purrr 0.2.0

New functions

• There are two handy infix functions:
  • x %||% y is shorthand for if (is.null(x)) y else x (#109).
  • x %@% "a" is shorthand for attr(x, "a", exact = TRUE) (#69).
• accumulate() has been added to handle recursive folding. It is shortand
  for Reduce(f, .x, accumulate = TRUE) and follows a similar syntax to
  reduce() (#145). A right-hand version accumulate_right() was also added.
• map_df() row-binds output together. It's the equivalent of plyr::ldply()
  (#127)
• flatten() is now type-stable and always returns a list. To return a simpler
  vector, use flatten_lgl(), flatten_int(), flatten_dbl(),
  flatten_chr(), or flatten_df().
• invoke() has been overhauled to be more useful: it now works similarly
  to map_call() when .x is NULL, and hence map_call() has been
  deprecated. invoke_map() is a vectorised complement to invoke() (#125),
  and comes with typed variants invoke_map_lgl(), invoke_map_int(),
  invoke_map_dbl(), invoke_map_chr(), and invoke_map_df().
• transpose() replaces zip2(), zip3(), and zip_n() (#128).
  The name more clearly reflects the intent (transposing the first and second
  levels of list). It no longer has fields argument or the .simplify argument;
  instead use the new simplify_all() function.
• safely(), quietly(), and possibly() are experimental functions
  for working with functions with side-effects (e.g. printed output,
  messages, warnings, and errors) (#120). safely() is a version of try()
  that modifies a function (rather than an expression), and always returns a
  list with two components, result and error.
• list_along() and rep_along() generalise the idea of seq_along().
  (#122).
• is_null() is the snake-case version of is.null().
• pmap() (parallel map) replaces map_n() (#132), and has typed-variants
  suffixed pmap_lgl(), pmap_int(), pmap_dbl(), pmap_chr(), and
  pmap_df().
• set_names() is a snake-case alternative to setNames() with stricter
  equality checking, and more convenient defaults for pipes:
  x %>% set_names() is equivalent to setNames(x, x) (#119).

Row based functionals

We are still figuring out what belongs in dplyr and what belongs in purrr. Expect much experimentation and many changes with these functions.

• map() now always returns a list. Data frame support has been moved
  to map_df() and dmap(). The latter supports sliced data frames
  as a shortcut for the combination of by_slice() and dmap():
  x %>% by_slice(dmap, fun, .collate = "rows"). The conditional
  variants dmap_at() and dmap_if() also support sliced data frames
  and will recycle scalar results to the slice size.
• map_rows() has been renamed to invoke_rows(). As other
  rows-based functionals, it collates results inside lists by default,
  but with column collation this function is equivalent to
  plyr::mdply().
• The rows-based functionals gain a .to option to name the output
  column as well as a .collate argument. The latter allows to
  collate the output in lists (by default), on columns or on
  rows. This makes these functions more flexible and more predictable.

Bug fixes and minor changes

• as_function(), which converts formulas etc to functions, is now
  exported (#123).
• rerun() is correctly scoped (#95)
• update_list() can now modify an element called x (#98).
• map*() now use custom C code, rather than relying on lapply(), mapply()
  etc. The performance characteristcs are very similar, but it allows us greater
  control over the output (#118).
• map_lgl() now has second argument .f, not .p (#134).

Deprecated functions

• flatmap() -> use map() followed by the appropriate flatten().
• map_call() -> invoke().
• map_n() -> pmap(); walk_n() -> pwalk().
• map3(x, y, z) -> map_n(list(x, y, z)); walk3(x, y, z) ->pwalk(list(x, y, z))`