6.17.5.1 Default Recognizers ¶

The Standard Forth text interpreter recognizes words in the search order (rec-nt), integer numbers (rec-num), and floating point numbers (rec-float). By default Gforth also recognizes syntaxes for

• strings, e.g., "mystring", with rec-string
• complex numbers, e.g., 0e+1ei, with rec-complex
• storing a value or changing a defered word, e.g., ->myvalue, with rec-to
• the xt representing the interpretation semantics of a word, e.g., `dup, with rec-tick
• the nt of a word, e.g., ``mysynonym, with rec-dtick
• an address in the body of a word, e.g., <myarray+8>, with rec-body
• an access to an environment variable of the operating system, e.g., ${HOME}, with rec-env
• a word in a vocabulary, e.g., myvoc1:myvoc2:myword, with rec-scope
• using a specific recognizer to recognize something, e.g., float?1., , with rec-meta

You can use locate (see Locating source code definitions) to determine which recognizer recognizes a piece of source code. E.g.:

defer mydefer
locate ->mydefer

will show that rec-to recognizes ->mydefer. However, if the recognizer recognizes a dictionary word (e.g., the scope recognizer), locate will show that word.

You can see which recognizers are used and the order of recognizers with

.recognizers ( –  ) gforth-experimental “dot-recognizers”

Print the current recognizer order, with the first-searched recognizer leftmost (unlike .order). The inverted ~ is displayed instead of rec-, which is the common prefix of all recognizers.

Recognizers are typically designed to avoid matching the same strings as other recognizers. E.g., rec-env (the environment variable recognizer) requires braces to avoid a conflict with the number recognizer for input strings like $ADD. There are a few exceptions to this policy, however:

• Word names can be anything, so they can conflict with any other recognizer (and the search order is searched before other recognizers).

  However, they tend not to start with 0 (and if they do, they contain special characters), so if your base is hex, it is a good practice to let your numbers start with 0.

  In the code bases we have looked at, starting words with ' (quote aka tick) is much more common than starting them with ` (backquote aka backtick), so the recognizers for the xt and the nt use ` to reduce the number of conflicts.

• Both the integer recognizer rec-num and the floating-point recognizer rec-float recognize, e.g., 1.. Because rec-num is (by default) first, 1. is recognized as a double-cell integer. If you change the recognizer order to use rec-float first, 1. is recognized as a floating-point number, but loading code written in Standard Forth may behave in a non-standard way.

  In any case, it’s a good practice to avoid that conflict in your own code as follows: Always write double-cell integers with a number prefix, e.g., #1.; and always write floating-point numbers with an e, e.g., 1e.

• We have seen a few word names that start with ->. You can avoid a conflict by using to myvalue or to?->myvalue (the latter works with postpone).