Proposed enhancement FUNCTION or DEF FN

[marianodominguez]

This is a thing lacking in all Atari Basic implementations, ability to define functions. (same as PROC defines rubroutines)

https://zxbasic.readthedocs.io/en/docs/function/

I propose using the stack for being able to do recursive functions, if local variables are too complex, this implementation uses only parameter copy.

That allows for shorter code. eg

160 XX=X+1:EXEC HS:HS1=V:XX=X-1:EXEC HS:HS2=V:XX=X+3:EXEC HS:HS3=V:XX=X-3:EXEC HS:HS4=V
170 YT=(H-L)*HS1+(R-H)*HS2+(L-W)*HS3+(W-R)*HS4+W
becomes
170 YT=(H-L)*HS(X+1)+(R-H)*HS(X-1)+(L-W)*HS(X+3)+(W-R)*HS(X-3)+W

[dmsc]

Note that your example is better written using a procedure with parameters:

  @HS X+1 : YT = V*(H-L) : @HS X-1 : YT = YT + V*(R-H) : @HS X+3 : YT = YT + V*(L-W) : @HS X-3 : YT = YT + V*(W-R)
  PROC HS COORD
    V = ........ ' your expression here
  ENDPROC  

The problem with functions is that, for the 6502 compiler to work, you can´t write to variables or call statements from a function - and also you could not nest USR() functions.

Having a limited DEF FN implementation could work - it has the benefit of having a standard naming convention for functions ( FN* ).