Re: [Haskell-cafe] How to write a pure String to String function in Haskell FFI to C++

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 wrote:

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On Sun, Jun 2, 2013 at 8:01 PM, Thomas Davie wrote:

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On 2 Jun 2013, at 16:48, Brandon Allbery wrote:

(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 the 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 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 http://sinenomine.net

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