes
a socklen_t*
argument. (Let us assume that socklen_t
is equivalant to int
for this example.) How is this argument best
described by a Haskell type? When working with pinned memory,
the best option
.
It works equally well regardless of whether we originally had an
array of Ptr
CInt
CInt
or a pointer to a single CInt
. This works because
of functions like advancePtr
and
plusPtr
that effectively index into an array.
Unpinned memory, however, is trickier. We want to have the full
flexibility (handling both a single-element or
multi-element buffer) that
affords. We cannot
offset into a Ptr
CInt
MutablePrimArray
to get a new one like we
could with Ptr
. (Such a function is not possible because
unpinned memory can be relocated.) So, the offseting must
be done in the C function wrapped by the unsafe
FFI. This
means that the offset must be passed together with the
MutablePrimArray
. This is the precisely the product that
MutablePrimArrayOffset
represents. In a type signature, it
provides additional clarity about the meaning of the offset.
This library is used in the extensively in the posix-api
library to clarify intent in a number of type signatures.
Synopsis
- data PrimArrayOffset a = PrimArrayOffset {}
- data MutablePrimArrayOffset s a = MutablePrimArrayOffset {
- array :: !(MutablePrimArray s a)
- offset :: !Int
- indexOffset :: Prim a => PrimArrayOffset a -> a
- readOffset :: (PrimMonad m, Prim a) => MutablePrimArrayOffset (PrimState m) a -> m a
Types
data MutablePrimArrayOffset s a Source #
A mutable primitive array and an index into the array.
MutablePrimArrayOffset | |
|
Resolution
indexOffset :: Prim a => PrimArrayOffset a -> a Source #
Recover the element in the primitive array.
readOffset :: (PrimMonad m, Prim a) => MutablePrimArrayOffset (PrimState m) a -> m a Source #
Recover the element in the mutable primitive array.