irmen
[Feature Request] `const sub`
I would like to see something similar to C++'s constexpr functions. As Prog8's const is more similar to constexpr, I would suggest this be spelled as const sub. Due to the difference's between the languages and out of understanding that it is not something trivial to implement, I think restricting it to the same limitations constexpr functions had in C++11 would make sense: such a subroutine could only consist of a single return statement, which itself can be evaluated as a constant expression as long as all arguments are themselves constants.
If a const subroutine is called with constant arguments, the entire expression should be evaluated at compile time. If a const subroutine is called with variable arguments, the return statement would instead be inlined to the call site. I believe this should avoid the issues inlining regular subroutines had in earlier versions of Prog8.
Examples:
; const sub that replicates the common BIT macro in C
const sub BIT(ubyte shift) -> uword {
return 1 << shift
}
const uword foo = BIT(3) ; at compile time the right hand size would evaluate to 0x08
ubyte shiftvar = 3
uword bar = BIT(shiftvar) ; the right hand side would be expanded to 1 << shiftvar
ubyte shiftvar = 3
const uword bar = BIT(shiftvar) ; compilation error
const ubyte shiftconst = 3
uword baz = BIT(shiftconst) ; at compile time the right hand size would evaluate to 0x08
If this is seen as something desirable, I can try implementing it myself when I have some time.
inline sub with the same restrictions but allowing for referencing variables in the return statement could also be useful.
I'll have to think about this some more, but notice that inlining subroutine calls on 6502 is problematic in general due to code size blowup. We only have a few tens of Kilobytes program size to work with
That is one of the reasons I'm suggesting limiting the bodies to only a single return statement and nothing else (and I suppose defining constants inside them wouldnt hurt anything either).
Just a heads-up, all time is currently being spent on adding structs and typed pointer support. Thinking about the const sub feature has to wait till after.