diff --git a/README.md b/README.md
index 57ebafb..1942da3 100644
--- a/README.md
+++ b/README.md
@@ -13,21 +13,35 @@ The idea is that you define an expression in another Rust crate with a proc_macr
 That macro can have the following attributes:
 
 - `output_type` -> to define the output type of that expression
-- `type_func` -> to define a function that computes the output type based on input types.
+- `output_type_func` -> to define a function that computes the output type based on input types.
 
 Here is an example of a `String` conversion expression that converts any string to [pig latin](https://en.wikipedia.org/wiki/Pig_Latin):
 
 ```rust
-fn pig_latin_str(value: &str, output: &mut String) {
+fn pig_latin_str(value: &str, capitalize: bool, output: &mut String) {
     if let Some(first_char) = value.chars().next() {
-        write!(output, "{}{}ay", &value[1..], first_char).unwrap()
+        if capitalize {
+            for c in value.chars().skip(1).map(|char| char.to_uppercase()) {
+                write!(output, "{c}").unwrap()
+            }
+            write!(output, "AY").unwrap()
+        } else {
+            let offset = first_char.len_utf8();
+            write!(output, "{}{}ay", &value[offset..], first_char).unwrap()
+        }
     }
 }
 
+#[derive(Deserialize)]
+struct PigLatinKwargs {
+    capitalize: bool,
+}
+
 #[polars_expr(output_type=Utf8)]
-fn pig_latinnify(inputs: &[Series]) -> PolarsResult<Series> {
+fn pig_latinnify(inputs: &[Series], kwargs: PigLatinKwargs) -> PolarsResult<Series> {
     let ca = inputs[0].utf8()?;
-    let out: Utf8Chunked = ca.apply_to_buffer(pig_latin_str);
+    let out: Utf8Chunked =
+        ca.apply_to_buffer(|value, output| pig_latin_str(value, kwargs.capitalize, output));
     Ok(out.into_series())
 }
 ```
@@ -46,11 +60,12 @@ class Language:
     def __init__(self, expr: pl.Expr):
         self._expr = expr
 
-    def pig_latinnify(self) -> pl.Expr:
+    def pig_latinnify(self, capatilize: bool = False) -> pl.Expr:
         return self._expr._register_plugin(
             lib=lib,
             symbol="pig_latinnify",
             is_elementwise=True,
+            kwargs={"capitalize": capatilize}
         )
 ```
 
@@ -58,7 +73,7 @@ Compile/ship and then it is ready to use:
 
 ```python
 import polars as pl
-from expression_lib import Language
+import expression_lib
 
 df = pl.DataFrame({
     "names": ["Richard", "Alice", "Bob"],
diff --git a/example/derive_expression/expression_lib/expression_lib/__init__.py b/example/derive_expression/expression_lib/expression_lib/__init__.py
index 9b3be74..58b13eb 100644
--- a/example/derive_expression/expression_lib/expression_lib/__init__.py
+++ b/example/derive_expression/expression_lib/expression_lib/__init__.py
@@ -10,11 +10,12 @@ class Language:
     def __init__(self, expr: pl.Expr):
         self._expr = expr
 
-    def pig_latinnify(self) -> pl.Expr:
+    def pig_latinnify(self, capitalize: bool = False) -> pl.Expr:
         return self._expr._register_plugin(
             lib=lib,
             symbol="pig_latinnify",
             is_elementwise=True,
+            kwargs={"capitalize": capitalize},
         )
 
     def append_args(
@@ -77,12 +78,12 @@ def haversine(
             cast_to_supertypes=True,
         )
 
+
 @pl.api.register_expr_namespace("date_util")
 class DateUtil:
     def __init__(self, expr: pl.Expr):
         self._expr = expr
 
-
     def is_leap_year(self) -> pl.Expr:
         return self._expr._register_plugin(
             lib=lib,
diff --git a/example/derive_expression/expression_lib/src/expressions.rs b/example/derive_expression/expression_lib/src/expressions.rs
index 429013d..ca764f2 100644
--- a/example/derive_expression/expression_lib/src/expressions.rs
+++ b/example/derive_expression/expression_lib/src/expressions.rs
@@ -4,16 +4,30 @@ use pyo3_polars::derive::polars_expr;
 use serde::Deserialize;
 use std::fmt::Write;
 
-fn pig_latin_str(value: &str, output: &mut String) {
+#[derive(Deserialize)]
+struct PigLatinKwargs {
+    capitalize: bool,
+}
+
+fn pig_latin_str(value: &str, capitalize: bool, output: &mut String) {
     if let Some(first_char) = value.chars().next() {
-        write!(output, "{}{}ay", &value[1..], first_char).unwrap()
+        if capitalize {
+            for c in value.chars().skip(1).map(|char| char.to_uppercase()) {
+                write!(output, "{c}").unwrap()
+            }
+            write!(output, "AY").unwrap()
+        } else {
+            let offset = first_char.len_utf8();
+            write!(output, "{}{}ay", &value[offset..], first_char).unwrap()
+        }
     }
 }
 
 #[polars_expr(output_type=Utf8)]
-fn pig_latinnify(inputs: &[Series]) -> PolarsResult<Series> {
+fn pig_latinnify(inputs: &[Series], kwargs: PigLatinKwargs) -> PolarsResult<Series> {
     let ca = inputs[0].utf8()?;
-    let out: Utf8Chunked = ca.apply_to_buffer(pig_latin_str);
+    let out: Utf8Chunked =
+        ca.apply_to_buffer(|value, output| pig_latin_str(value, kwargs.capitalize, output));
     Ok(out.into_series())
 }
 
diff --git a/example/derive_expression/run.py b/example/derive_expression/run.py
index 92c094b..78a0cca 100644
--- a/example/derive_expression/run.py
+++ b/example/derive_expression/run.py
@@ -16,6 +16,7 @@
 
 out = df.with_columns(
     pig_latin=pl.col("names").language.pig_latinnify(),
+    pig_latin_cap=pl.col("names").language.pig_latinnify(capitalize=True),
 ).with_columns(
     hamming_dist=pl.col("names").dist.hamming_distance("pig_latin"),
     jaccard_sim=pl.col("dist_a").dist.jaccard_similarity("dist_b"),
@@ -26,7 +27,7 @@
         integer_arg=93,
         boolean_arg=False,
         string_arg="example",
-    )
+    ),
 )
 
 print(out)
@@ -35,11 +36,12 @@
 # Tests we can return errors from FFI by passing wrong types.
 try:
     out.with_columns(
-    appended_args=pl.col("names").language.append_args(
-        float_arg=True,
-        integer_arg=True,
-        boolean_arg=True,
-        string_arg="example",
-    ))
+        appended_args=pl.col("names").language.append_args(
+            float_arg=True,
+            integer_arg=True,
+            boolean_arg=True,
+            string_arg="example",
+        )
+    )
 except pl.ComputeError as e:
-    assert "the plugin failed with message" in str(e)
\ No newline at end of file
+    assert "the plugin failed with message" in str(e)