Node:Wordlists and Search Order Tutorial, Previous:Compilation Tokens Tutorial, Up:Tutorial
The dictionary is not just a memory area that allows you to allocate
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
because this wordlist was created anonymously with
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.
>order ( wid -- )with adds
widas 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.