3 files changed, 44 insertions, 1 deletions

diff --git a/src/gleam/dynamic.gleam b/src/gleam/dynamic.gleam
index 6b54df1..f1c8f2d 100644
--- a/src/gleam/dynamic.gleam
+++ b/src/gleam/dynamic.gleam
@@ -1,3 +1,4 @@
+import gleam/bit_string.{BitString}
 import gleam/list as list_mod
 import gleam/atom
 import gleam/map.{Map}
@@ -26,6 +27,20 @@ pub external fn from(a) -> Dynamic =
 pub external fn unsafe_coerce(Dynamic) -> a =
   "gleam_stdlib" "identity"
 
+/// Check to see whether a Dynamic value is a bit_string, and return the bit_string if
+/// it is.
+///
+/// ## Examples
+///
+///    > bit_string(from("Hello")) == bit_string.from_string("Hello")
+///    True
+///
+///    > bit_string(from(123))
+///    Error("Expected a BitString, got `123`")
+///
+pub external fn bit_string(from: Dynamic) -> Result(BitString, String) =
+  "gleam_stdlib" "decode_bit_string"
+
 /// Check to see whether a Dynamic value is a string, and return the string if
 /// it is.
 ///
diff --git a/src/gleam_stdlib.erl b/src/gleam_stdlib.erl
index a5dd804..fc53294 100644
--- a/src/gleam_stdlib.erl
+++ b/src/gleam_stdlib.erl
@@ -9,7 +9,8 @@
          decode_element/2, parse_int/1, parse_float/1, compare_strings/2,
          string_pop_grapheme/1, string_starts_with/2, string_ends_with/2,
          string_pad/4, decode_tuple2/1, decode_map/1, bit_string_int_to_u32/1,
-         bit_string_int_from_u32/1, bit_string_append/2, bit_string_part_/3]).
+         bit_string_int_from_u32/1, bit_string_append/2, bit_string_part_/3,
+         decode_bit_string/1]).
 
 should_equal(Actual, Expected) -> ?assertEqual(Expected, Actual).
 should_not_equal(Actual, Expected) -> ?assertNotEqual(Expected, Actual).
@@ -60,6 +61,9 @@ decode_map(Data) -> decode_error_msg("a map", Data).
 decode_atom(Data) when is_atom(Data) -> {ok, Data};
 decode_atom(Data) -> decode_error_msg("an atom", Data).
 
+decode_bit_string(Data) when is_binary(Data) -> {ok, Data};
+decode_bit_string(Data) -> decode_error_msg("a bit_string", Data).
+
 decode_string(Data) when is_binary(Data) -> {ok, Data};
 decode_string(Data) -> decode_error_msg("a string", Data).
 
diff --git a/test/gleam/dynamic_test.gleam b/test/gleam/dynamic_test.gleam
index 410ae0e..8a43f69 100644
--- a/test/gleam/dynamic_test.gleam
+++ b/test/gleam/dynamic_test.gleam
@@ -1,3 +1,4 @@
+import gleam/bit_string
 import gleam/dynamic
 import gleam/atom
 import gleam/list
@@ -5,6 +6,29 @@ import gleam/should
 import gleam/result
 import gleam/map
 
+
+pub fn bit_string_test() {
+  ""
+  |> dynamic.from
+  |> dynamic.bit_string
+  |> should.equal(Ok(bit_string.from_string("")))
+
+  "Hello"
+  |> dynamic.from
+  |> dynamic.bit_string
+  |> should.equal(Ok(bit_string.from_string("Hello")))
+
+  1
+  |> dynamic.from
+  |> dynamic.bit_string
+  |> should.equal(Error("Expected a bit_string, got an int"))
+
+  []
+  |> dynamic.from
+  |> dynamic.bit_string
+  |> should.equal(Error("Expected a bit_string, got a list"))
+}
+
 pub fn string_test() {
   ""
   |> dynamic.from