On Tue Oct 4 2016 Stefan Monnier wrote:
> There are two ways to do that with lexical-binding:
> - use (eval <form> `((<var1> . ,<val1>) (<var2> . ,<val2>) ...)
> which will not give the exact same behavior but works well in many
> cases (i.e. depends on the code put in diary-date-forms).
> - use
>
> (defvar <var1>) (defvar <var2>) ...
> (let ((<var1> <val1>)
> (<var2> <val2>)
> ...)
> (eval <form> t))
>
> which will preserve backward compatibility.
Thank you, I did not know that! - It appears to me that the first
approach is the cleaner way to go. So I've tested it with
bbdb-anniv.el, and it seems to do exactly the right thing. Under
what circumstances could there be a problem? (The forms in
diary-date-forms are lists containing symbols and strings. Then
`eval' is applied to the list elements in order to replace the
symbols by their let-bound values. So there is nothing fancy
happening here. Possibly there is yet a completely different
strategy for this problem? I just copied the relevant code from
diary-list-entries-2, assuming that this should be the most reliable
strategy for bbdb-anniv even if I do not know too much about
diary-lib.el.)
Actually, I am a bit confused about the second approach you
suggested. I believe I understand its logic, though I find it odd
to use defvar in such a context. Interestingly, the elisp manual
says about `defvar':
If SYMBOL is already lexically bound (e.g., if the ‘defvar’ form
occurs in a ‘let’ form with lexical binding enabled), then ‘defvar’
sets the dynamic value. The lexical binding remains in effect
until its binding construct exits. *Note Variable Scoping::.
This suggests to me that in your second approach the defvar's could
also go inside the let, which appears rather odd to me. Or am I
misinterpreting this paragraph? What would be an example for this
usage of defvar in a let form?
Thanks again,
Roland
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