iavor.diatchki:
Hello, The "packCString" function (and other similar functions) in the ByteString library break referential transperancy, which is one of the big selling points of Haskell (and its libraries). Here is an example:
main = do x <- newCString "Hello" let s = packCString x h1 = B.head s print s -- print h1 poke x (toEnum 97) print s let h2 = B.head s print h1 print h2 Output: "Hello" "aello" 97 97
This is already confusing because the "pure" value 's' has magically
Right, in order to support zero-copy string between C and Haskell, for efficiency, the default packCString just wraps up the Ptr CChar to look like a ByteString. It doesn't copy it. If you the mutate the C String, well, as you see above. Here you should be using: copyCString :: CString -> IO ByteString The functions should document this behaviour better. Of course, you're paying with poke and C strings so you should be careful anyway. I'll correct the documentation to explain all this. Thanks for noticing! -- Don