Path: utzoo!mnetor!uunet!husc6!think!barmar
From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: lisp environments
Message-ID: <13906@think.UUCP>
Date: 17 Dec 87 22:51:03 GMT
References: <487@PT.CS.CMU.EDU> <460@cresswell.quintus.UUCP> <13731@think.UUCP> <471@cresswell.quintus.UUCP>
Sender: usenet@think.UUCP
Reply-To: barmar@sauron.think.com.UUCP (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge, MA
Lines: 48

In article <471@cresswell.quintus.UUCP> pds@quintus.UUCP (Peter Schachte) writes:
>In article <13731@think.UUCP>, barmar@think.COM (Barry Margolin) writes:
>> ...The editor
>> maintains a partial parse of the program in an auxiliary data structure.
>> It uses event counts to update the parse as the user changes the code.
>
>So the Symbolics editor is a text AND structure editor!  This approach
>seems like it has the best of both worlds.  But how partial is partial?

Well, in the case of Lisp it is pretty simple, because Lisp has very
simple syntax.  Each line of the buffer is tagged with the lexical
state at the beginning of the line; this specifies how deeply nested
it is, whether it is in the middle of a string, and the function
definition (or other top-level form) the line is part of.  There are
also some heuristics that speed things up, especially useful when the
file isn't totally syntactically correct; for example, the parse state
is reset whenever a line that begins with an open parenthesis is seen,
so that leaving off a close paren will not screw up the parsing of the
rest of the file.

In the case of other languages such as Pascal, C, and Ada, a real
parse tree is maintained, but I don't know much about its
implementation.

>And how long does it take to start up an edit on a fairly large file?

The extra time it takes to parse the buffer is noticeable, but not
intolerable, since it is interleaved with reading the file in from the
file server.

>Will it let you know you have a syntax error before you get far afield?
>Or do you have to explicitly ASK the system?

It generally only does syntax checking when you perform an operation
on a syntactic unit.  For example, when you ask to compile the current
Lisp function it checks that the parentheses are balanced within that
definition.  There is a command that searches the buffer looking for
unbalanced parentheses.  The only automatic syntax check I'm aware of
is done when a Lisp source file is being written out; it checks
parentheses as it goes, and if it notices that they aren't balanced it
asks whether you want to continue writing.

---
Barry Margolin
Thinking Machines Corp.

barmar@think.com
seismo!think!barmar