herkforth deferred words

herkforth now (as of herkforth-0.7) supports deferred words. This document describes how they work, and how to use them in a way that makes it clear to programmers (including yourself) that it is deferred, and each place that it is deferred.

Creating a deferred word is something that should be done very rarely. In the entire sources for herkforth, I have only deferred one word (-emit). For these reasons, I have not created convenience words to make this process any less technical. So you'll have to understand how things work.

Conventions

This is critical, as it makes it so someone browsing the source sees that it's deferred, and can find the places where it is changed.

When you define a word do be deferred to all the following:

1) Define the word to be changed with no definition:

: -emit

2) On the next line, create a word which changes what definition it uses:

: is-emit ` -emit [ dict->cfa ] lit swap -is ;

And don't forget to set a default definition for it:

` output-char [ dict->cfa is-emit

How it works

` -emit puts a pointer to the dictionary entry on the stack at compile time. [ dict->cfa takes this dictionary pointer and returns its CFA (code field address). ] -is takes two CFAs and makes the former call the latter.

To define a word that changes a definition, be sure to get a lit in there to make the CFA available at runtime, not just compiletime.