When a word is redefined, is it possible to access the old word?
Imagine there is a word foo defined and redefined
: foo ( n -- 2*n ) 2* ; ok
: foo ( n -- 2*n+1 ) foo 1+ ; redefined foo ok
10 foo . 21 ok
foo here executed both definitions.
Is it possible to execute the first definition ("second-foo")?
21 second-foo . 42 ok
To see it?
see second-foo
: foo
2* ; ok
A simple way to access a shadowed definition is to create a synonym for the definition before creating a new definition with the same name.
synonym old-foo foo
: foo ... ;
Another way is to use introspection tools, namely the word traverse-wordlist. Using this word we can define a word find-name-nth-in that is similar to the standardized word find-name-in ( c-addr u wid -- nt|0 ), but it finds n-th shadowed word, as follows:
: find-name-nth-in ( c-addr1 u1 u wid -- nt|0 )
>r 1+
[: ( sd u nt -- sd u true | sd nt 0 false )
2>r 2dup r@ name>string compare if rdrop r> true exit then
r> r> 1- dup if nip then dup 0<>
;] r> traverse-wordlist ( sd u | sd nt 0 )
if 2drop 0 exit then nip nip
;
This find-name-nth-in word returns an nt for the word that has the given name in the given word list, and after which exactly u words with the same name (case-sensitively) were defined in the word list, or 0 otherwise.
A test case:
: ?name ( nt|0 -- nt ) dup 0= -13 and throw ;
: foo ." old" ;
: foo ." new" ;
"foo" 0 forth-wordlist find-name-nth-in ?name name>interpret execute
\ prints "new"
"foo" 1 forth-wordlist find-name-nth-in ?name name>interpret execute
\ prints "old"
In Gforth the word xt-see can be used as:
"foo" 1 forth-wordlist find-name-nth-in ?name name>interpret xt-see