[Haskell-cafe] Quickest way to pass Text to C code

Yves Parès yves.pares at gmail.com
Wed Mar 21 21:45:54 CET 2012

Okay, eventually it boils down to this:

import Data.Text
import Data.Text.Encoding (encodeUtf32LE)
import Data.ByteString.Unsafe (unsafeUseAsCString)

textAsPtrW32 :: Text -> (Ptr Word32 -> IO a) -> IO a
textAsPtrW32 t = unsafeUseAsCString (encodeUtf32LE $ t `snoc` '\0') . (.

As the function passed copies or at least does not store the pointer, I can
use unsafeUseAsCString, but then I have to manually append the

Le 21 mars 2012 13:09, Antoine Latter <aslatter at gmail.com> a écrit :

> On Wed, Mar 21, 2012 at 3:35 AM, Yves Parès <yves.pares at gmail.com> wrote:
> > Hello,
> >
> > I have to interact with a C++ library that accepts as string types
> (putting
> > c++ strings aside) pointers of wchar_t (CWString in Haskell) or unsigned
> > 32-bit int (Ptr Word32 for UTF-32 codepoints).
> >
> > I have read what text, bytestring and base provide, but Text can only be
> > directly converted to (Ptr Word16), and if I use encodeUTF32 to get a
> > ByteString, then I only get useAsCString, no direct conversion to
> CWString
> > or Ptr WordXX is possible.
> A CString is a (Ptr CChar). You can then use castPtr to get whichever
> pointer type you need, if you believe the underlying buffer has the
> representation you want (in this case, UTF-32).
> It still won't be null-terminated, however.
> Antoine
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