Path: utzoo!attcan!uunet!husc6!rutgers!noao!arizona!mike
From: mike@arizona.edu (Mike Coffin)
Newsgroups: comp.lang.misc
Subject: Re: Turing programming language.
Message-ID: <8861@megaron.arizona.edu>
Date: 24 Jan 89 16:42:35 GMT
References: <12@euteal.UUCP>
Organization: U of Arizona CS Dept, Tucson
Lines: 34

From article <12@euteal.UUCP>, by mart@euteal.UUCP (Mart van Stiphout):

>  Let me explain my objections to the Turing loop syntax.  My main
>  objection is the location of the exit criterion. It is inside the
>  loop and generally will be surrounded by (lots ?) of code. As we
>  all know, the correctness of loops can be shown by pre- and
>  postconditions and the invariant relation.  In my view, the stop
>  criterion should not be located somewhere in the middle of the
>  loop. Furthermore, in my opinion your Pascal examples of loops
>  proof their superiority to the Turing loops: the loop type is clear
>  in a glance. I think it's ok to use different notations for
>  different loops
I'm not sure what the pre- and post-condition have to do with where
the loop exit is.  The loop invariant has nothing to do with where the
exit is, and the other part of the post-condition is exactly the exit
condition.  Again, it doesn't matter where the exit is.

In a practical sense, it is very useful to exit a loop from the middle
sometimes.   Almost all programming languages recognize this and
provide some way to do it:  C has break, Pascal has goto, etc.  Turing
just attempts to make this less of an exception.  With proper
formatting, it is just as obvious what kind of loop it is.

>  Why can't there be a programming language with type list that
>  automatically allocates memory if needed and allows treatment of
>  list in a similar way as arrays.

There are lots of them.  CommonLisp, Scheme, Icon, etc.


-- 
Mike Coffin				mike@arizona.edu
Univ. of Ariz. Dept. of Comp. Sci.	{allegra,cmcl2}!arizona!mike
Tucson, AZ  85721			(602)621-2858