Re: expose strlen from Foreign.C.String

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 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 mailto:ietf-dane@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. _______________________________________________ Libraries mailing list Libraries@haskell.org mailto:Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries