gleam/os add timestamp function (#117)

Author: rvcas

CHANGELOG.md

@@ -9,6 +9,7 @@
 - The `result` module gains the `all` function.
 - The `dynamic` module gains the `option` function.
 - The `uri` module gains the `percent_encode` and `percent_decode` functions.
+- The `os` module gains the `erlang_timestamp` function.
 
 ## v0.11.0 - 2020-08-22
 

src/gleam/os.gleam

@@ -58,3 +58,9 @@ pub type TimeUnit {
 /// https://erlang.org/doc/apps/erts/time_correction.html#OS_System_Time
 pub external fn system_time(TimeUnit) -> Int =
   "os" "system_time"
+
+/// Return the current OS system time as a tuple of Ints
+///
+/// http://erlang.org/doc/man/os.html#timestamp-0
+pub external fn erlang_timestamp() -> tuple(Int, Int, Int) =
+  "os" "timestamp"

test/gleam/os_test.gleam

@@ -26,3 +26,15 @@ pub fn system_time_test() {
   { os.system_time(os.Millisecond) < june_12_2020 * 1000000 }
   |> should.equal(True)
 }
+
+pub fn erlang_timestamp_test() {
+  // in microseconds
+  let june_12_2020 = 1591966971000000
+  let tuple(mega_seconds, seconds, micro_seconds) = os.erlang_timestamp()
+
+  let stamp_as_micro =
+    { mega_seconds * 1_000_000 + seconds } * 1_000_000 + micro_seconds
+
+  { stamp_as_micro > june_12_2020 }
+  |> should.be_true()
+}