Re: Effect of lexical binding upon function paramaters

[Emanuel Berg]

Heime wrote:

>>> What I can see from this test below, formal parameters
>>> ("arguments" in standard information interchange) are
>>> always dynamic under dynabound, and always static under
>>> lexical
>> 
>> Yup. Same holds for the var bound by `condition-case`.
>> The binding constructs that can be "either/or" are `let`
>> and `let*` (via `lexical-let` for the dynbound dialect and
>> via `defvar` for the lexbound dialect).
>
> So what are we to do

Use lexbound, so far by adding this, as you know

;;; -*- lexical-binding: t -*-

as the first line of every Elisp source file.

Byte-compile the source, it will warn you for unused lexical
variables for example.

Avoid global lexical variables (created with `setq' in the
absence of another variable with the same name), instead use
let-closures for the 'persistent value' (state) and
share-between-functions use cases.

If you desire to create a global variable in the "option"
sense, i.e. you want to introduce something like the
`fill-column' variable, because you imagine it will be used
across a range of functions and different settings (ha), even
be used by future functions - then use `defvar'.

Use `let'/`let*' for function-local variables to organize the
code neatly and break up computations into smaller steps, this
will be the number one use case, however let/let* can also be
used in the "with-option-temporarily-as" sense, in that case
that variable must already exist and be dynamic/special, so
either it is an Emacs option already _or_ you have created it
as described above, with `defvar'.

Note that you can mix this up, let/let* handles it all for you
transparently, just spell everything correctly, okay?

In the example below, we see that `fill-column' is already
a dynamic/special variable, i.e. a global option.
In a lexbound `let', we create "a" and "b" and since they
don't exist they default to lexical. `fill-column' OTOH is
temporarily assigned a new value and remains dynamic/special.

;;; -*- lexical-binding: t -*-

fill-column ; 62

(special-variable-p 'fill-column) ; t, i.e. dynamic/special

(let ((a 1) ; lexical/static
      (b 2) ; lexical/static
      (fill-column 10) ) ; dynamic/special
  (fill-paragraph) ) ; eval me, then change `fill-column' to say 99 and retry

fill-column ; 62

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