modify.md
In purrr: Functional Programming Tools

modfiying data.frame preserves type and size

Code
  modify(df1, ~ integer())
Condition
  Error in `modify()`:
  ! Can't recycle `out$x` (size 0) to size 2.
Code
  modify(df1, ~ 1:4)
Condition
  Error in `modify()`:
  ! Can't recycle `out$x` (size 4) to size 2.
Code
  modify_at(df1, 2, ~ integer())
Condition
  Error in `modify_where()`:
  ! Can't recycle `out$y` (size 0) to size 2.
Code
  modify2(df1, list(1, 1:3), ~.y)
Condition
  Error in `modify2()`:
  ! Can't recycle `out$y` (size 3) to size 2.

zap gives clear error

Code
  modify_at(1, 1, ~ zap())
Condition
  Error in `map_vec()`:
  ! `out[[1]]` must be a vector, not a <rlang_zap> object.
Code
  modify_at(list(1), 1, ~ zap())
Condition
  Error in `modify_at()`:
  ! Can't use `zap()` to change the size of the output.
Code
  modify_at(data.frame(x = 1), 1, ~ zap())
Condition
  Error in `modify_at()`:
  ! Can't use `zap()` to change the size of the output.
Code
  modify_at(lm(mpg ~ wt, data = mtcars), 1, ~ zap())
Condition
  Error in `modify_at()`:
  ! Can't use `zap()` to change the size of the output.

bad type has useful error

Code
  modify(1:3, ~"foo")
Condition
  Error in `map_vec()`:
  ! Can't convert `<list>[[1]]` <character> to <integer>.
Code
  modify_at(1:3, 1, ~"foo")
Condition
  Error in `map_vec()`:
  ! Can't convert `<list>[[1]]` <character> to <integer>.
Code
  modify_if(1:3, is_integer, ~"foo")
Condition
  Error in `map_vec()`:
  ! Can't convert `<list>[[1]]` <character> to <integer>.
Code
  modify2(1:3, "foo", ~.y)
Condition
  Error in `map2_vec()`:
  ! Can't convert `<list>[[1]]` <character> to <integer>.

modify2() recycles arguments

Code
  modify2(1:3, integer(), `+`)
Condition
  Error in `map2()`:
  ! Can't recycle `.x` (size 3) to match `.y` (size 0).
Code
  modify2(1:3, 1:4, `+`)
Condition
  Error in `map2()`:
  ! Can't recycle `.x` (size 3) to match `.y` (size 4).

modify_if() requires predicate functions

Code
  modify_if(list(1, 2), ~NA, ~"foo")
Condition
  Error in `modify_if()`:
  i In index: 1.
  Caused by error:
  ! `.p()` must return a single `TRUE` or `FALSE`, not `NA`.

user friendly error for non-supported cases

Code
  modify(mean, identity)
Condition
  Error in `modify()`:
  ! `.x` must be a vector, list, or data frame, not a function.
Code
  modify_if(mean, TRUE, identity)
Condition
  Error in `modify_if()`:
  ! `.x` must be a vector, list, or data frame, not a function.
Code
  modify_at(mean, "x", identity)
Condition
  Error in `modify_at()`:
  ! `.x` must be a vector, list, or data frame, not a function.
Code
  modify2(mean, 1, identity)
Condition
  Error in `modify2()`:
  ! `.x` must be a vector, list, or data frame, not a function.