What Dialects Need From Binding

Development Binding
2

So the point we're trying to get at is that every dialect becomes an interpreter in its own right. When you pass it a block, it wants to do more with that block than just delegate to the evaluator's rules by DO-ing it. It wants to enumerate over it with something like a FOR-EACH... making decisions based on the logic of whatever its mini-language rules wants.

In the past, if one of these dialect languages used FOR-EACH and came across a WORD!, it would be able to GET that word as a variable...thanks to binding passes that walked the blocks deeply before the dialect function received it.

The weaknesses of that binding strategy are articulated in detail elsewhere (weak enough that I consider it broken and unfit for purpose, hence the search for new ideas). But it can seem powerful at first glance.

Imagine a silly Rebol2 dialect that just takes SET-WORD!s and INTEGER!s in pairs, and assigns double the value:

rebol2>> double-assigner: func [block] [
             foreach [sw i] block [
                 assert [(set-word? sw) (integer? i)]
                 set sw 2 * i
             ]
             return none
        ]

rebol2>> x: none y: none
== none

rebol2>> double-assigner [x: 10 y: 20]
== none

rebol2>> x
== 20

rebol2>> y
== 40

I'll throw in an example which might be interesting to you: the DESTRUCTURE dialect.

The idea that you can do such things has hinged on a very different idea from "the currency of code is largely unbound, with bindings at the tips affecting evaluative products of non-quoted material", which is what is currently under consideration.

If that takes effect, then the dialect must mirror the evaluator's logic of propagating bindings stored in blocks as it goes where applicable... making decisions at each point. Because by default, any PICKs or FOR-EACHs will just be giving back the literal elements with no binding. (Unless they happen to be one of the relatively rare items which have been explicitly bound.)

Brett's example of "write your own COMPOSE", e.g. REPLACE-HOLES, has a similar problem of finding a GROUP! via enumeration as opposed to evaluation... then wanting to DO it, when the enumeration led to getting it with no binding.

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