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
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