About `read'

About `read'

[Xue Fuqiao]

I have a question about read:

(read "(I am) xfq")
     =>(I am)

(read "I am xfq")
     =>I

Why are they different?  When does `read' skip whitespace characters?
-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao

Re: About `read'

[Teemu Likonen]

Xue Fuqiao [2013-01-04 21:35:03 +0800] wrote:

> I have a question about read:
>
> (read "(I am) xfq")
>      =>(I am)
>
> (read "I am xfq")
>      =>I
>
> Why are they different? When does `read' skip whitespace characters?

Function READ reads one Lisp expression. In your first example the first
expression is a list of two symbols. In the second example the first
expression is symbol named "I".


    read is a built-in function in `C source code'.

    (read &optional STREAM)

    Read one Lisp expression as text from STREAM, return as Lisp object.
    If STREAM is nil, use the value of `standard-input' (which see).
    STREAM or the value of `standard-input' may be:
     a buffer (read from point and advance it)
     a marker (read from where it points and advance it)
     a function (call it with no arguments for each character,
         call it with a char as argument to push a char back)
     a string (takes text from string, starting at the beginning)
     t (read text line using minibuffer and use it, or read from
        standard input in batch mode).

Re: About `read'

[Xue Fuqiao]

On Fri, 04 Jan 2013 15:44:16 +0200
Teemu Likonen <tlikonen@iki.fi> wrote:

> Function READ reads one Lisp expression. In your first example the first
> expression is a list of two symbols. In the second example the first
> expression is symbol named "I".
Thanks, I see.

>      a function (call it with no arguments for each character,
>          call it with a char as argument to push a char back)
When STREAM is a function called with one argument, what does `push a char back' mean?  Where will the char be pushed?
-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao

Re: About `read'

[Barry Margolin]

In article <mailman.16693.1357308229.855.help-gnu-emacs@gnu.org>,
 Xue Fuqiao <xfq.free@gmail.com> wrote:

> On Fri, 04 Jan 2013 15:44:16 +0200
> Teemu Likonen <tlikonen@iki.fi> wrote:
> 
> > Function READ reads one Lisp expression. In your first example the first
> > expression is a list of two symbols. In the second example the first
> > expression is symbol named "I".
> Thanks, I see.
> 
> >      a function (call it with no arguments for each character,
> >          call it with a char as argument to push a char back)
> When STREAM is a function called with one argument, what does `push a char 
> back' mean?  Where will the char be pushed?

Read often has to check an extra character beyond the representation of 
an object, to detect a delimiter.  Since this character isn't actually 
part of the object, it has to be made available to the next caller. So 
read calls the function again, to "unread" the character.

Where it gets pushed is up to the design of the function.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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