as the others have said, if you want to have text data go between ghc and
c++, please use Text or Bytestring,
String... would get weird.
If you seriously want to experiment with writing low level code
manipulating the String type, it *MIGHT* be possible using the GHC C minus
minus (CMM). This would be very very very subtle to do correctly, and also
just be really really complicated and hard.
Likewise, for writing a "pure" looking ffi function, a good example is in
the lz4hs lib, where all the allocation occurs on the haskell side, and the
ffi is only mutating freshly allocated memory. Subject to this,
unsafePerformIO can be safely used to give a safe pure thread safe api.
cheers
-Carter
On Sun, Jun 2, 2013 at 10:55 PM, Chris Wong wrote: The C++/C function (e.g. toUppers) is computation-only and as pure as cos
and tan. The fact that marshaling string incurs an IO monad in current
examples is kind of unintuitive and like a bug in design. I don't mind
making redundant copies under the hood from one type to another.. If you can guarantee that the call is pure, then you can execute it
directly using `unsafePerformIO`. Simply call the external function as
usual, then invoke `unsafePerformIO` on the result. See <
http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/System-IO-... . On another note, if you really care about performance, you should use
the `bytestring` and `text` packages instead of String. They are
implemented in terms of byte arrays, instead of linked lists, hence
are both faster and more FFI-friendly. On Sun, Jun 2, 2013 at 8:08 PM, Brandon Allbery On Sun, Jun 2, 2013 at 8:01 PM, Thomas Davie wrote: On 2 Jun 2013, at 16:48, Brandon Allbery (String is a linked list of Char, which is also not a C char; it is a
constructor and a machine word large enough to hold a Unicode codepoint. And because Haskell is non-strict, any part of that linked list can be an
unevaluated thunk which requires forcing the evaluation of arbitrary
Haskell
code elsewhere to "reify" the value; this obviously cannot be done in middle of random C code, so it must be done during marshalling.) I'm not convinced that that's "obvious" – though it certainly requires
functions (that go through the FFI) to grab each character at a time. I think you underestimate the complexity of the Haskell runtime and the
interactions between it and the FFI. Admittedly it is probably not
"obvious"
in the sense of "anyone can tell without knowing anything about it that
it
can't possibly work", but it should be at least somewhat obvious to
someone
who sees why there needs to be an FFI in the first place that the
situation
is not trivial, and that they probably should not blindly assume that wrote:
the
the only reason one can't just pass Haskell values directly to C is that
some
GHC developer was feeling lazy at the time. --
brandon s allbery kf8nh sine nomine
associates
allbery.b@gmail.com
ballbery@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad
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