[R] matrix from list

[Olivier Lefevre]

It's true one may have to set some rules but I think you are blowing it up.

First, it is true one would have to agree for list[[vec]] to always return 
a matrix but it is the useful behaviour since you can already get a vector 
with unlist(list[vec]).

Second, as to the raggedness, matrix(), array() and probably other methods 
already have rules to deal with it, i.e., they cycle and post a warning if 
the desired length is not a multiple of the length of the supplied 
"prototype". You'd get a matrix of the length of the longest selected list 
element; anything else would be baroque.

Third, as to heterogeneity, matrix() and as.matrix() already face the same 
issue and so have a set of coercion rules to deal with it; when they can't 
they throw an error.

Lastly, you are correct that I had overlooked the problem of nested lists 
and that one need (at least I think so) to make some non-obvious rules to 
decided how to flatten them. So you'd throw an error in that case.

In short I think array [[]] indexing would still be well-defined for data 
frames and non-nested lists, with no need for new and special rules. Given 
the existence of alternatives it might or might not be very useful but it 
would remove the shock factor of an indexing operator that does not take 
vector arguments.

Regards,

-- O.L.