You have to worry about json having a floating point number or an integer that is too large to be represented in a Word32, although you could also just let it overflow if you don't care too much. There's probably an easier way to do this, but this is what I came up with. instance FromJSON CUid where parseJSON (Number n) = do case floatingOrInteger n of Right i | inrange i -> return . CUid . fromIntegral $ i -- not an integer, or not in range _ -> mempty where inrange :: Integer -> Bool inrange i = fromIntegral i >= (minBound @Word32) && fromIntegral i <= (maxBound @Word32) -- not a number parseJSON _ = mempty On Thu, Aug 9, 2018 at 11:37 AM, PICCA Frederic-Emmanuel < frederic-emmanuel.picca@synchrotron-soleil.fr> wrote:
Hello, I try to write the instance for the CUid[1] type But I do not know what to do.
instance FromJSON CUid where parseJSON = ... {-# INLINE parseJSON #-}
Thansk for your help
[1] https://hackage.haskell.org/package/base-4.11.1.0/docs/ System-Posix-Types.html#t:CUid _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners