Error: add-to-list cannot use lexical var ...

Error: add-to-list cannot use lexical var ...

[Jambunathan K]

I happened to make some modifications to vc-dir.el recently and tried
using `add-to-list' as part of these modifications.

Add the below snippet somewhere in vc-dir.el and byte-compile it.
    (let (l)
      (add-to-list 'l "testing"))

You will see an error like.
    vc-dir.el:54:1:Warning: Unused lexical variable `l'
    vc-dir.el:54:7:Error: add-to-list cannot use lexical var `l'

I should say the results are not along the expected lines.  Can someone
explain it to me?
-- 

Re: Error: add-to-list cannot use lexical var ...

[Stefan Monnier]

>     (let (l)
>       (add-to-list 'l "testing"))

Functions like add-to-list and `set' rely on an equivalence between
symbol objects and variables which is incompatible with lexical scoping.

> You will see an error like.
>     vc-dir.el:54:1:Warning: Unused lexical variable `l'
>     vc-dir.el:54:7:Error: add-to-list cannot use lexical var `l'

> I should say the results are not along the expected lines.  Can someone
> explain it to me?

When interpreted, add-to-list could be made to work by looking up the
lexical environment, searching for the binding of `l'.
But when byte-compiled there's no such option because the byte-code
doesn't even remember that the local var was named `l'.

You can use CL's pushnew (with :test 'equal) instead.


        Stefan