[R] Why a list of NULL's are reduced to NULL?

[Don MacQueen]

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.wickham at 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