R, shiny and source()

This article is originally published at https://logfc.wordpress.com

This one cost me more time to figure out than it should have. The reason being, it turns out that I never properly understood what the source() function does.

So here is the story: I was setting up a shiny server for a student based on her code. She was running the shiny app from within RDesktop, and so before starting the app with runApp() she would load all necessary object and source() a file called helpers.R with some common calculations.

In order to put the app on a server, I have moved these pre-runApp() initializations into ui.R and server.R. Suddenly, weird errors appeared. The functions in the helpers.R no longer seemed to be able to find anything in the parent environment — object X not found! Even though I called source() immediately after loading the necessary objects into the environment:

# file server.R
load("myobjects.rda")
source("helpers.R")

The solution was, as usual, to read documentation. Specifically, documentation on source():

local   TRUE, FALSE or an environment, determining where the 
        parsed expressions are evaluated. FALSE (the default) 
        corresponds to the user's workspace (the global 
        environment) and TRUE to the environment from which 
        source is called.

The objects which I have load()-ed before were not in the global environment, but instead in another environment created by shiny. However, the expressions from helpers.R were evaluated in the global environment. Thus, a new function defined in helpers.R could be seen from inside server.R, but an object loaded from server.R could not be seen by helpers.R.

It is the first time that I have noticed this. Normally, I would use a file such as helpers.R only to define helper functions, and actually call them from server.R or ui.R. However, I was thinking that source() is something like #include in C, simply calling the commands in the given file as if they were inserted at this position into the code — or called from the environment from which source() was called.

This is not so.

Thanks for visiting r-craft.org
This article is originally published at https://logfc.wordpress.com
Please visit source website for post related comments.