Now that I've organized the compiler a bit, it's starting to get a little
easier to see how we could do an [LLVM](http://llvm.org) backend for felix. One
of the many challenges we'll have to deal with for that is what do we do for a
foreign function interface. For those who don't know, one of the neat things
felix does is compile into C++ code. This lets us easily define adds like this:
{{{
:::felix
fun add: int * int -> int = "$1 + $2";
fun sin: float -> float = "std::sin($1)";
}}}
Where we will just pass the string through the program, translating the
`$`-prefixed values into the proper variable names in the output source. We can
even do complex functions like this:
{{{
:::felix
fun foo: int * int * int -> int = "$1 + $2 - $3";
}}}
And still have it work.
For [LLVM](http://llvm.org), though, we'd be directly compiling against their
API, since that would be more efficient. So, what can we do? One possibility
would be to use [Ocs Scheme](http://will.iki.fi/software/ocs/) and our
S-Expression Library, sex:
{{{
:::felix
fun add: int * int -> int = "`(add ,_1 ,_2)";
fun sin: float -> float = "`(ccall std::sin ,_1)";
fun foo: int * int * int -> int = "`(add ,_1 `(sub ,_2 ,_3))";
}}}
Then just implement in the backends how to translate these simple calls into
the target format. The nice thing about this is that it stays reasonably
generic. I was worried that we'd have to sprinkle `#IFDEF`s throughout our
source, but this seems to avoid that in most situations.
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