6.10.10.6 User-defined 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