Question

I’m writing FFI to pdflib. Pdflib C API has lots of functions that return and/or take various handles (document, page, image, font) as plain Integer (not pointer).

In order to ensure i do not accidentally pass the wrong param to a function i create a bunch of newtypes in the form of:

newtype PdiDoc = PdiDoc Int
newtype PdiPage = PdiPage Int
newtype PdfImage = PdfImage Int
newtype PdfFont = PdfFont Int

Now i need to provide a marshaller for those types.

image2c (PdfImage i) = fromIntegral i
font2c (PdfFont f) = fromIntegral f
pdipage2c (PdiPage i) = fromIntegral i

As you see the marshallers are exactly the same, just for different types.

So my question is, is there some kind of type magic, SYB vodoo trick that i can use to have just one function to marshall all those types, or do i have to write same functions again and again for different newtypes ?

EDIT: I accepted Don’s answer, because it solved my problem.

I switched on

GeneralizedNewtypeDeriving 

added

deriving (Eq, Ord, Num, Enum, Real, Integral)

to each of my newtypes, and now i can use standard fromIntegral to marshall all of them.

Nathan Howell’s answer is also correct one, i upvoted it. But unfortunately his solution would mean giving up on FFI preprocessors like c2hs i am using.

Answer 1

You can derive ‘Num’ for your types using GeneralizedNewtypeDeriving, this helps you a bit with literals and operators.

For the marshalling, I’d use a FFI preprocess, such as hsc2hs, which can automate the wrapping and unwrapping of newtypes.

An example from RWH: