Quoted-Parse Idea

Part of the LGPL:ed PFE

Trying to get new people into the forth arena has a big problem: forth is an unusual language. That makes people afraid to get in contact with it. Some people that I've talked to said that they can not program very well but they've learned some language once and now they can atleast "read" program sources and understand what is going on, perhaps even fix little bugs and correct misleading comment blocks.

In today's world, most people will have learned a language being somewhat C style - may that be even Java or Javascript or C++, even Perl has borrowed some syntax from this arena. That lowers the fences to get involved - so people just see the source code and ask "what's different".

When people look at a forth program first, they won't see much things they already know. They will feel some more comfortable if you remind them of a programmable desktop calculator (including the unix standard program "dc"/"bc") with its numbers and the operators in postfix position. People will then be able to grasp the idea of values on a stack. So far so good - soon they can read forth sources, as they see "3 2 *" and know it's 6.

However, at one point they are strongly mislead - with the forth string literals. Even languages being not C have gone to adopt a common notion of writing string literals - being enclosed in quotes and non-printables (and quotes themselves) can be embedded by prepending that char with a backslash. One has to point that this is the case for almost all contemporary programming languages in use.

But forth is different. One of the most Frequently Asked Question is "how can I embed a doublequote into a string literal". And the other is that people to constantly make programming errors as they forget to include the leading space after the leading doublequote. It is pretty obvious that forth as a builtin notion of integer literals but not of string literals - string literals are not in fact outer-interpreter literals but the result of parsing words.

(hint: openboot presented "H# F01A" to express hex number literals with parsing words, so it was a deliberate choice to include the integer literals into the outer-interpreter but leaving out the string literals).

Well, the deed is done. Including a string-literal notion into the outer-interpreter woulf make forth not being quite forth anymore but just another forthish variant. After some attempts of extending the outer-interpreter, I came up with the idea of a quoted-parse extension that can be even switched on/off as one wishes.

The normal PARSE word ist left as is. But for the use in the outer interpreter, most ANS Forth system will have implemented a form of PARSE-WORD (described in the annex of the forth94 standard) which skips leading spaces and parses away the next word. This word is extended with the quoted-parse idea. Currently, a PARSE-WORD will stop at (a) the end-of-line, leaving IN just after the last valid char (b) the next whitespace, leaving IN just after the whitespace (and one char beyond the last valid char). With Quoted-Parse it will also (c) stop just after the next doublequote char returning including it into the returned string - which is different from the whitespace handling which is not returned even that the IN pointer is left pointer after it.

  S" hello world" cr type
  hello world ok
  true to QUOTED-PARSE?
  S" hello world" cr type
   hello world ok

One can of course make-over the S" and C" implementation to chop away the leading blank they get when the Quoted-Parse mode is enabled, just to be fully backward compatible with traditional forth sources. However, one is free to define similar words that will leave the space that follows the opening doublequote, so they people "will get what they see" between the doublquotes.

Many forth systems implement zero-terminated string literals to make it easier to interface with a C-based operating environment. They even implement backslash-interpretation - the two words are mostly called Z" and Z\" respectivly. Now imagine that we make a word similar to the latter one, and let it do a true Quote-Parse. A text could then look like

  cr 1 .H "x" ztype 2 .H
  1x2 ok
  cr "hello\nworld" ztype
  world ok
  cr " you see \t this?" ztype
   you see       this? ok

All this is compatible with traditional forth - the ["]-word is not part of the outer-interpreter, it is still a parsing word but one with a name that ends in a doublequote and therefore ensures that the next char in the input buffer to the outer-interpreter is not chopped away automatically, so that the next PARSE can still find it there ready to be taken.

The only incompatibility that arises: you can not define colon-words with a doublequote in the middle or starting with it. You can only define new colon-words that end with a doublequote and all these do have the implication that the outer-interpreter will leave the IN pointer just after that word when being encountered in the input stream. That will affect a (small?) fraction current forth programs but it should be easy to fix the places that are in conflict with the Quoted-Parse idea.

On the upside you are free to implement a whealth of new parsing words that will look like type-prefixed string-literals. Even newer C-type languages know the U-prefix for strings to denote unicode- encoded string literals. That can be achieved with a parsing word in forth - no need to extend the outer interpreter and you still can make it look just alike the model in those languages that known unicode string literals in its basic syntax.

  : U" [char] " PARSE (unicode-literal) COMPILE, DUP W,
  true to QUOTED-PARSE?
  U"hello world" cr wTYPE
  hello world

  ( and a definition for H" to bring in a hex-literal )
  H"F01A" cr .H

As a last note: when you have enabled Quoted-Parse, be sure to remember it also on creating new colon-words as the ":"-word will usually not call PARSE but a form of PARSE-WORD to have all leading spaces be skipped and not included in the resulting NAME of the created word. It stops after the first doublequote.

  : add"1 + ;
  ( this is not similar to )
  : add-1 + ;
  ( but it is similar to )
  : add- 1 + ;
  ( and here you might get a "redefined" warning )
  : C":: [char] ; PARSE ... ;

The question is now: when leaving quoted-parse the default on startup of a system, can it be called still to be basically a normal forth? Do some propaganda to make it the default? A default that can be checked for with an environment-query in standard-forth of 2006 ?