Re: Building blocks (Was: Design By Contract)

["Neil Wilson" <nobody@nobody.com>]

> In article <34316EC3.5B62@dynamite.com.au>, aebrain@dynamite.com.au
wrote:
> Yes, it's quite wrong to use a flag to terminate a while loop, instead of
> just using exit with a loop.  The idiom is, use an exit when the
> termination depends on what you read, eg
> 

[SNIP]

> 
> The classic example is, read some integers until the user types in the
value 0:
> 
> loop
> 
>    Read (N);
>    exit when N = 0;
> 
>    <process N>
> 
> end loop;

This can be fixed by using a 'half unroll'. Eiffel supports this:

loop
	file.read_integer
while file.last_integer != 0
	x.process (file.last_integer)
	file.read_integer
end -- loop

The introduction of the syntactically redundant 'loop' marker allows you to
flag the start of the loop initialisation section (technically the section
that sets up the loop invariant). The 'while' section is the loop itself
(which has to maintain the invariant, move forward the variant and do the
correct processing). 

Duplicating the 'read' command can be a bit of a pain, but in many
instances initialising the structure can be a different operation from the
actual iteration itself.


-- 
Neil Wilson (neil at aldur dot demon dot co dot uk)
Aldur Systems Ltd, UK