[Haskell-cafe] How to write a pure String to String function in Haskell FFI to C++

adam vogt vogt.adam at gmail.com
Wed Jun 5 04:08:02 CEST 2013

On Sun, Jun 2, 2013 at 10:19 PM, Ting Lei <tinlyx at gmail.com> wrote:
> Thanks for your answers so far.
> It seems that the laziness of String or [char] is the problem.
> My question boils then down to this. There are plenty of Haskell FFI
> examples where simple things like sin/cos in <math.h> can be imported into
> Haskell as pure functions. Is there a way to extend that to String without
> introducing an IO (), but maybe sacrificing laziness?
> If String has to be lazy, is there another Haskell data type convertible to
> String that can do the job?
> The C++/C function (e.g. toUppers) is computation-only and as pure as cos
> and tan. The fact that marshaling string incurs an IO monad in current
> examples is kind of unintuitive and like a bug in design. I don't mind
> making redundant copies under the hood from one type to another..

Hi Ting,

In the Foreign.C.String there is a function that converts String to an
array (CString = Ptr CChar) which can be handled on the C side:

withCString :: String -> (CString -> IO a) -> IO a

peekCString :: CString -> IO String

It's slightly more convenient to use these functions through the
preprocessor c2hs, as in the following example
<http://code.haskell.org/~aavogt/c_toUpper_ffi_ex/>. c2hs also has a
'pure' keyword which makes it add the unsafePerformIO, but for
whatever reason the side-effects were not done in the right order (the
peekCString happened before the foreign function was called).


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