Iavor Diatchki wrote:
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 changed. Also notice that the evaluation order of the program affects the output. If we include the commented out statement (which forces h1 to be evaluated earlier) the output becomes: "Hello" 72 "aello" 72 97
I think that because of this we should either remove these functions, or at least follow the convention of other libraries and give them "unsafe" names.
-Iavor
The Data.ByteString type is really a fancy interface to ForeignPtr, which leads to these issues. I mostly agree. Perhaps the pack* function should have unsafe* names. Or at least very large warnings in the documentation.
From looking at the Haddock documentation:
The "safe" versions already exist as copyCString and copyCStringLen, which look like (copy . packCString) and (copy . packCStringLen). And Data.ByteString.useAsCStringLen also looks "unsafe", but Data.ByteString.useAsCString is safe. Even more oddly Data.ByteString.useAsCStringLen looks identical in the documentation to Data.ByteString.Base.unsafeUseAsCStringLen but does not call itself "unsafe" and neither have the same dire warnings as Data.ByteString.Base.unsafeUseAsCString. -- Chris