Add a document about optional arg (#247)

book/src/SUMMARY.md

@@ -12,7 +12,8 @@
 - [`#[savvy]` macro](./savvy_macro.md)
 - [Handling Vector Input](./input.md)
 - [Handling Vector Output](./output.md)
-- [Handling Scalars](./scalar.md)
+- [Handling Scalar](./scalar.md)
+- [Optional Argument](./optional_arg.md)
 - [Type-specific Topics](./type-overview.md)
     - [Integer, Real, String, Logical, And Complex](./atomic_types.md)
     - [List](./list.md)

book/src/optional_arg.md

@@ -0,0 +1,31 @@
+# Optional Argument
+
+To represent an optional argument, you can wrap it with `Option`. Then, the
+corresponding R function sets the default value of `NULL` on the argument.
+
+``` rust
+#[savvy]
+fn default_value_vec(x: Option<IntegerSexp>) -> savvy::Result<Sexp> {
+    if let Some(x) = x {
+        x.iter().sum::<i32>().try_into()
+    } else {
+        (-1).try_into()
+    }
+}
+```
+
+``` r
+function(x = NULL) {
+  .Call(savvy_default_value_vec__impl, x)
+}
+```
+
+This function works with or without the argument.
+
+``` r
+default_value_vec(1:10)
+#> [1] 55
+
+default_value_vec()
+#> [1] -1
+```

book/src/savvy_macro.md

@@ -42,6 +42,7 @@ function must satisfy the following conditions:
     * a corresponding Rust type for scalar (e.g., `i32` and `f64`)
     * a user-defined struct marked with `#[savvy]` (`&T`, `&mut T`, or `T`)
     * a user-defined enum marked with `#[savvy]` (`&T`, or `T`)
+    * any of above wrapped with `Option` (this is translated as an optional arg)
 * The function's return value must be either
     * `savvy::Result<()>` for the case of no actual return value
     * `savvy::Result<savvy::Sexp>` for the case of some return value of R object

book/src/scalar.md

@@ -1,4 +1,4 @@
-# Handling Salars
+# Handling Scalar
 
 ## Input