Is there really any implementation of a Ada compiler/cross-compiler/interpreter for 6502? Link:
There is an Ada for the 6502 for the Commodore 64 but it is sold out at The Computer Bargain Store: Abacus Software for Commodore 64 & 128. It looks like they once had a used copy but sold it. The Ada Training Course is still available for $69.
If you are interested in other languages that generate efficient 6502 code or are similar to Ada, try these:
The most efficient compiler is likely Action! for the Atari 8-bit computers. It's syntax is derived from ALGOL so is a bit like Ada. It doesn't have FP built in but I believe there is an FP library available.
Another efficient 6502 compiler I've seen is Betterworking's Power C for the C64 which seems to be only 40% slower than assembly, but it is only a partial implementation of C and not very good for more than small projects. Abacus' Super C is a more complete implementation but is only 25% the speed of Power C.
The closest thing to Ada would be Pascal, such as KYAN Pascal but the Commodore 64 version is 30% slower than Power C. It is also available for the Atari 8-bit computers and the Apple II.
PROMAL for the C64 is twice as fast as KYAN Pascal. PROMAL is like C except it's syntax is more like Python, although Python didn't come along until later.