[R] More user-friendly error message needed.

[Tony Plate]

It's actually not that easy to provide the meaningful kind of error message 
you are asking for.  (Though it would be nice.)
Consider what happens in the case you give:

(1) The argument of plot() of evaluated, and has the value NULL
(2) plot() is called with a single argument, which is NULL

All the information available inside plot() is that its argument has the 
value NULL (without getting into meta programming).  However, it would be 
possible for plot(), or xy.coords(), to print a more meaningful error 
message like "called with NULL value for x".  This disadvantage of doing 
this is that if you do it everywhere, a considerable volume of the code 
becomes tests and error messages, which can make the code less readable and 
more prone to errors (because of the possibility that some of the tests are 
buggy themselves).  The weak type checking in the S language exacerbates 
these problems.  As it currently is, many functions are simple and concise, 
but provide very cryptic error messages when something goes wrong.  This is 
the tradeoff.

One might argue that x$z should cause an error, but it is a 
long-established behavior that the "$" operator returns NULL when the list 
element does not exist or is not uniquely identified (this might even be in 
the written definition of the S language).

It might be an interesting CS grad student project to build a system that 
matches cryptic error messages and stack traces against a library of common 
errors in order to provide less cryptic error messages.

-- Tony Plate

At Wednesday 01:07 PM 4/7/2004, Shin wrote:
>When I tried the following commands, I got a strange message.
>
>
> > x<-data.frame(y=c(1:10))
> > plot(x$z)
>Error in xy.coords(x, y, xlabel, ylabel, log) :
>         x and y lengths differ
>
>"The data frame, x, does not have a field named z."
>may be better user-friendly message for this kind of common error.
>
>
>Daehyok Shin
>
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