Add getDataDir proc (#21408)

* Add getDataDir()

* Update lib/std/private/osappdirs.nim

---------

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

lib/std/private/osappdirs.nim

@@ -12,6 +12,7 @@ proc getHomeDir*(): string {.rtl, extern: "nos$1",
   ## for the convenience of processing paths coming from user configuration files.
   ##
   ## See also:
+  ## * `getDataDir proc`_
   ## * `getConfigDir proc`_
   ## * `getTempDir proc`_
   ## * `expandTilde proc`_
@@ -24,6 +25,30 @@ proc getHomeDir*(): string {.rtl, extern: "nos$1",
   when defined(windows): return getEnv("USERPROFILE") & "\\"
   else: return getEnv("HOME") & "/"
 
+proc getDataDir*(): string {.rtl, extern: "nos$1"
+  tags: [ReadEnvEffect, ReadIOEffect].} =
+  ## Returns the data directory of the current user for applications.
+  ## 
+  ## On non-Windows OSs, this proc conforms to the XDG Base Directory
+  ## spec. Thus, this proc returns the value of the `XDG_DATA_HOME` environment
+  ## variable if it is set, otherwise it returns the default configuration
+  ## directory ("~/.local/share" or "~/Library/Application Support" on macOS).
+  ## 
+  ## See also:
+  ## * `getHomeDir proc`_
+  ## * `getConfigDir proc`_
+  ## * `getTempDir proc`_
+  ## * `expandTilde proc`_
+  ## * `getCurrentDir proc`_
+  ## * `setCurrentDir proc`_
+  when defined(windows):
+    result = getEnv("APPDATA")
+  elif defined(macosx):
+    result = getEnv("XDG_DATA_HOME", getEnv("HOME") / "Library" / "Application Support")
+  else:
+    result = getEnv("XDG_DATA_HOME", getEnv("HOME") / ".local" / "share")
+  result.normalizePathEnd(trailingSep = true)
+
 proc getConfigDir*(): string {.rtl, extern: "nos$1",
   tags: [ReadEnvEffect, ReadIOEffect].} =
   ## Returns the config directory of the current user for applications.
@@ -38,6 +63,7 @@ proc getConfigDir*(): string {.rtl, extern: "nos$1",
   ##
   ## See also:
   ## * `getHomeDir proc`_
+  ## * `getDataDir proc`_
   ## * `getTempDir proc`_
   ## * `expandTilde proc`_
   ## * `getCurrentDir proc`_
@@ -63,6 +89,7 @@ proc getCacheDir*(): string =
   ## * `getHomeDir proc`_
   ## * `getTempDir proc`_
   ## * `getConfigDir proc`_
+  ## * `getDataDir proc`_
   # follows https://crates.io/crates/platform-dirs
   when defined(windows):
     result = getEnv("LOCALAPPDATA")