Re: as_ly_scm_list

[Dan Eble]

On Jul 25, 2022, at 16:09, Jean Abou Samra <jean@abou-samra.fr> wrote:
> 
> Now, the next question. Suppose I just want to iterate over the list, nothing 
> else. (This is actually the only case I have found myself in.) Suppose I get 
> this list from an lvalue. The compiler won’t let me use ly_scm_list (lvalue). 
> Is there a way to use ly_scm_list nevertheless and not as_ly_scm_list in 
> order not to put the reader in a mode where they look for in-place 
> modifications of the SCM lvalue?

Readers will know that you are just iterating if you use a range-based for loop.

If the variable declaration is a reference, assignment modifies the current 
list element (and readers will be expecting it).

    // set! all elements of the_list to #f
    if (SCM &s : as_ly_scm_list (the_list))
      s = SCM_BOOL_F;

If the variable declaration is a value, then it is a copy of the current list 
element, which can't be used to change the element.

    if (SCM s : as_ly_scm_list (the_list))
      {
        // s is read, but probably not assigned to.
        // If s is assigned to, it doesn't change the_list.
      }

However, if s above refers to a smob, the smob can still be modified via s even 
though the_list cannot.  For a homogeneous list of smobs, we can hamper that; 
for example,

    if (const auto *col : as_ly_smob_list<Paper_column> (the_list))
      ...

> Is there a way to use ly_scm_list ...

I am aware that I haven't actually answered this question.
— 
Dan