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Scope

This article is about R package development.

Motivation

As stated in the the Writing R Extensions manual and the Software for Data Analysis book (aka the R bible), packages should whenever possible use Imports instead of Depends, to avoid name collision (masking) and ensure trustworthy computations. See this discussion on Stackoverflow.

The problem

So after adopting best practices, and only using Imports in my packages, the problem is that some imported packages do not follow those best practices and rely on Depends for their dependencies, and consequently will not find their dependencies unless hacked.

Say we are writing a package MyPkg, that uses some functions from a CRAN package A, which lists a CRAN package B in its Depends.

MyPkg::f

f <- function() a()

A::a

a <- function() b()

B::b b <- function() { ... }

Executing f() will find the function a, explicitly imported from package A. But executing a() will die because function b can not be found.

analysis

Because B is listed in the Depends of package A, b() is normally found in the search path, where namespace B is attached. What can we do to work around this problem ?
Sadly, there is nothing we can do at the MyPkg package level, since the problem sits in the A namespace. We could of course lists B in the MyPkg Depends, but that is precisely what we want to avoid.

the dirty hack solution

the idea

I suggest that you first have a look at this very good blog How R Searches and Finds Stuff if you need some clarification about namespaces and imports.

Here is a diagram of the current state of the problem:

We want the a() call, which is executed in namespace A, to find the b symbol in namespace B. To achieve this, we will re-route the namespace A (actually its Imports namespace) to the B namespace:

nsa <- getNamespace('A')                      # package A environment
nsa_imp <- parent.env(nsa)                    # package A Imports
parent.env(nsa_imp) <- getNamespace('B')      # re-route A Imports to package B

Now our namespaces should look like this:

Notice that the link From A Imports to base has been re-routed towards package B (the arrow labelled "New")

Example

The above code is not straightforward to test because you need to write a package.

Here is some reproducible code that illustrates how the hack works, so that you can simulate its mechanism right now by copy-pasting the following code in your R console. This example uses package multcomp that Depends on mvtnorm.

## load multcomp package, and its imports, but do not attach them in the
## search path
l <- loadNamespace("multcomp")  # assignment to avoid annoying print

amod <- aov(breaks ~ wool + tension, data = warpbreaks)
wht <- multcomp::glht(amod, linfct = multcomp::mcp(tension = "Tukey"))
ci <- confint(wht)  # mvtnorm::qmvt is not found, because mvtnorm is not attached to the search path

# let's load the mvtnorm namespace
l <- loadNamespace("mvtnorm")
ci <- confint(wht)  # mvtnorm::qmvt still not found, mvtnorm is loaded but not attached

# hack hack hack: re-route package multcomp to mvtnorm
ns1 <- getNamespace("multcomp")
ns1_imp <- parent.env(ns1)
parent.env(ns1_imp) <- getNamespace("mvtnorm")

ci <- confint(wht)  # now should work
print(ci)

## 
##   Simultaneous Confidence Intervals
## 
## Multiple Comparisons of Means: Tukey Contrasts
## 
## 
## Fit: aov(formula = breaks ~ wool + tension, data = warpbreaks)
## 
## Quantile = 2.415
## 95% family-wise confidence level
##  
## 
## Linear Hypotheses:
##            Estimate lwr     upr    
## M - L == 0 -10.000  -19.354  -0.646
## H - L == 0 -14.722  -24.076  -5.369
## H - M == 0  -4.722  -14.076   4.631

implementation of the hack in MyPkg

Just put the re-routing code in your MyPkg .onLoad function, defined usually in R/zzz.R:

.onLoad <- function(libname, pkgname) {
    nsa <- getNamespace('A')
    nsa_imp <- parent.env(nsa)
    parent.env(nsa_imp) <- getNamespace('B')    
}

why dirty ?

First of all, it uses parent.env<-, and according to the documentation:

The replacement function parent.env<- is extremely dangerous as it can be used to destructively change environments in ways >that violate assumptions made by the internal C code. It may be removed in the near future.

It is a pity because it can be extremely useful.

why a hack ?

Unfortunately it does not work in the general case. Suppose you need two packages A and D, that both depend on B and C, but with different rules:

A -> B -> C
D -> C -> B

In that case, we would re-route A to B, then B to C. And this is incompatible with the way D is designed, resulting in wrong symbol picks if symbols with same name are both defined by B and C.

so what ?

We'll see in a future post a probably much better solution.

Karl Forner @ Quartz Bio

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