I tried to explain this in my reply in the Futhark issue, but I was on my way
to the airport so the answer was a bit brief. What you're seeing in
`AV_CHANNEL_LAYOUT_MASK` is an object constructor, however since C doesn't have
typed macros it doesn't tell us which object its constructing for. The
following `define` statements then expand this snippet of text and replaces
`nb`, and `m` in the template with the given arguments. We still don't have any
type information as these are only text expansions. Whenever you try to use
e.g. `AV_CHANNEL_LAYOUT_STEREO` it will then simply paste in this object
constructor, and if you are using it correctly this will be a place where such
an object constructor is allowed and it will create an object of whatever type
was required. Since Nim has fully typed macros and not just text expansion this
isn't really something you could easily do in Nim. However by looking at the
surrounding code (and comments), we are able to grok what this is actually
supposed to be doing and can create a template a bit something like this:
template AV_CHANNEL_LAYOUT_MASK(nb, m: untyped): AVChannelLayout =
AVChannelLayout(order: AV_CHANNEL_ORDER_NATIVE, nb_channels: nb, u:
AVChannelLayoutAnonUnion(mask: m), opaque: nil)
Run
This will create an object of kind `AVChannelLayout` which can then be used
just like the ones in the C code. The name of `AVChannelLayoutAnonUnion` might
of course vary, I don't remember exactly what Futhark generates in this case,
but it's something similar to that.
