pipe into return doesn't work as expected

PejoPhylo 0
Viewed 40

The following chunks of code should be doing exactly the same thing, but the first chunk unexpectedly prints "We reach here?" also when x = 2, which shouldn't be the case (I hope it's obviosu what I mean).

Chunk 1

library(tidyverse)
input <- c(1,2,3)
output <- map(input, function(x) {
  print(x)
  if (x == 2) {
    return(x)
  }
  print('We reach here?')
  return(x)
})

Output

This output is as expected, as the string doesn't print when x = 2.

[1] 1
[1] "We reach here?"
[1] 2
[1] 3
[1] "We reach here?"

Chunk 2

input <- c(1,2,3)
output <- map(input, function(x) {
  print(x)
  if (x == 2) {
    x %>% return() 
  }
  print('We reach here?')
  x %>% return()
})

Output

[1] 1
[1] "We reach here?"
[1] 2
[1] "We reach here?"
[1] 3
[1] "We reach here?"

What am I missing here? Is this a bug?

r
1 Answers

The return() function jumps up a level. But the magrittr pipe %>% pushes evaluation down a level using some tricky evaluation. So piping into return() jumps back to where it started, instead of up a level from the function where you wrote it.

I would have expected the base pipe x |> return() to work, because it really should be identical to return(x) (whereas the magrittr pipe is only a partial simulation of that code). However, it is not allowed for some reason.

Is this a bug? It's a limitation of the magrittr pipe for sure. I'm not sure if the magrittr documentation discusses it. (It does; see the edit below!) So avoid putting return() in a pipe, and just use

return(x)

Edited to add:

I looked carefully through the magrittr docs, and found that this is discussed in the "tradeoffs" vignette. They give alternative versions of the pipe, %!>% with "eager" semantics and %|>% with "nested" semantics to handle this. (The %|> pipe doesn't appear to be in magrittr currently, but it should match the base pipe fairly closely.)

input <- c(1,2,3)
output <- map(input, function(x) {
  print(x)
  if (x == 2) {
    x %!>% return() 
  }
  print('We reach here?')
  x %!>% return()
})

This prints the same output as your Chunk 1.

user25543300
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