Newsgroups: comp.lang.misc
Path: utzoo!utgpu!jarvis.csri.toronto.edu!csri.toronto.edu!vg
From: vg@csri.toronto.edu (Victor Greenberg)
Subject: Passing Values Through a "goto"
Message-ID: <8901070434.AA10471@yorkville.csri.toronto.edu>
Organization: University of Toronto, CSRI
References: <3300001@uxg.cso.uiuc.edu> <3290002@hpctdls.HP.COM> <4505@xenna.Encore.COM> <2583@ficc.uu.net> <9310@ihlpb.ATT.COM> <11359@haddock.ima.isc.com>
Date: Fri, 6 Jan 89 23:34:00 EST

karl@haddock.ima.isc.com (Karl Heuer) writes:
>Let's try to define EC, an extension of C with the property that every
>statement is an expression, yet retaining the property that every expression
>has a type.  All constructs with no reasonable value definition become void
>expressions.
>...

I once designed an expression-based variant of C called "D".
This language supported "compound expressions" of the form:
	( [expression|declaration] ; ... ; expression )
The final expression yields the result.
This means you can embed declarations within expressions:
	( int i = getint(); i * i )

A compound expression with no declarations has the form:
	( expr1; expr2; ...; exprn )
and yields exprn as its result.  This means that the "," operator is
unnecessary, as ";" can be used in its place.

A compound expression with a missing result expression has a value of
type "void".  Eg:
	( expr1; expr2; expr3; )
This means we don't need C's compound statements enclosed in {}, we
can use parentheses instead.

[None of this is new.  Algol 68 already does all of the above.  I wish
 C were as well designed.]

>Passing values through a goto doesn't seem to work well, since a label by
>itself is not a valid statement.  Well, I guess we could say that a labeled
>null statement has the value of the statement preceding the goto, but that's
>stretching the idea a bit too far, I think.

Although goto is questionable as a language feature, it isn't hard to
generalize for use in an expression based language.  Instead of labelled
statements, you want labelled *expressions*.  For example:
	printf("%d\n", foo:(a+b) * c)
In the above fragment, (a+b) is a sub-expression with the label "foo".
When this fragment is executed normally, the value printed is (a+b) * c.
However, elsewhere in the program, you can write:
	goto foo(x)
This will jump into the middle of the call to printf, and the value
printed is x * c.

Doug Moen.