Pep and Nom

the ℕ𝕠𝕞 "decrement" or "--" command

Decrement the tape pointer by one.

The -- command and the ++ (increment-tape-pointer) command are very important for accessing the attributes associated with a particular parse token. A standard procedure during parsing is to “match” a particular sequence of tokens, then access the text attributes for each token to build a new text attribute (which will be associated with the new parse token that we are about to build) and then put the new attribute into the tape at the correct location.

  (eof) { quit; }
  while [:space:]; clear; whilenot [:space:]; 
  "a","an","the", { put; clear; add "article*"; push; }
  "big","blue","small" { put; clear; add "adjective*"; push; }
  "cloud","tree","idea" { put; clear; add "noun*"; push; }
  clear;
  parse>
  pop;
  "nounphrase*" { swap; add ": is a "; get; print; clear; }
  pop;pop;
  "article*adjective*noun*" {
    # build the new attribute from the old attributes
    clear; get; add " "; ++; get; add " "; ++; get; --; --; put;
    # create the new parse token and push onto the stack.
    clear; add "nounphrase*"; push; .reparse
  }
  push;push;push;
 

The single character (cryptic) version of the -- command is < but you would only use it if you have some strange quota on the number of characters you are allowed to use.

If the current tape element is the first element of the tape then the tape pointer is not changed and the command has no effect. The tape pointer is also decremented by the “pop” command (if it is greater than zero).

pop and decrement tape pointer

The pop command performs an implicit -- operation, usually (see below for the important caveat ). This means that when you perform a pop on the ℙ𝕖𝕡 machine stack the tape pointer will be automagically decremented and this is a marvellous thing, because it means that the tape and the stack tend to stay “in sync” with each other.

The caveat which I mentioned above is that when the stack is empty, that is, it contains no parse tokens, then a pop operation will do nothing. Since there is nothing to pop out of the stack, nothing will come out, but what is more, the tape pointer to the current tape cell will not be decremented (and this is also a stroke-of-genius if I may say so myself).

You will have already guessed that a very similar behaviour applies to the push command and the ++ (increment tape pointer) command.