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Re: IDL wish list (continued)
firstname.lastname@example.org (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 email@example.com firstname.lastname@example.org
San Diego, CA