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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: Self-referential types
Date: 1999/10/18
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In article <38077EB3.E6911567@mitre.org>,
  "Robert I. Eachus" <eachus@mitre.org> wrote:
> Brian Rogoff wrote:
>
>    AFIAK, it is almost always poor engineering to declare
procedures
> within other procedures in Ada. The chief exception is
recursive descent
> parsers.

What an *amazing* statement. I am completely flabbergasted!

First, I don't see recursive descent parsers as being
relevant here, and see no reason for nesting there.

There are however MANY MANY examples where nesting makes
excellent sense, and IMO any language that does not permit
nested procedures is badly broken,

There are zillions of reasons for nesting, but let me just
give two.

1. Localization of names. If you have a small procedure that
is just called a couple of times within a single procedure,
it is FAR better to restrict its name space by nesting it
within the calling procedure, and it is indeed very bad
style to unnest it in this case. One particular case is
where you have a recursive implementation of a non recursive
procedure interface for the client, often referencing one
of the parameters of the outer procedure non-locally.

2. Nested procedures makes it FAR easier to make things thread
safe, because you can have variables that are "global" to a set
of cooperating procedures, but are still on a local
thread-specific stack of the enclosing procedurers.
In a language without nested procedures, you often see real
global variables used for this purpose, which is a common
source of non-thread safeness. Yes, you can take the position
that you should never have any non-local references, but this
often obfuscates the code and is error prone.


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