Reference Guide

2-26 RPL Programming Examples
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NAMES is demonstrated in the program VFY.
VFY (Verify Program Argument)
VFY verifies that an argument on the stack is either a name or a list that contains exactly two names.
Level 1
→
Level 1
'name'
→
'name'
{ valid list }
→
{ valid list }
{ invalid list }
→
{ invalid list } (and error message in status area)
invalid object
→
invalid object (and error message in status area)
Techniques used in VFY
 Utility programs. VFY by itself has little use. However, it can be used with minor modifications by other
programs to verify that specific object types are valid arguments.
 CASE…END case structure). VFY uses a case structure to determine if the argument is a list or a name.
 Structured programming. If the argument is a list, VFY calls NAMES to verify that the list contains exactly
two names.
 Local variable structure. VFY stores its argument in a local variable so that it can be passed to NAMES if
necessary.
 Logical function. VFY uses NOT to display an error message.
Required Programs
NAMES
 NAMES verifies that a list argument contains exactly two names.
VFY program listing
Program: Comments:
«
DUP
Copies the original argument to leave on
the stack.
DTAG
Removes any tags from the argument for
subsequent testing.
→ argm
Stores the argument in local variable argm.
«
Begins the defining procedure.
CASE
Begins the case structure.
argm TYPE 5. SAME
Tests if the argument is a list.
THEN
argm NAMES
END
If so, puts the argument back on the stack
and calls NAMES to verify that the list is
valid, then leaves the CASE structure.