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Re: Terminology question

From: Jean-Christophe Helary
Subject: Re: Terminology question
Date: Wed, 27 Apr 2016 13:05:12 +0900

Thank you John for the reply.

Let me ask a few more questions. Apologies if I sound picky, I just want to 
make sure I understand.

> 2016/04/26 23:48、John Wiegley <address@hidden> のメール:
>>>>>> Jean-Christophe Helary <address@hidden> writes:
>> I cannot find a clear definition of "form", "expression", "symbolic
>> expression", "sexp" and whether the terms are equivalent or not.
> A "form" is a piece of data in Lisp that represents a program, meaning it can
> be evaluated without error.
>  3       is a simple, self-evaluating form
>  (3)     is not a form
>  (+ 1 2) is a form

Ok, that corresponds to 1.3.3:

"A Lisp expression that you can evaluate is called a form. Evaluating a form 
always produces a result, which is a Lisp object."

> All of these are Lisp expressions, meaning sentences that are legal according
> to the grammar of the Lisp language.

But in 2.1 we find this:

"In Lisp, an expression is primarily a Lisp object and only secondarily the 
text that is the object’s read syntax."

Which seems to suggest that there is no distinction between "form" and 
"expression" and thus contradicts 1.3.3.

And that is confirmed by 9.1:

"A Lisp object that is intended for evaluation is called a form or 

But then we seem to have a circular definition...

As a conclusion, according to 1.3.3 if evaluating rose produces an error 
message because no value has been assigned to it, it is not a form, but 
according to 9.1, if I intend to evaluate rose, which is syntactically valid 
Lisp, it is a form...

So I am still confused. Maybe it is because I do not understand what "evaluate" 
means in the context of Lisp. In the Introduction 1.2, there is: 

"If you run it (for which the Lisp jargon is evaluate), the computer will do 
one of three things: do nothing except return to you the list itself; send you 
an error message; or, treat the first symbol in the list as a command to do 

Again, in Introduction 1.3 there is:

"What we will do is evaluate a list that is not quoted and does not have a 
meaningful command as its first element."

So evaluation can produce an error in which case 9.1 is the correct definition 
of form and expression: a form is syntactically valid lisp regardless of 
whether it produces a Lisp object or not (unless we consider that errors can be 
considered Lisp objects, but that does not seem to be the case).

The obvious conclusion is that there is a confusion in the use of the term 
"evaluate". In 1.3.3 it means "produce a lisp object", in 9.1 and in the 
Introduction it means "see if it produces a lisp object". That meaning seems to 
be the one chosen in the Emacs manual (for ex in 27.7 Executing Lisp 

I am still confused...

Jean-Christophe Helary 

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