No instance for (Fractional Int32)

[Gabor Greif]

Okay, here is a compile-time check whether a C type is float or integral:

cat <<EOF > detect-float-compile-time.c
typedef char testarray_f[(float)0.4 - (float)0.8 < 0 ? 1 : -1];
typedef char testarray_d[(double)0.4 - (double)0.8 < 0 ? 1 : -1];
typedef char testarray_x[(long double)0.4 - (long double)0.8 < 0 ? 1 : -1];
typedef char testarray_i[(int)0.4 - (int)0.8 < 0 ? 1 : -1];
EOF

$ gcc detect-float-compile-time.c -c -o /dev/null
detect-float-compile-time.c:4: error: size of array ‘testarray_i’ is negative

Note that the fourth line errored out, since 'int' is not a float type.
This test should be robust enough for any cross target.

I'll try to fix the configury tomorrow and submit a patch.

Cheers,

    Gabor


On 2/8/13, Gabor Greif <ggreif at gmail.com> wrote:
> On 2/8/13, Stephen Paul Weber <singpolyma at singpolyma.net> wrote:
>> Somebody claiming to be Christopher Vollick wrote:
>>>On Fri, Feb 08, 2013 at 01:20:53PM +0100, Gabor Greif wrote:
>>>> libraries/base/Foreign/C/Types.hs:162:72:
>>>>     No instance for (Fractional Int32)
>>>>       arising from the 'deriving' clause of a data type declaration
>>>>     Possible fix:
>>>>       use a standalone 'deriving instance' declaration,
>>>>         so you can specify the instance context yourself
>>>>     When deriving the instance for (Fractional CFloat)
>>>>
>>>> Has anybody seen this with HEAD?
>>
>> This may be a side effect of the fix for
>> <http://hackage.haskell.org/trac/ghc/ticket/7569>
>
> Yeah, something with the test that contains the literal double 1.4
> When configure runs, it does not succeed to set HTYPE_IS_INTEGRAL
> to "" and this leads it down the signed/unsigned road.
>
> Will check tomorrow what is going on here.
> Possibly we need to change the test to C++,
> where compile-time detection of these things
> is a breeze (because of overloading).
>
> Cheers and thanks,
>
>     Gabor
>
>>
>> --
>> Stephen Paul Weber, @singpolyma
>> See <http://singpolyma.net> for how I prefer to be contacted
>> edition right joseph
>>
>> _______________________________________________
>> ghc-devs mailing list
>> ghc-devs at haskell.org
>> http://www.haskell.org/mailman/listinfo/ghc-devs
>>
>