[Haskell-cafe] converting prefixes of CString <-> String

[Malcolm Wallace]

On 25 Apr 2011, at 08:16, Eric Stansifer wrote:

> Let 'c2h' convert CStrings to Haskell Strings, and 'h2c' convert
> Haskell Strings to CStrings.  (If I understand correctly, c2h . h2c
> === id, but h2c . c2h is not the identity on all inputs;

That is correct.  CStrings are 8-bits, and Haskell Strings are 32-bits.  Converting from Haskell to C loses information, unless you use a multi-byte encoding on the C side (for instance, UTF8).

>  or perhaps c2h is not defined for all CStrings.

Rather, h2c is not necessarily well-defined for all Haskell Strings.  In particular, the marshalling functions in Foreign.C.String simply truncate any character larger than one byte, to its lowest byte.

I suggest you look at the utf8-string package, for instance Codec.Binary.UTF8.String.{encode,decode}, which convert Haskell strings to/from a list of Word8, which can then be transferred via the FFI to wherever you like.

Regards,
    Malcolm