Re: Expressive Case Statements (was: Software landmines)

[Brian Rogoff <bpr@shell5.ba.best.com>]

On Tue, 1 Sep 1998, Richard D Riehle wrote:
>  I realize this was supposed to be about optimizations. But this is the
>  year of changing the subject what with tails wagging dogs, etc. Ted's
>  mention of "case statements" reminds me of a problem with which I have
>  been struggling for quite a while; one for which I finally cry, "Help!"
> 
>  There is a language called COBOL in which the ANSI 1995 standard 
>  introduced a fascinating construct called the EVALUATE statement. It
>  is the most powerful form of a case statement I have seen in any
>  programming language. More powerful than Ada. More powerful than Eiffel.
>  More powerful than C++ or Java. 

Unfortunately, I don't COBOL, but I think I can read your case statement
below, and I wonder if the following, in Objective Caml, does the trick:
> 
>  One of the forms the EVALUATE statement (there are several options), 
>  allows the programmer to directly represent a decision-table for a
>  set of conditions with syntax such as,
> 
>         EVALUATE condition-1 ALSO condition-2 ALSO condition-3 
>                              
>            WHEN TRUE ALSO TRUE ALSO FALSE 
>                           PERFORM 
>                               some action-stub statements
>                           END-PERFORM
>            WHEN TRUE ALSO FALSE ALSO FALSE
>                           PERFORM 
>                               some action-stub statements
>                           END-PERFORM
>            WHEN FALSE ALSO FALSE ALSO TRUE
>                           PERFORM 
>                               some action-stub statements
>                           END-PERFORM
>         END-EVALUATE

let get_val b1 b2 b3 =  
	match (b1, b2, b3) with 
	| (true, true, false) ->
	perform1	
	| (true, false, false) ->
	perform2
	| (false, false, true) ->
	perform3
	| (_,_,_) ->  perform_default;;

I think ML style pattern matching is pretty powerful compared to Ada's,
and Prolog's even more so. 

> In business data process software, such as banking and insurance, the
> decision table is useful for representing complex sets of conditions
> and their corresponding actions.  In military command and control
> systems we also see these kinds of complex condition sets.
> 
> It is certainly easy to represent any set of conditions with a sequence
> of if ... elsif statements but I am seeking something else.  What I have
> been trying to accomplish is some generalized algorithm in Ada that 
> allows me to design a package that approximates the power of the EVALUATE
> statement in COBOL.  Some of the new features of Ada 95 such as generic
> formal package parameters, access to subprogram, etc. have helped a
> little, but ...

As I'm sure you know, you need to create mapping functions to an
enumerated type or something similar in order to use Ada's case statement. 
I don't think there is a general solution, you can't even use tags in 
case statements. Yeah, I prefer ML for its more powerful pattern matching,
but Ada has her charms too. I guess we Ada fans will have to do this by 
hand; we also have to do it by hand if we want to translate our close 
cousin language VHDL's case statement to Ada.

-- Brian