From maechler at stat.math.ethz.ch  Fri Sep  1 14:03:22 2017
From: maechler at stat.math.ethz.ch (Martin Maechler)
Date: Fri, 1 Sep 2017 14:03:22 +0200
Subject: [Rd] side-effect of calling functions via `::`
In-Reply-To: <986041A4-19BE-4500-AD7E-4B2A32504D76@rstudio.com>
References: <59A93D09.8010006@gipsa-lab.fr>
 <986041A4-19BE-4500-AD7E-4B2A32504D76@rstudio.com>
Message-ID: <22953.19594.227858.248280@stat.math.ethz.ch>

>>>>> Lionel Henry <lionel at rstudio.com>
>>>>>     on Fri, 1 Sep 2017 13:47:07 +0200 writes:

    > A package should probably never register a S3 method unless it owns
    > either the generic or the class.

I agree... (and typically it does "own" the class)

    > Here `formula.tools` owns neither.

i.e., it neither defines as.character() nor class "formula".

    > Instead of registering the method, it should export it like a regular
    > function. This way S3 dispatch is based on lexical scoping rather than
    > session-wide side effect.

I don't the 2nd sentence above is quite correct.  S3 method
registration should be done (in the case it should) and S3
dispatch is not just based on lexical scoping but also on S3
method registration.

    > Lionel

It is still the case that :: silently loads the namespace if
needed, and that "things may behave differently" after the use '::', because
loading a namespace does have an effect on the R session ...,
(and I still think  `::`  is much  "over used")

Martin



    >> On 1 sept. 2017, at 12:57, Simon Barthelm? <simon.barthelme at gipsa-lab.fr> wrote:
    >> 
    >> Dear list
    >> 
    >> I'm not sure whether this is a bug or an unavoidable consequence of the way packages are loaded, but there can be surprising side effects of calling a function via package::function. Here's an example using the formula.tools package:
    >> 
    >> form <- a ~ b
    >> as.character(form)
    >> formula.tools::lhs(form)
    >> as.character(form)
    >> 
    >> The first call to as.character returns:
    >> [1] "~" "a" "b"
    >> The second returns:
    >> [1] "a ~ b"
    >> 
    >> The reason being that formula.tools has:
    >> S3method(as.character,formula)
    >> in its namespace, which quietly supersedes the default one. In my case it led to a bug that was rather hard to track down because it looked like non-deterministic behaviour.
    >> Shouldn't there at least be a warning about such side effects, the way library() tells you about masking?
    >> 
    >> Best
    >> 
    >> Simon Barthelme
    >> 
    >> ______________________________________________
    >> R-devel at r-project.org mailing list
    >> https://stat.ethz.ch/mailman/listinfo/r-devel

    > ______________________________________________
    > R-devel at r-project.org mailing list
    > https://stat.ethz.ch/mailman/listinfo/r-devel



[Rd] side-effect of calling functions via `::`
Martin Maechler