Re: add-to-list with lexical variables

[Stefan Monnier]

>>> 1. when I set `lexical-binding' to t and byte-compile the file, it
>>> would report this error:
>>> 
>>> add-to-list cannot use lexical var `env-list'

add-to-list is a function, and functions have access to the
dynamically-bound vars of its caller, but not to the lexically-bound
vars of its caller.

The warning was added because add-to-list is a frequent use-case where
Elisp code presumes dynamic binding.

>>> 2. And when I using `lexical-let*' instead, there would be an warning:
>>> 
>>> Warning: assignment to free variable `env-list'

lexical-let is an old way to get lexical binding.  It was added long
before add-to-list was invented, and it was only used at those few
places where people really wanted lexical binding, and those people
generally knew that add-to-list couldn't possibly work there, so the use
of add-to-list on a var bound with lexical-let was so rare that it
didn't occur to anybody to try and provide an ad-hoc check and warning.
instead it just notices that the call to `add-to-list' uses a (dynamic)
variable `env-list' which is not declared (i.e. is "free").

> However I still don't know the difference `lexical-binding' and
> `lexical-let' brings. Are there some authoritative introductions/tutorials?

lexical-let is an old hack made obsolete by lexical-binding.


        Stefan