switch part to return result

3 files changed, 38 insertions, 12 deletions

diff --git a/src/gleam/binary.gleam b/src/gleam/binary.gleam
index a45ae5d..c36b941 100644
--- a/src/gleam/binary.gleam
+++ b/src/gleam/binary.gleam
@@ -7,20 +7,25 @@ pub external type Binary
 pub external fn from_string(String) -> Binary =
   "gleam_stdlib" "identity"
 
+/// Returns an integer which is the number of bytes in the binary.
+pub external fn byte_size(Binary) -> Int =
+"erlang" "byte_size"
+
 /// Extracts the part of a binary.
 ///
 /// Binary part will start at given position and continue up to specified length.
 /// A negative length can be used to extract bytes at the end of a binary:
-pub external fn part(string: Binary, position: Int, length: Int) -> Binary =
-  "binary" "part"
-
-/// Returns an integer which is the number of bytes in the binary.
-pub external fn byte_size(Binary) -> Int =
-  "erlang" "byte_size"
+pub external fn part(string: Binary, position: Int, length: Int) -> Result(Binary, Nil) =
+"binary_native" "part"
 
-// Not sure these will be necessary with bit syntax for for now I think they are
+/// Convert integer to unsigned 32 bits.
+///
+/// Returns an error if integer is less than zero or equal to or larger than 2^32.
 pub external fn int_to_u32(Int) -> Result(Binary, Nil) =
   "binary_native" "int_to_u32"
 
+/// Convert unsigned 32 bits to Integer
+///
+/// Returns an error if the binary is not 32 bits in length.
 pub external fn int_from_u32(Binary) -> Result(Int, Nil) =
   "binary_native" "int_from_u32"
diff --git a/src/gleam/binary_native.erl b/src/gleam/binary_native.erl
index d7e774c..1969602 100644
--- a/src/gleam/binary_native.erl
+++ b/src/gleam/binary_native.erl
@@ -1,5 +1,10 @@
 -module (binary_native).
--export ([int_to_u32/1, int_from_u32/1]).
+-export ([int_to_u32/1, int_from_u32/1, part/3]).
+
+part(Bin, Pos, Len) ->
+  try {ok, binary:part(Bin, Pos, Len)} catch
+    error:badarg -> {error, nil}
+  end.
 
 int_to_u32(I) when 0 =< I, I < 4294967296 ->
   {ok, <<I:32>>};
diff --git a/test/gleam/binary_test.gleam b/test/gleam/binary_test.gleam
index cf56076..20b7cc2 100644
--- a/test/gleam/binary_test.gleam
+++ b/test/gleam/binary_test.gleam
@@ -12,19 +12,35 @@ pub fn length_test() {
 pub fn part_test() {
   binary.from_string("hello")
   |> binary.part(0, 5)
-  |> should.equal(binary.from_string("hello"))
+  |> should.equal(Ok(binary.from_string("hello")))
 
   binary.from_string("hello")
   |> binary.part(0, 0)
-  |> should.equal(binary.from_string(""))
+  |> should.equal(Ok(binary.from_string("")))
 
   binary.from_string("hello")
   |> binary.part(2, 2)
-  |> should.equal(binary.from_string("ll"))
+  |> should.equal(Ok(binary.from_string("ll")))
 
   binary.from_string("hello")
   |> binary.part(5, -2)
-  |> should.equal(binary.from_string("lo"))
+  |> should.equal(Ok(binary.from_string("lo")))
+
+  binary.from_string("")
+  |> binary.part(0, 0)
+  |> should.equal(Ok(binary.from_string("")))
+
+  binary.from_string("hello")
+  |> binary.part(6, 0)
+  |> should.equal(Error(Nil))
+
+  binary.from_string("hello")
+  |> binary.part(-1, 1)
+  |> should.equal(Error(Nil))
+
+  binary.from_string("hello")
+  |> binary.part(1, 6)
+  |> should.equal(Error(Nil))
 }
 
 pub fn u32_test() {