to and defer@ ¶Gforth’s values have several operators: to (to which is is
an alias, and defer! takes an xt instead of a name in the input
stream), +to, addr, and action-of (where
defer@ takes an xt instead of a name in the input stream).
Gforth allows you to change the (to) action of a word.
(to) ( val operation xt – ) gforth-1.0 “paren-to”
xt is of a value like word name. Stores val to
name. operation selects between to, +to,
addr, and action-of.
to-table: ( "name" "xt1" .. "xtn" – ) gforth-experimental “to-table-colon”
create a table with entries for TO, +TO,
ADDR, and ACTION-OF. Use n/a to mark
unsupported operations.
to-method: ( xt table "name" – ) gforth-experimental “to-method-colon”
create a to-method, where xt computes the address to access the field, and table contains the operators to store to it.
set-to ( to-xt – ) gforth-1.0 “set-to”
Sets the implementation of the (to) ( val xt -- ) method
of the current word to to-xt.
n/a ( – ) gforth-experimental “not-available”
this word can be ticked, but throws an “Operation not supported” exception on interpretation and compilation. Use this for methods and alike that aren’t supported.
(to) is a word used inside to: it stores the value at
run-time. The general stack effect of (to) method is (
val operation xt -- ), where xt identifies the word stored into,
operation the actual variant of to-like operations, and
val is the value (of appropriate type) stored there.
You shall implement the to-method using to-table: to
create a type-specific table of operations (not specified slots at the
end of the table are filled up with n/a), and using
to-method: together with the operation to get from the xt of
the value to its data field (usually >body for values in the
dictionary, but value-style data can also reside in structures or user
areas).
E.g., one can implement fvalue as follows:
to-table: f!-table f! f+! ' >body f!-table to-method: fvalue-to : fvalue ( r "name" -- ; name: -- r ) create f, ['] f@ set-does> ['] fvalue-to set-to ; 5e fvalue foo : bar foo 1e f+ to foo ; see bar