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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