You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: cast.rs
    path of file: ./repos/nushell/crates/nu-cmd-dataframe/src/dataframe/eager
    
    the code of the file until where you have to start completion: use crate::dataframe::values::{str_to_dtype, NuExpression, NuLazyFrame};

use super::super::values::NuDataFrame;
use nu_engine::CallExt;
use nu_protocol::{
    ast::Call,
    engine::{Comm
    
    fn signature(&self) -> Signature {
        Signature::build(self.name())
            .input_output_types(vec![
                (
                    Type::Custom("expression".into()),
                    Type::Custom("expression".into()),
                ),
                (
                    Type::Custom("dataframe".into()),
                    Type::Custom("dataframe".into()),
                ),
            ])
            .required(
                "dtype",
                SyntaxShape::String,
                "The dtype to cast the column to",
            )
            .optional(
                "column",
                SyntaxShape::String,
                "The column to cast. Required when used with a dataframe.",
            )
            .category(Category::Custom("dataframe".into()))
    }