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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Using a string as a binary operator
Date: Wed, 15 Oct 2008 08:22:36 -0700 (PDT)
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Date: 2008-10-15T08:22:36-07:00
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On Oct 14, 11:51 am, "Jeffrey R. Carter"
<spam.jrcarter....@spam.acm.org> wrote:
> Joe wrote:
>
> > I'm trying to build a simple stack the evaluates expressions in
> > postfix notation (i.e. "1 2 +").  I can't find a way to use the
> > operator directly when I get to it.  When I get to the  "+", but how
> > do I apply this string to 1 and 2?  The best I can do is make a case
> > statement that has a case for each binary operator, but this seems
> > very klunky.  I know you can write "+"(1,2) to return 3 but how do I
> > get Ada to recognize the string as an operator?  Here's a short
> > example of what I would like to do:
>
> The short answer is that you can't. You have to evaluate "1" and "2" to get
> values of some numeric type and push them on a stack, then find a branch in an
> if-elsif-else chain

If-elsif-elsif-elsif-elsif-elsif-else chain?  That's not very
imaginative!  Why not:

    type Operator_Descriptor is record
        Name      : access String;
        Operation : access function (Left, Right : Integer)
                             return Integer;
    end record;
    type Operator_Desc_Array is
        array (natural range <>) of Operator_Descriptor;

    Available_Operators : constant Operator_Desc_Array :=
        ((new string' ("+"), Plus'access),
         (new string' ("-"), Minus'access), .......);

Then you can search the array for an element whose Name.all matches
the operator string, and then call the Operation function of the
element.  You'll have to write Plus, Minus, etc., since you can't take
the 'access of an intrinsic function.  If you want to mix some unary
operators in the array, you might use a variant record, or with a
tagged type that has "Unary_Operation" or "Binary_Operation" type
extensions, or something along those lines.

Although it isn't always possible, I definitely prefer to set things
up in an array or some sort of data structure when possible, rather
than to rely on a 67-branch IF statement or even a CASE statement.

                               -- Adam