Re: Dynamic unquote ( , )?

Ulrich Hobelmann wrote:
Now the problem is that I don't want VAL to be pushed, i.e. I want
something more like
(emit-action `(do-stuff-with ,val)) but then the emit-action macro
won't execute that code, but expand to (if *bla* quoted-do-stuff-exp
...), so it will only *return* that expression, instead of executing it.
Again I'm in a situation where I need to eval stuff :(

What EMIT-ACTION will do is push the expanded backquote object onto
*ACTIONS*. And so when later you want to evaluate that action, that
action will want to evaluate the VAL variable, in the wrong context.
I.e. what you have here is exactly the same as:

(emit-action (list 'do-stuff-with val))

but the LIST is not evaluated, since this is a macro. So what ends up
happening is that this expands to the code:

(if *execute-mode*
(list 'do-stuff-with val)
(progn (collect-emit-buf) (push '(list 'do-stuff-with val)
*actions*))

In excute mode, the (LIST 'DO-STUFF-WITH VAL) doesn't do the right
thing now. Although it can correctly resolve the value of VAL from the
surrounding lexical environment, all it does is compute the expression
and throw it away.

In delayed mode, the expression (LIST 'DO-STUFF-WITH VAL) is evaled,
but it can't resolve the correct value of VAL.

Any ideas?

This isn't a macro problem, but a problem of wanting to evaluate the
same expression in two different contexts, yet allow it to resolve
variables in the same way. That is done using closures, rather rhan
backquote substitution tricks.

Yes, the two branches of the IF do need a different solution, but that
solution can be based on exactly the same piece of source code that
comes in as a macro argument.

You want this code to come out of the macro:

(if *execute-mode*
(do-stuff-with val)
(progn
(collect-emit-buf)
(push (lambda () (do-stuff-with val)) *actions*)))

From this pattern, it should be obvious how to write the macro which
build this out of (EMIT-ACTION (DO-STUFF-WITH-VAL)).

So *ACTIONS* isn't a list containing fragments of source code. It's a
list of first-class funtions (which are compiled if the places that
generate them are compiled!)

You call these first class functions using FUNCALL, rather than EVAL.

.

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