entry at other than main (was want to know)

[Chris Torek]

In article <10147 at csli.Stanford.EDU> poser at csli.Stanford.EDU (Bill Poser)
writes:
>Chris Torek says that in Cobol subroutines must be declared before
>use but that program execution starts at the top.

Not COBOL: SNOBOL.  Utterly different languages.

>Does this mean that you can't use subroutines,

Not at all.

>or that [SNOBOL] allows declarations, which as non-executable statements
>can precede the top-level function,

It does not.  In SNOBOL, declarations are executable statements.

>separate from the actual subroutine definitions?

They must be together (although it is permissible to branch out and
back in, thus weaving the subroutine and the main program together into
one big ugly piece, as I recall).  It has been too long since I wrote
anything in SNOBOL IV, and I never wrote much (3 programs?), so I do
not recall the syntax.  By suitable snooping about, however, I have
found someone's `snobolhmwk' file---a homework assignment from 1984.
(Being at a large university has its advantages :-) .)  It includes
several example programs:

  1	* program 1
  2	      X = 1
  3	      DEFINE('P()X')
  4	      DEFINE('Q()','A')        :(G)
  5	A     X = X + 1                :(RETURN)
  6	P     X = 5
  7	      Q()
  8	      OUTPUT = X               :(RETURN)
  9	G     P()
 10	      OUTPUT = X
 11	END

(the line numbers are mine, inserted for reference).

As I recall, this creates a global `x' and sets it to one, then defines
P() as a procedure which will be found when called by looking for label
`P'.  (Labels go in the left column.)  It also defines Q(), which
begins at A (not Q).  I am not sure what the X after DEFINE('P() means,
but the obvious guess is that it is a local variable.  The :(G) means
`goto label G' (unconditionally).  Thus, by line 3, the interpreter
knows about three variables: X (a value) and P and Q (procedures that
begin at labels P and A respectively).  It jumps to line 9, which calls
P(); P() sets its local X to 5 and calls Q(); Q() increments an X
(whether the local one or the global, I am not now sure; I suspect it
is the most recent one, i.e., the local) and returns; P() prints the
current value of X (by assigning to the pseudo-variable `OUTPUT'), and
returns; then the main program prints the value of the global X.

Here is a more interesting SNOBOL program:

  1	     X = 'Z'
  2	     Y = 'X'
  3	     Z = 'Y'                :(R)
  4	S    Z = 'Y'
  5	     $X = $Z '0'
  6	     $Y = $Y '1'
  7	     OUTPUT = X ',' Y ',' Z
  8	     EQ(A,0)                :S(RETURN)
  9	     DEFINE('Q(Y,A)Z','S')
 10	     Q('Y',A-1)             :(RETURN)
 11	R    DEFINE('P(X,A)','S')
 12	     P('Z',1)
 13	END

This sets X to "Z", Y to "X", Z to "Y", and branches to R (line 11).
This defines procedure P (beginning at S) with arguments X and A.  Line
12 then calls P with the (now local) X set to "Z" and the local A set
to 1.  At line 4, Z (global) is set to "Y"; at line 5, the variable
named by X---and X is "Z", so this means the global Z---is set to
whatever is named by Z (here the global Y) concatenated with the string
"0", so this sets the global Z to "X0".  Line 6 sets concatenates the
string "1" into whatever variable Y names (here X), so this changes
the local X from "Z" to "Z1".  It then prints X, Y, and Z, which should
produce the output
	Z1,X,X0
At line 8, if A is equal to zero, we return (the notation :S(RETURN)
means `return if the expression on the left did not fail).  Since A
is 1, not zero, the comparison fails and the return is ignored, and
we fall through to line 9, which suddenly declares Q as a procedure
which has two arguments (Y and A) and one local variable (Z) and
which begins at label S (line 4).  We then call Q, passing "Y" and
A-1 (0), and when (if) Q returns, we return from P().  Tracing the
execution of Q is left to *you*....
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris