Pep and Nom

the ℕ𝕠𝕞 "pop" command

pop a parse token off the stack into the front of the workspace buffer

The “pop” command pops the last item from the pep virtual machine stack and places it’s contents, without modification, at the beginning of the workspace buffer, and at the same time decrements the tape pointer. If the stack is empty, then the pop command does nothing (and the tape pointer is unchanged).

pop is typically used before a “grammar rule” block to see if a certain parse-token sequence is on the stack

Below is ℕ𝕠𝕞 script-fragment to recognise a LISP -like language

    pop;pop;pop;pop;pop;
    # grammar rule:
    #   expression := '(' operator expression expression ')' ;
    "(*operator*expression*expression*)* {
      clear; add "expression* ??"; push; .reparse
    }
    push;push;push;push;push;
  

 pop;

 pop;pop; "word* ??colon* ??" { clear; add 'command*'; push; }

The pop command is usually employed in the parsing phase of the script (not the “lexing” phase); that is, after the parse> label. The pop command is the converse machine operation of the push command, but it is important to realise that a command sequence of pop;push; is not equivalent to the nop (no-operation) command.

If the pep/nom parser language is being used to parse and translate a language then the script writer needs to ensure that each token ends with a delimiter character (by default “*") in order for the push and pop commands& ??rdquo; to work correctly.

The pop command also affects the tape of the ℙ𝕖𝕡 virtual machine in that the tape-pointer is automatically decremented by one once for each pop command. This is convenient because it means that the tape pointer will be pointing to the corresponding element for the token. In other words in the context of parsing and compiling a “formal language” the tape will be pointing to the “value” or “attribute” for the token which is currently in the workspace.