Re: Difference between cons and dot

[tpeplt <tpeplt@gmail.com>]

Heime <heimeborgia@protonmail.com> writes:

> Am adding an elemnt to an alist using either the cons or dot method.
> Have noticed that with cons I do not need to call list 
>
> Thusly 
>
> (push (cons kfrz lnum) tema-lugar)
>
> rather than
>
> (push '(cons kfrz lnum) tema-lugar)
>
> But with the dot method I have to use 
>
> (push '(kfrz . lnum) tema-lugar)
>
> and not 
>
> (push (kfrz . lnum) tema-lugar)
>
> What is the reason for this difference ?

1. The reason for the difference is that, in Lisp, a function’s arguments
   are expressions that are evaluated to obtain a value before the
   function is called.  And your two examples yield different values
   when the argument expressions are evaluated.

   So, the expression ‘(cons kfrz lnum)’ is evaluated to obtain a value
   before the function ‘push’ is called.  And the expressions ‘kfrz’ and
   ‘lnum’ are evaluated before the function ‘cons’ is called, (Which
   means that you probably meant to write ‘(cons 'kfrz 'lnum)’ instead).
   If the expression ‘(cons 'kfrz 'lnum)’ is evaluated, it yields the value
   ‘(kfrz . lnum)’, which is the value passed to ‘push’.

   Likewise, the expression (kfrz . lnum) is evaluated for a value before
   ‘push’ is called.  In order to evaluate (kfrz . lnum), the two
   arguments ‘.’ and ‘lnum’ would need to be evaluated before calling the
   function ‘kfrz’.  Of course, you likely did not mean for ‘kfrz’ to be a
   function.  Rather, you meant for ‘push’ to receive the value (kfrz
   . lnum), which is what (cons 'kfrz 'lnum) returns when evaluated.  It is
   also what the expression '(kfrz . lnum) returns when it is evaluated.
   The apostrophe or single-quote in the expression is a Lisp shorthand for
   the ‘quote’ function (C-h f quote for more detail).  So, you could
   write:

   (push (quote (kfrz . lnum)) tema-lugar)

   (quote (kfrz . lnum)) yields the same value as '(kfrz . lnum).  Enter
   each of these two expressions after M-: if you would like to see this
   confirmed.

2. If you have read the "Introduction to Emacs Lisp", you will want to read
   parts of it again to increase your understanding of Lisp.  In
   particular, read parts of the chapter on List Processing:

   (info "(eintr) List Processing")

   including

   (info "(eintr) Evaluation")
   (info "(eintr) Arguments")
   (info "(eintr) Summary")

-- 
The lyf so short, the craft so long to lerne.
- Geoffrey Chaucer, The Parliament of Birds.