6.11.10.10 Const-does> ¶

A frequent use of create...does> is for transferring some values from definition-time to run-time. Other ways of achieving this are closures (see Closures), and with colon definitions (see User-defined defining words with colon definitions), but another way of achieving this is to use

const-does> ( run-time: w*uw r*ur uw ur "name" –  ) gforth-obsolete “const-does”

Defines name and returns.

name execution: pushes w*uw r*ur, then performs the code following the const-does>.

A typical use of this word is:

: curry+ ( n1 "name" -- )
1 0 CONST-DOES> ( n2 -- n1+n2 )
    + ;

3 curry+ 3+

Here the 1 0 means that 1 cell and 0 floats are transferred from definition to run-time.

The advantages of using const-does> compared to create}...@word{does> are:

• You don’t have to deal with storing and retrieving the values, i.e., your program becomes more writable and readable.
• When using does>, you have to introduce a @ that cannot be optimized away automatically (because the system does not know whether you allow to access the data with >body...!). You can address this problem with set-optimizer (see User-defined compile,), but const-does> avoids it; however, the current implementation is still not particularly efficient.

A Standard Forth implementation of const-does> is available in compat/const-does.fs.