3.37 Wordlists and Search Order

The dictionary is not just a memory area that allows you to allocate memory with allot, it also contains the Forth words, arranged in several wordlists. When searching for a word in a wordlist, conceptually you start searching at the youngest and proceed towards older words (in reality most systems nowadays use hash-tables); i.e., if you define a word with the same name as an older word, the new word shadows the older word.

Which wordlists are searched in which order is determined by the search order. You can display the search order with order. It displays first the search order, starting with the wordlist searched first, then it displays the wordlist that will contain newly defined words.

You can create a new, empty wordlist with wordlist ( -- wid ):

     wordlist constant mywords

Set-current ( wid -- ) sets the wordlist that will contain newly defined words (the current wordlist):

     mywords set-current
     order

Gforth does not display a name for the wordlist in mywords because this wordlist was created anonymously with wordlist.

You can get the current wordlist with get-current ( -- wid). If you want to put something into a specific wordlist without overall effect on the current wordlist, this typically looks like this:

     get-current mywords set-current ( wid )
     create someword
     ( wid ) set-current

You can write the search order with set-order ( wid1 .. widn n -- ) and read it with get-order ( -- wid1 .. widn n ). The first searched wordlist is topmost.

     get-order mywords swap 1+ set-order
     order

Yes, the order of wordlists in the output of order is reversed from stack comments and the output of .s and thus unintuitive.

Assignment: Define >order ( wid -- ) with adds wid as first searched wordlist to the search order. Define previous ( -- ), which removes the first searched wordlist from the search order. Experiment with boundary conditions (you will see some crashes or situations that are hard or impossible to leave).

The search order is a powerful foundation for providing features similar to Modula-2 modules and C++ namespaces. However, trying to modularize programs in this way has disadvantages for debugging and reuse/factoring that overcome the advantages in my experience (I don't do huge projects, though). These disadvantages are not so clear in other languages/programming environments, because these languages are not so strong in debugging and reuse.

Reference: Word Lists.