R namespace access and match.fun

Question

I'm working on an R package where one of the functions contains a `match.fun` call to a function in a package that's imported to the package namespace. But on loading the package, the `match.fun` call can't find the function name. From Hadley Wickham's description I think I'm doing everything right, but this is clearly not the case. Example: ``` # in the package file header, for creation of the NAMESPACE via roxygen2: ##` @import topicmodels # The function declaration in the package ModelTopics <- function(doc.term.mat, num.topics, topic.method="LDA"){ topic.fun <- match.fun(topic.method) output <- topic.fun(doc.term.mat, k=num.topics) return(output) } ``` And then in R: ``` > library(mypackage) > sample.output <- ModelTopics(my.dtm, topic.method="LDA", num.topics=5) Error in get(as.character(FUN), mode = "function", envir = envir) : object 'LDA' of mode 'function' was not found ``` From my understanding of namespaces, the `match.fun` call should have access to the package namespace,…

Accepted Answer

I believe this an issue of scope. Although you have imported `topicmodels` and thus `LDA`, you haven't exported these functions, so they aren't available in the search path.

From `?match.fun`:

`match.fun` is not intended to be used at the top level since it will perform matching in the parent of the caller.

Thus, since you are using `ModelTopics` in the global environment, and `LDA` isn't available in the global environment, the `match.fun` call fails.

It seems to me you have two options:

Option 1: use `get`

An alternative would be to use `get` where you can specify the environment. Consider this: try to use `match.fun` to find `print.ggplot` in the package `ggplot2`:

```
match.fun("print.ggplot")
Error in get(as.character(FUN), mode = "function", envir = envir) : 
  object 'print.ggplot' of mode 'function' was not found
```

Since `print.ggplot` isn't exported, `match.fun` can't find it.

However, `get` does find it:

```
get("print.ggplot", envir=environment(ggplot))
function (x, newpage = is.null(vp), vp = NULL, ...) 
{
    set_last_plot(x)
    if (newpage) 
        grid.newpage()
    data <- ggplot_build(x)
    gtable <- ggplot_gtable(data)
    if (is.null(vp)) {
        grid.draw(gtable)
    }
    else {
        if (is.character(vp)) 
            seekViewport(vp)
        else pushViewport(vp)
        grid.draw(gtable)
        upViewport()
    }
    invisible(data)
}
<environment: namespace:ggplot2>
```

Option 2: Export the functions from `topicmodels` that you need

If you make the necessary functions from `topicmodels` available in your NAMESPACE, your code should also work.

Do this by either:

- Selective exporting `LDA` and other functions to the namespace using `@export`

- Declare a dependency, using `Depends: topicmodels` - this is the same as calling `library(topicmodels)` in the global environment. This will work, but is possibly a bit of a brute force approach. It also means that any subsequent `library` call can mask your `LDA` function, thus leading to unexpected results.

R namespace access and match.fun

Solution

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