[Haskell-cafe] Storables and Ptrs

Antoine Latter aslatter at gmail.com
Mon Dec 6 06:46:44 CET 2010

On Sun, Dec 5, 2010 at 10:45 PM, Tyler Pirtle <teeler at gmail.com> wrote:
> Hi cafe,
> I'm just getting into Foreign.Storable and friends and I'm confused
> about the class storable. For GHC, there are instances of storable for
> all kinds of basic types (bool, int, etc) - but I can't find the
> actual declaration of those instances.
> I'm confused that it seems that all Storable instances operate on a
> Ptr, yet none of these types allow access to an underlying Ptr. I
> noticed that it's possible via Foreign.Marshal.Utils to call 'new' and
> get a datatype wrapped by a Ptr, but this isn't memory managed - I'd
> have to explicitly free it? Is that my only choice?

The Storable class defines how to copy a particular Haskell type to or
from a raw memory buffer - specifically represented by the Ptr type.
It is most commonly used when interacting with non-Haskell (or
'Foreign') code, which is why a lot of the tools look like they
require manual memory management (because foreign-owned resources must
often be managed separately anyway).

Not all of the means of creating a Ptr type require manual memory
management - the 'alloca' family of Haskell functions allocate a
buffer and then free it automatically when outside the scope of the
passed-in callback (although 'continuation' or 'action' would be the
more Haskell-y way to refer to the idea):

alloca :: Storable a => (Ptr a -> IO b) -> IO b

This can be used to call into C code expecting pointer input or output
types to great effect:

wrapperAroundForeignCode :: InputType -> IO OutputType
wrapperAroundForeignCode in =
  alloca $ \inPtr ->
  alloca $ outPtr -> do
    poke inPtr in
    c_call inPtr outPtr
    peek outPtr

The functions 'peek' and 'poke' are from the Storable class, and I
used the 'alloca' function to allocate temporary storage for the
pointers I pass into C-land.

Is there a particular problem you're trying to solve? We might be able
to offer more specific advice. The Storable and Foreign operations may
not even be the best solution to what you're trying to do.

Take care,

> Is there a way that given just simply an Int I could obtain a Ptr from
> it, and then invoke the storable functions on it? Or for that matter,
> if I go and create some new data type, is there some generic
> underlying thing (ghc-only or otherwise) that would let me have a Ptr
> of it?
> Thanks,
> Tyler
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