diff --git a/docs/writeups/0013-calling-rust-functions.md b/docs/writeups/0013-calling-rust-functions.md
new file mode 100644
index 0000000..6a435a1
--- /dev/null
+++ b/docs/writeups/0013-calling-rust-functions.md
@@ -0,0 +1,20 @@
+# WU0013: Calling Rust functions
+
+## Syntax experimentation
+
+```rust
+fn foo() -> i32 { 15 }
+fn bar(a: i32, c: i32) -> i32 { a + c }
+```
+
+```rust
+where rust = source("rust");
+where i32 = rust.i32;
+where Fn = rust.Fn;
+
+where foo_type = rust.fn("foo", return_type: i32).build();
+where bar_type = rust.fn("bar", args: (i32, i32), return_type: i32).build();
+
+where foo_type = Fn[(), i32](name: "foo");
+where bar_type = Fn[(i32, i32), i32](name: "bar");
+```