Bug: accumulate2 returns a list, not a vector

[jonathan-g]

The documentation for accumulate and accumulate2 says that both functions return vectors ("A vector the same length of .x with the same names as .x"). In fact, accumulate returns a vector, but accumulate2 returns a list.

Expected behavior

accumulate2 returns a vector

Observed behavior

accumulate2 returns a list

Reprex

library(purrr)

f <- function(x, y, z) {
  x + y + z
}

accumulate(seq(5), f, 1)
#> [1]  1  4  8 13 19

accumulate2(seq(5), seq(4), f)
#> [[1]]
#> [1] 1
#> 
#> [[2]]
#> [1] 4
#> 
#> [[3]]
#> [1] 9
#> 
#> [[4]]
#> [1] 16
#> 
#> [[5]]
#> [1] 25

^{Created on 2020-07-10 by the reprex package (v0.3.0)}

Session Info

> sessionInfo()
R version 4.0.2 (2020-06-22)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 18363)

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252 LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] reprex_0.3.0

loaded via a namespace (and not attached):
 [1] ps_1.3.3               digest_0.6.25          packrat_0.5.0          crayon_1.3.4           R6_2.4.1               lifecycle_0.2.0       
 [7] magrittr_1.5           evaluate_0.14          pillar_1.4.4           rlang_0.4.6            rstudioapi_0.11.0-9000 fs_1.4.2              
[13] callr_3.4.3            whisker_0.4            vctrs_0.3.1            ellipsis_0.3.1         rmarkdown_2.3.2.9002   tools_4.0.2           
[19] purrr_0.3.4            processx_3.4.3         xfun_0.15              compiler_4.0.2         pkgconfig_2.0.3        clipr_0.7.0           
[25] htmltools_0.5.0        knitr_1.29             tibble_3.0.2          

[lionel-]

It's actually the intended behaviour because accumulate() should have returned a list all along. But since we're unlikely to ever make such a breaking change, it's probably better to make accumulate2() consistent with accumulate() instead.