unreachable_code

Added in 0.4.0

What it does

Detects code that can never be executed because it appears after control flow statements like return, break, or next, or in branches that cannot be reached.

Why is this bad?

Unreachable code indicates a logic error or dead code that should be removed. It clutters the codebase, confuses readers, and may indicate unintended behavior.

R Markdown and Quarto

The chunks of an .Rmd/.qmd document run one after another in a single R session, so a stop() in one chunk does make the code in the chunks after it unreachable. This is not the case if at least one the following two options are specified:

eval = FALSE (or #| eval: false), because the chunk never runs at all;
error = TRUE (or #| error: true), because the chunk prints the error but continues to evaluate the subsequent chunks.

Such a chunk is left out of the analysis entirely, so the following also wouldn’t be reported:

```
#| eval: false
stop("a")
1 + 1 # unreachable but not reported
```

Note that unreachable code not at the top-level would still be reported, e.g.:

```
#| eval: false
f <- function() {
  stop("a")
  1 + 1 # reported as unreachable
}
```

A chunk whose evaluation is decided at render time (e.g. #| eval: run_it) is considered evaluated.

Configuration options

The operation of the unreachable_code rule can be customised in the configuration file.

Use stopping-functions to fully replace the default list of functions that are considered to stop execution (never return). Use extend-stopping-functions to add to the default list. Specifying both is an error.

Function names in stopping-functions or extend-stopping-functions also match namespaced calls, e.g. stopping-functions = ["abort"] will consider abort() and rlang::abort() as stopping functions.

Default values

stopping-functions = [
    "stop",
    ".Defunct",
    "abort",
    "cli_abort",
    "q",
    "quit"
]

TOML settings

[lint]
...

[lint.unreachable_code]
# Add a custom function to the list of stopping functions
extend-stopping-functions = ["my_custom_stop"]

Examples

if (x > 5) {
  stop("hi")
} else {
  stop("bye")
}
1 + 1 # unreachable

foo <- function(x) {
  return(x + 1)
  print("hi")  # unreachable
}

foo <- function(x) {
  for (i in 1:10) {
    x <- x + 1
    if (x > 10) {
       break
       print("x is greater than 10") # unreachable
    }
  }
}