I hit a roadblock trying to understand linear types, because I couldn't figure out what is meant to "use" a variable. I eventually think I figured it out, by reading "Linear types can change the world!", but it was not easy to do so. I think a better description of what "use" means would be very helpful for those unfamiliar with linear types, so I took a stab at it. Hope it helps.
Following a fairly detailed discussion on slack, the feeling is
generally that it's better to have a single interface. While precision
is nice, it doesn't appear to buy us anything here. If that turns out to
be wrong, or limiting somehow, we can revisit it later. Also:
- it's easier for backend authors if the type of IO operations is
slightly less restrictive. For example, if it's in HasIO, that limits
alternative implementations, which might be awkward for some
alternative back ends.
- it's one less extra detail to learn. This is minor, but there needs to
be a clear advantage if there's more detail to learn.
- It is difficult to think of an underlying type that can't have a Monad
instance (I have personally never encountered one - if they turns out
to exist, again, we can revisit!)
The output directory was previously called the executable directory, but I changed it because the output is not always an executable (depending on the code generator).
The code generators can now distinguish between where to place the (temporary) build files and the resulting output files.
I'm playing with some linear structures and finding these useful a lot,
so good to have a consistent syntax for it. '#' is chosen because it's
short, looks a bit like a cross if you look at it from the right angle
(!) and so as not to clash with '@@' in preorder reasoning syntax.
