I would study the third party function and learn all the types it can return.
After all if you don't know what type it's going to return, how can
you possibly make *any* confident use of its output?
Do you actually have this situation? A third part function that is so
poorly documented that you can't (1) predict the type of its return,
and (2) learn what its possible outputs are? Is this function from an
R package downloaded from CRAN?
-Don
At 12:24 PM -0600 12/11/09, Peng Yu wrote:
On Fri, Dec 11, 2009 at 12:05 PM, hadley wickham <h.wick...@gmail.com> wrote:
A very common situation is that the users don't know all the possible
return types of 'some_third_party_function()'. If the users don't know
all the return types, he/she can not make sure the return type of
function(x) {...} be always the same. How do you deal with this case?
It's not that common. It's pretty bad practice to return different
types from a function depending on the input parameters. In many
languages this isn't even possible.
I know this is a bad practice. But R doesn't have a way to forbid such
thing happen. To program defensively, I have to test even uncommon
case, unless it is impossible. When you use a third party software in
your code, do you just ignore the possibility that a function could
return different types?
The solution is to write a function that takes the output from the
first function, inspects it, and coerces all possibilities to the same
type.
How do you figure out all the possibilities?
You can look at its code. You can try the entire range of inputs you
anticipate giving to it (you are in charge of the inputs, so you can
do this). You can talk to the person who wrote it (if that person is
not available, and the function is just some anonymous thing you got
from somewhere, you probably shouldn't place any confidence it its
correctness).
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--
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA
925-423-1062
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