Add a path argument to path_abs

Fixes #87

NEWS.md

@@ -1,5 +1,8 @@
 # fs 1.2.1
 
+* `path_abs()` gains a `start` argument, to specify where the absolute path
+  should be calculated from (#87).
+
 * Fix for a memory issue reported by ASAN and valgrind.
 
 # fs 1.2.0

R/path.R

@@ -120,14 +120,17 @@ path_join <- function(parts) {
 
 #' @describeIn path_math returns a normalized, absolute version of a path.
 #' @export
-path_abs <- function(path) {
+path_abs <- function(path, start = ".") {
+  if (!is_absolute_path(start)) {
+    start <- path_norm(path(getwd(), start))
+  }
   is_abs <- is_absolute_path(path)
   path[is_abs] <- path_norm(path[is_abs])
-  cwd <- getwd()
-  path[!is_abs] <- path_norm(path(cwd, path[!is_abs]))
+  path[!is_abs] <- path_norm(path(start, path[!is_abs]))
   path_tidy(path)
 }
 
+
 #' @describeIn path_math collapses redundant separators and
 #' up-level references, so `A//B`, `A/B`, `A/.B` and `A/foo/../B` all become
 #' `A/B`. If one of the paths is a symbolic link, this may change the meaning
@@ -170,7 +173,7 @@ path_norm <- function(path) {
 #' @describeIn path_math computes the path relative to the `start` path,
 #'   which can be either a absolute or relative path.
 #' @export
-#' @param start A starting directory to compute relative path to.
+#' @param start A starting directory to compute the path to.
 # This implementation is partially derived from
 # https://github.com/python/cpython/blob/9c99fd163d5ca9bcc0b7ddd0d1e3b8717a63237c/Lib/posixpath.py#L446
 path_rel <- function(path, start = ".") {

tests/testthat/test-path.R

@@ -269,7 +269,7 @@ describe("path_common", {
 
 # derived from https://github.com/python/cpython/blob/6f0eb93183519024cb360162bdd81b9faec97ba6/Lib/test/test_posixpath.py#L483
 describe("path_rel", {
-  it("works for posix paths", {
+  it("works for POSIX paths", {
     cur_dir <- path_file(getwd())
     expect_equal(path_rel("a"), "a")
     expect_equal(path_rel(path_abs("a")), "a")
@@ -308,6 +308,12 @@ describe("path_rel", {
   })
 })
 
+describe("path_abs", {
+  it("uses the start parameter", {
+    expect_equal(path_abs("b", "a"), path_abs("a/b"))
+  })
+})
+
 describe("path_home", {
   # The trailing slash is needed to ensure we get path expansion
   # on POSIX systems when readline support is not built in. (#60)