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5.9.9.2 The gory details of CREATE..DOES>

DOES> ( compilation colon-sys1 – colon-sys2  ) unknown “DOES>”

This means that you need not use CREATE and DOES> in the same definition; you can put the DOES>-part in a separate definition. This allows us to, e.g., select among different DOES>-parts:

: does1 
DOES> ( ... -- ... )
    ... ;

: does2
DOES> ( ... -- ... )
    ... ;

: def-word ( ... -- ... )
    create ...
    IF
       does1
    ELSE
       does2
    ENDIF ;

In this example, the selection of whether to use does1 or does2 is made at definition-time; at the time that the child word is CREATEd.

In a standard program you can apply a DOES>-part only if the last word was defined with CREATE. In Gforth, the DOES>-part will override the behaviour of the last word defined in any case. In a standard program, you can use DOES> only in a colon definition. In Gforth, you can also use it in interpretation state, in a kind of one-shot mode; for example:

CREATE name ( ... -- ... )
  initialization
DOES>
  code ;

is equivalent to the standard:

:noname
DOES>
    code ;
CREATE name EXECUTE ( ... -- ... )
    initialization
>body ( xt – a_addr  ) core “to-body”

Get the address of the body of the word represented by xt (the address of the word’s data field).