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Re: IDL wish list (continued)

rfinch@water.ca.gov (Ralph Finch) wrote:
> > This is part of the fundamental nature of IDL and would be
> > difficult if not impossible to resolve without the use of either
> > explicit or implicit typing.
>Not really, all you have to do is arrange for the interpreter to
>warn users that a variable they are using has not been explicitly set
>to a value.  MATLAB does this and doesn't require typing.

I don't know anything about MATLAB, but IDL already warns the user if he 
attempts to use an undefined variable:

IDL> print, a
% PRINT: Variable is undefined: A.
% Execution halted at:  $MAIN$                 

Iarla's post said:

>How about an IDL that would not create an undefined variable whenever
>something gets mistyped...

Allocating a data name when an IDL line is compiled is part of the basic 
language system.  IDL first compiles the code and then executes it.  
Part of compilation includes putting entries into the data name table.  
In order to prevent the variable from being created, the line would have 
to be compiled, and if the above error occurred when the line was 
executed, the variable would have to be removed from the name table.  
IMHO, this would be either difficult or impossible to implement and 
certainly not cost effective.  Also, would it be backwards compatible?

Iarla's post doesn't give a lot of information, but as I see it, the two 
problems that occur when undefined variables are created are:

1.  The user mistypes a variable name interactively at the main program 
level and receives the above warning.  In addition, a new variable is 
created which counts against the local variable limit and clutters up 
the HELP listing.  This could be helped by allowing more local 
variables.  The user can also use the DELVAR procedure to delete the 
undefined variables; however, this has the side effect of erasing the 
current main program.


2.  The user creates a function or a procedure and mistypes the variable 
name.  The function or procedure compiles correctly, but when it is 
executed, a run time error similar to the above occurs.  The user then 
has to edit, compile, and run again.  This cycle continues until no more 
run time errors of this type occur.

Number one is of very little concern to me personally, but two is 
extremely annoying and wastes a lot of my time.  The way that most 
compiled languages get around this problem is to require static typing.  
This enables these problems to be discovered at compilation time and 
enables numerous errors to be discovered in one compilation cycle.  IDL 
could be improved as an application development language by:

1.  Providing an option to do strong typing.  Probably not cost 

2.  Making the compiler more intelligent so that it can recognize this 
type of problem and issue a warning.

3.  Providing a lint type of tool to check source code for potential 
problems that should be looked into.

Ken Knighton        knighton@gav.gat.com   knighton@cts.com
Fusion Division
General Atomics
San Diego, CA