Pep and Nom

the ℕ𝕠𝕞 "until" command

Read the input stream until the given text is encountered, or until the end of the input stream (EOF) is encountered.

The until command is an important “tokenising” command that converts blocks of text in the input-stream into grammar or parse tokens. Other tokenising commands are while and whilenot

There are 2 formats of the until command

the 2 types of until



   until 'abcd';
   until;

In the 1^st version of until the input stream is read until the workspace buffer ends with the text “abcd” . In the 2^nd form of the command, the input-stream is read until the workspace buffer ends with the text that is in the current tape cell.

The until command also ignores the end-text in the input stream if it is 'escaped' (that is preceded with '\\'). The following example illustrates parsing quoted text with the until command.

tokenise and count quoted text



    read; 
    '"' { 
      a+; until '"'; put; clear; add "quoted*"; 
      push; .reparse
    }
    clear;
  parse>
    pop; pop;
    "quoted*quoted*" {
      clear; get; add "\n"; ++; get; --; put; 
      clear; add "quoted*"; push; .reparse
    }
    push; push;
    (eof) { 
      add "Found "; count; add " quoted text blocks \n"; print; clear;
      pop; "quoted*" { clear; get; print; }
      quit;
    }

examples

print any text that occurs between '<' and '>' characters

 /</ { until ">"; print; } clear;

print only text between quote characters (excluding the quotes)

 r; '"' { until '"'; clip; clop; print; } clear;

create a parse token 'quoted' from quoted text

 r; '/' { until '"'; clip; clop; put; add 'quoted*'; push; } clear;

print quoted text, reading past escaped quotes (\")

 /"/ { until '"'; print; } clear;

The while and whilenot commands are similar to the until command but they depend on the value of the 'peep' virtual machine buffer (which is a single-character buffer) rather than on the contents of the 'workspace' buffer like the until command.

notes

The 'until' command usually will form part of the 'lexing' phase of a script. That is, the until command permits the script to turn text patterns into 'tokens'. While in traditional parsing tools (such as Lex and Yacc) the lexing and parsing phases are carried out by separate tools, with the 'pep' tool the two functions are combined.

The until command essentially performs multiple read operations or commands and after each read checks to see whether the workspace meets the criteria specified in the argument.