R devel - Sep 2020 - more Matrix weirdness

>     > "These operators are also implicit S4 generics, but as
>     > primitives, S4 methods will be dispatched only on S4
>     > objects ?x?."
> Yes, exactly,  very well found, Georgi!
I'm sorry Martin, but I don't understand your point here.

I'm assuming that you want the (S3) matrix, x, to be converted to an
(S4) Matrix.

However, this is not a question of method dispatch, as such.
But rather a question of type conversion (integer to numeric to complex, etc).

Specifically, can/should automatic type conversion, convert an S3 data
type to an S4 data type, even where user-defined data types are
involved?

Abby, my answer was too concise. The thrust is that even if you define a method
for
"[<-" with signature x="matrix" and value
="Matrix", for example, it will never be used since "matrix"
is S3.
If instead x="someS4class" then the S4 method will be invoked.

There may be cases when changing the class of the left-hand side make sense
(such as one subclass of "Matrix" to another) but certainly not for
the base R vector classes.


Georgi Boshnakov


-----Original Message-----
From: Abby Spurdle <spurdle.a at gmail.com> 
Sent: 11 September 2020 03:03
To: Martin Maechler <maechler at stat.math.ethz.ch>
Cc: Georgi Boshnakov <georgi.boshnakov at manchester.ac.uk>; r-devel at
r-project.org
Subject: Re: [Rd] more Matrix weirdness
>     > "These operators are also implicit S4 generics, but as
>     > primitives, S4 methods will be dispatched only on S4
>     > objects ?x?."
> Yes, exactly,  very well found, Georgi!
I'm sorry Martin, but I don't understand your point here.

I'm assuming that you want the (S3) matrix, x, to be converted to an
(S4) Matrix.

However, this is not a question of method dispatch, as such.
But rather a question of type conversion (integer to numeric to complex, etc).

Specifically, can/should automatic type conversion, convert an S3 data type to
an S4 data type, even where user-defined data types are involved?

> There may be cases when changing the class of the left-hand side make sense
(such as one subclass of "Matrix" to another) but certainly not for
the base R vector classes.
I'm not sure what you mean by "not for the base R vector classes".
Historically, the simpler class (or mode) gets coerced to the more
complex class (or mode).

    x <- y <- 1:10
    y [1] <- 1

    (class (x) == class (y) ) #FALSE

Also, I note the behavior of multiplication of a matrix with a Matrix.

    library (Matrix)

    m <- matrix (1:16, 4, 4)
    M <- Matrix (1:16, 4, 4)

    as.vector (class (m * M) )   #dgeMatrix
    as.vector (class (M * m) )   #dgeMatrix
    as.vector (class (m %*% M) ) #dgeMatrix
    as.vector (class (M %*% m) ) #dgeMatrix

So, here also, the output is a Matrix, regardless of the type of
multiplication, or the order of the operands.

But the following surprised me:

    k <- m
    mode (k) <- "complex"
    k %*% M