Yihui Xie

You Should Rarely Need eval(parse())

Yihui Xie / 2023-02-07

Many programming languages have a function to evaluate a string as code or an expression, e.g., eval() in R. Once you learn it, it becomes tempting to abuse it. Instead of writing code naturally and directly, you start to think about writing code that constructs code to be evaluated. For example, instead of writing 1 + 1, you may write a monster like this:

eval(parse(text = paste(c('1', '1'), collapse = '+')))

This is obviously just a silly example, but I have seen people abuse eval() in R many times in the past, often due to the lack of understanding of a function. One example is a recent Stack Overflow question. The OP wants to include a series of figures via knitr::include_graphics(), but didn’t realize that this function can just take a vector of figure paths, and then the OP wrote code to construct code consisting of a series of knitr::include_graphics() calls.

This way of thinking not only often leads to clumsy code, but also brings security risks when you, as a developer, allow users to input any string to be evaluated.

Of course, eval(parse()) has its great value and I’m not saying it should be banned. For example, the whole knitr package is built on top of it (for evaluating code chunks as code strings). However, I feel it’s almost surely the wrong way to go in your daily programming job. There should be a better way.