newRPL - Updated to build 1510 [official build remains at 1487]

[3298]

I believe that a custom case structure is an overengineered solution, the simple command CKARG appears more reasonable (a building block that can be used in a normal case structure, or in a wholly different manner if you want).

If I may, I'd like to request replacing the simple true/false flag with an index of which type in the list matched (with 0 as placeholder for no match, like POS does). In RPL anything that isn't 0 counts as true anyway, so the value can be used for the same purposes, and performance-wise it should be a freebie, but in situations where the cases are similar but not quite the same the programmer could use this index to pull the type-specific handling code out of a list, then follow up with the common code. For instance, if your program shall accept a list or a vector and treat either as coordinates, that would allow something like this, based around AXL to coerce the input into one canonical format:

Code:

... CKARGTF
IF DUP NOT
THEN DROP "Bad Argument Type" DOERR
ELSE { AXL \<< \>> } SWAP GET EVAL
  ...
END

(NOP would be nice to have by the way, here I used an empty program instead. And yes, I'm using traditional RPL trigraphs instead of newRPL's Unicode glyphs; should be understandable either way.)
This kind of dispatching also opens the door to a third command that returns the index but retains the error-out on no match.

In the end, CKARGTF would arrive at something like this UserRPL implementation:

Code:

\<<
  SWAP
  1. + (adjusting stack level parameter for the types list in the way)
  PICK (fetch the object for examination; also serves as a stack depth guard)
  TYPE POS
\>>

This one doesn't have auto-detagging and such, but it should serve as a prototype to experiment with before deciding on whether to implement it as a native command. Additional features can be prototyped as wrappers around this, e.g. the error-checking CKARG would be

Code:

\<<
  IF CKARGTF NOT
  THEN "Bad Argument Type" DOERR
  END
\>>

As a hint for a tag-stripping wrapper, DTAG would be the key command; though I didn't check if newRPL has that one yet.


Side note: I'm a SysRPL programmer, but in >90% of my programs type-checking doesn't use CK&Dispatch1 or its siblings. It's not as awesome as it appears to be. Checking each stack level (or group of them, in some cases) in turns is where it's at.