Subject: Best of DATAMATION: GOTO-less
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Date: Thu, 14 Aug 2008 16:34:22 +0100

We don't know where to GOTO if we don't know where we've COME FROM. This
linguistic innovation lives up to all expectations.

A Linguistic Contribution of GOTO-less Programming

By R. Lawrence Clark*

From DATAMATION, December, 1973

Nearly six years after publication of Dijkstra's now-famous letter, [1]
the subject of GOTO-less programming still stirs considerable controversy.
Dijkstra and his supporters claim that the GOTO statement leads to difficulty in
debugging, modifying, understanding and proving programs. GOTO advocates argues
that this statement, used correctly, need not lead to problems, and that it
provides a natural straightforward solution to common programming
procedures.

Numerous solutions have been advanced in an attempt to resolve this debate.
Nevertheless, despite the efforts of some of the foremost computer scientists,
the battle continues to rage.

The author has developed a new language construct on which, he believes, both
the pro- and the anti-GOTO factions can agree. This construct is called the COME
FROM statement. Although usage of the COME FROM statement is independent of the
linguistic environment, its use will be illustrated within the FORTRAN language.

box 1

Unconditional COME FROM statement:
General Form
COME FROM xxxxx
Where: xxxxx is the number of an executable statement in the same program unit.

This statement causes control to be transferred to the next statement (the statement immediately following the COME FROM upon completion of the designated statement.
Example:
10 J=1
11 COME FROM 20
12 WRITE (6,40) J STOP
13 COME FROM 10
20 J=J+2
40 FORMAT (14)

Explanation:

In this example, J is set to 1 by statement 10. Statement 13 then causes
control to be passed to statement 20, which sets J to 3. Statement 11 then
causes control to be passed to statement 12, which writes the current value of
J. The STOP statement then terminates the program.

box 2

Conditional COME FROM statement:
General Form
IF (cond) COME FROM xxxxx
Where: cond is any logical expression.
xxxxx is the number of an executable statement in the same program unit.

This statement causes control to be transferred to the next statement whenever the condition cond is true and the designated statement has just been completed.
Example:
I = 1
IF (I .LT. 10) COME FROM 50
I = I+1
50 WRITE (6,60) I
STOP
60 FORMAT (14)

Explanation:

The COME FROM takes effect only while I is less than 10. Thus when I is equal
to 10, the program continues past statement 50 and terminates. This is
equivalent to the now-obsolete formulations:
I = 1
30 I = I+1
WRITE (6,60) I
IF (I .LT. 10) GO TO 30
STOP
60 FORMAT (14)
or
DO 50 I = 2, 10
50 WRITE (6,60) i
STOP
60 FORMAT (14)

Note how much cleaner is the intent of the code containing the COME FROM
construct.

box 3:

Computed COME FROM statement
General Form
COME FROM (x1, x2, x3....,xn), i
Where: Each x is the number of an executable statement in the same program unit.
i is an integer variable.

This statement causes control to be transferred to the next statement
whenever any of the following conditions holds:
* statement x1 has just been executed and i is equal to 1
* statement x2 has just been executed and i is equal to 2
* statement x3 has just been executed and i is equal to 3
* statement xn has just been executed and i is equal to n

If, when statement xj is executed, i has any value other than j, this
statement has no effect.

Example:
DO 200 INDEX=1,10
10 X=1.
20 X=X*2.
30 X=X*3.
40 X=X*4.
50 X=X*5.
60 X=X*6.
70 X=X*7.
80 X=X*8.
90 X=X*9.
100 X=X*10.
COME FROM (10,20,30,40,50,60,70,80,90,100),INDEX
WRITE (6,500) INDEX,X
200 CONTINUE
STOP
500 FORMAT (14,2X,F12.0)

Example:

This program illustrates the power of the computed COME FROM by providing a
compact algorithm for computing factorials. On the first iteration (INDEX=1), as
soon as statement 10 has been executed, control passes to the WRITE statement.
As a more general case, consider the fifth iteration: X is set to 1, and the
multiplied by 2., 3., 4. and 5. before control passes to the WRITE statement.

box 4:

Assign and assigned COME FROM statements
General Form
ASSIGN xxxxx TO m
*
*
*
COME FROM m, (x1,x2,x3,...,xn)
Where: xxxxx is the number of an executable statement. It must be one of the numbers x1,x2,x3...,xn
Each xx if the number of an executable statement in the same program unit.
m is an integer variable which is assigned one of the statement numbers x1,x2,x3,...,xn.

The assigned COME FROM causes control to be transferred to the next statement
upon completion of the statement whose number is currently assigned to m. This
provides a convenient means of passing control to a common point from a variety
of points in the program unit. The actual point from which control is to be
passed can be selected under program control.

Example:
DO 60 I=6,32
20 X=I*6+14
IF (X-20.) 10, 30, 50,
10 ASSIGN 40 TO JUMP
30 Y=2*X**2.-17.4
COME FROM JUMP, (40,20,30)
ASSIGN 30 TO JUMP
X=X*Y-X**2
40 ASSIGN 40 TO JUMP
IF (Y-X) 20, 60, 50
50 ASSIGN 20 TO JUMP
60 CONTINUE

Explanation:

This example is self-explanatory.

The author feels that the COME FROM will prove an invaluable contribution to
the field of computer science. It is confidently predicted that this solution
will be implemented in all future programming languages, and will be retrofitted
into existing languages. Although it is clear that the COME FROM statement
fulfills most of the requirements of the advocates of GOTO-less programming, it
remains for the practitioners of automatic programming to evaluate just how much
this construct contributes to the development of automatic proofs of program
correctness. Having at last put to rest to GOTO controversy, we now may enter
the era of the COME FROM conundrum.

* The author is indebted to G.F. Groner and N.A. Palley for a valuable
discussion which took place in New Haven, Conn.

[1] E. W. Dijkstra, "GOTO Statement Considered Harmful," Letter of the
Editor, Communications of the ACM, March 1968, pp. 147-148.

Mr. Clark is a programmer/analyst in the computation center of the Rand Corp.
He has been active in the development of user-oriented, interactive computer
systems, especailly graphics systems. He has a BS in mathematics from
Pennsylvania State Univ.

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