[Rd] model.matrix.default() silently ignores bad contrasts.arg

[Ben Bolker]

An lme4 user pointed out <https://github.com/lme4/lme4/issues/491> that
passing contrasts as a string or symbol to [g]lmer (which would work if
we were using `contrasts<-` to set contrasts on a factor variable) is
*silently ignored*. This goes back to model.matrix(), and seems bad
(this is a very easy mistake to make, because of the multitude of ways
to specify contrasts for factors in R  - e.g. options(contrasts=...);
setting contrasts on the specific factors; passing contrasts as a list
to the model function ... )

The relevant code is here:

https://github.com/wch/r-source/blob/trunk/src/library/stats/R/models.R#L578-L603

The following code shows the problem: a plain-vanilla model.matrix()
call with no contrasts argument, followed by two wrong contrasts
arguments, followed by a correct contrasts argument.

data(cbpp, package="lme4")
mf1 <- model.matrix(~period, data=cbpp)
mf2 <- model.matrix(~period, contrasts.arg="contr.sum", data=cbpp)
all.equal(mf1,mf2) ## TRUE
mf3 <- model.matrix(~period, contrasts.arg=contr.sum, data=cbpp)
all.equal(mf1,mf3)  ## TRUE
mf4 <- model.matrix(~period, contrasts.arg=list(period=contr.sum),
data=cbpp)
isTRUE(all.equal(mf1,mf4))  ## FALSE


  I've attached a potential patch for this, which is IMO the mildest
possible case (if contrasts.arg is non-NULL and not a list, it produces
a warning).  I haven't been able to test it because of some mysterious
issues I'm having with re-making R properly ...

  Thoughts?  Should I submit this as a bug report/patch?

  cheers
   Ben Bolker



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