expose strlen from Foreign.C.String

[Eric Mertens]

I’m on board with this import, but we’ll need to get the type right if we’re going to bind to libc’s strlen directly

foreign import unsafe "strlen"
  cstringLength :: CString -> IO CSize

> On Jan 22, 2021, at 6:04 PM, chessai <chessai1996 at gmail.com> wrote:
> 
> I agree with Andrew, let's just export the lifted ffi call
> 
> This suits my needs, but, regardless of my needs, seems like a perfectly sensible addition to Foreign.C.String
> 
> Concrete addition:
> 
> foreign import unsafe "strlen"
>   cstringLength :: CString -> IO Int
> 
> 
> On Fri, Jan 22, 2021, 17:09 Viktor Dukhovni <ietf-dane at dukhovni.org <mailto:ietf-dane at dukhovni.org>> wrote:
> 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 <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.
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