September 2001 - Glasgow-haskell-users

Re: Ptr and ForeignPtr Questions
by Ashley Yakeley 24 Sep '01

24 Sep '01

At 2001-09-23 19:14, Manuel M. T. Chakravarty wrote: >Hmm, we must be misunderstanding each other. Given that you >have > > foreign import strcat :: Ptr CChar -> Ptr CChar -> IO (Ptr CChar) > >How do you want to know whether the C prototype is > > char *strcat(char *dest, const char *src); > >or > > char *strcat(char *dest, char *src); I was just hoping for GHC to be able to spit out headers for 'foreign import' functions that the user could then define. This merely means a map from some restricted set of Haskell function types to C types. But it looks like you want to be able to arbitrarily import any C library function. Of course this means a map from C functions to Haskell functions. In the case you gave Haskell could get away with calling strcat using this: char* strcat(char*,char*); ...even if the function had actually been defined like this: char* strcat(char*,const char*); This is quite safe from a 'const' point of view. More difficult is this: const char* foo() { return "hello"; } ...since the best the FFI can do is this: foreign import foo :: IO (Ptr CChar) ...which is certainly _not_ const-safe. Perhaps one could have ConstPtrs, which didn't allow writes, and allow those in FFI. At 2001-09-23 19:21, Manuel M. T. Chakravarty wrote: >If you are writing a Haskell binding to a large C library >that has hundreds of functions, it is extremely unattractive >to write "impedance matchers" like `MyModule__StringCopy' >manually. I'm taking an approach similar to JNI, in which the user defines in C what has been declared in Haskell. But what you want to do is to allow the user to declare in Haskell what has been defined in C, obviously a more ambitious goal. Const pointers are not your only worry; how would you deal with structs? -- Ashley Yakeley, Seattle WA

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Re: Ptr and ForeignPtr Questions
by Ashley Yakeley 23 Sep '01

23 Sep '01

At 2001-09-23 04:04, Marcin 'Qrczak' Kowalczyk wrote: >It would be impossible then to directly call a C function with a >parameter declared as a const pointer. It's illegal in C to have >mismatching prototypes of the same function. You can always do this: module MyModule where { foreign import StringCopy :: Ptr Int8 -> Ptr Int8 -> IO (); } which autogenerates this header: extern "C" { void MyModule__StringCopy(signed char* a,signed char* b); } which the user can implement like this: char* strcat(char* dest, const char* src); void MyModule__StringCopy(char* dest,char* src) { strcat(dest,src); } (assuming char and signed char are identical). -- Ashley Yakeley, Seattle WA

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Re: Ptr and ForeignPtr Questions
by Ashley Yakeley 23 Sep '01

23 Sep '01

At 2001-09-23 04:02, Manuel M. T. Chakravarty wrote: >> I would assume that Ptr types would always be mapped to non-const >> pointers. Do you have an example of a Haskell type for a foreign import >> function, for which the corresponding C function type would be ambiguous? > >Take, for example, the following excerpt of the Linux man >pages But do you have an example of a Haskell type for a foreign import function, for which the corresponding C function type would be ambiguous? At 2001-09-21 02:20, Manuel M. T. Chakravarty wrote: >One could attempt to generate C prototypes from the Haskell >type declarations or similar things, btu it doesn't work >out. Surely one can still do this? This was what we were discussing, was it not? A C type for any given Haskell function type, not necessarily the other way around... -- Ashley Yakeley, Seattle WA

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Re: Ptr and ForeignPtr Questions
by Ashley Yakeley 23 Sep '01

23 Sep '01

At 2001-09-23 15:56, Marcin 'Qrczak' Kowalczyk wrote: >> But do you have an example of a Haskell type for a foreign import >> function, for which the corresponding C function type would be ambiguous? > >Ptr CChar (as an argument). It could be either char * or const char *. It's always char*. A Haskell function passed a Ptr CChar is not prevented from modifying the contents of the pointer simply due to its type-declaration. In C, a char* can be implicitly converted to a const char* where necessary (but not the other way around). -- Ashley Yakeley, Seattle WA

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Re: Ptr and ForeignPtr Questions
by Marcin 'Qrczak' Kowalczyk 23 Sep '01

23 Sep '01

Sun, 23 Sep 2001 15:41:41 -0700, Ashley Yakeley <ashley(a)semantic.org> pisze: > But do you have an example of a Haskell type for a foreign import > function, for which the corresponding C function type would be ambiguous? Ptr CChar (as an argument). It could be either char * or const char *. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Re: Alternative Design for Finalisation
by Ashley Yakeley 23 Sep '01

23 Sep '01

At 2001-09-21 09:40, Marcin 'Qrczak' Kowalczyk wrote: >> (apologies for the different spelling of finalize - apparently both are >> correct and I randomly settled on the 'z' version some time ago). > >I guess 's' is British and 'z' is American. Chambers (of Cambridge, England) has both. -- Ashley Yakeley, Seattle WA

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Re: Ptr and ForeignPtr Questions
by Marcin 'Qrczak' Kowalczyk 23 Sep '01

23 Sep '01

Sun, 23 Sep 2001 02:04:49 -0700, Ashley Yakeley <ashley(a)semantic.org> pisze: > I would assume that Ptr types would always be mapped to non-const > pointers. It would be impossible then to directly call a C function with a parameter declared as a const pointer. It's illegal in C to have mismatching prototypes of the same function. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Re: Ptr and ForeignPtr Questions
by Ashley Yakeley 23 Sep '01

23 Sep '01

At 2001-09-23 00:45, Manuel M. T. Chakravarty wrote: >No, unfortunately not. Consider, for example, that storage >qualifiers would have to be generated correctly and there is >no way to determine form a Haskell type whether a `const' >modifier needs to be added. I would assume that Ptr types would always be mapped to non-const pointers. Do you have an example of a Haskell type for a foreign import function, for which the corresponding C function type would be ambiguous? -- Ashley Yakeley, Seattle WA

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Re: Ptr and ForeignPtr Questions
by Ashley Yakeley 23 Sep '01

23 Sep '01

At 2001-09-21 02:20, Manuel M. T. Chakravarty wrote: >One could attempt to generate C prototypes from the Haskell >type declarations or similar things, btu it doesn't work >out. Why not? Isn't there a straightforward type map? -- Ashley Yakeley, Seattle WA

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foreign funciton invokation
by Chen Yu 23 Sep '01

23 Sep '01

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