expose strlen from Foreign.C.String

[Viktor Dukhovni]

On Fri, Jan 22, 2021 at 04:56:33PM -0500, Andrew Martin wrote:

> This will eventually be used to great effect in bytestring. See
> https://github.com/haskell/bytestring/pull/191.

Yes, you might recall that I'm well aware of that (already merged) PR,
indeed that's how I happened to recall that cstringLength# is present
in 9.0.

> To get back to the original question, I think that any user-facing
> cstringLength function should probably be:
> 
>     cstringLength :: CString -> IO Int
> 
> We need a separate FFI call that returns its result in IO to
> accomplish this. But this just be done in base rather than ghc-prim.
> There are no interesting rewrite rules that exist for such a function.

So I guess your suggestion in response to @chessai's original post:

>> On Wed, Jan 20, 2021 at 09:54:30AM -0800, chessai wrote:
>> 
>> I've wanted the following before:
>> 
>> foreign import ccall unsafe "strlen"
>>   cstringLength# :: Addr# -> Int#
>> 
>> cstringLength :: CString -> Int
>> cstringLength (Ptr s) = I# (cstringLength# s)
>> 
>> A natural place for this seems to be Foreign.C.String.

would be to instead directly implement the lifted FFI variant:

    foreign import ccall unsafe "strlen"
      cstringLength :: CString -> IO Int

which probably would not need a wrapper and can be exported directly.

    module Main (main) where
    import Control.Monad ( (>=>) )
    import Foreign.C.String (CString, withCString)

    foreign import ccall unsafe "strlen"
      cstringLength :: CString -> IO Int

    main :: IO ()
    main = withCString "Hello, World!" $ cstringLength >=> print

The cost of this safety net is that it results in more sequencing than
is strictly necessary.  It is enough for the enclosing IO action to not
embed the length in its result in some not yet fully evaluated thunk.

I guess @chessai can let us know whether the more strictly sequenced
variant meets his needs.

-- 
    Viktor.