Macro decl_func 

macro_rules! decl_func {
    ( $identifier:ident, $func_type:expr, $predicate:expr, $target:expr ) => { ... };
}

Given a function name, a FunctionType, a predicate and a target ValueType declares a function. It generates a wrapper that handles types and unbox parameters.

The predicate expects a Value as parameter and an EvalResult<Value> as output.

Examples

// Remember to import everything under the `function` module.
use num_parser::{*, function::*};

decl_func!(
    // Function name
    hypotenuse,
    // Function type
    FunctionType::Std,
    // Predicate
    |v: Value| {
        // Read the contained data using read_vec_values
        read_vec_values!(v, a, b);
        // Convert the data to the desired type
        let a_as_number = a.as_float()?;
        let b_as_number = b.as_float()?;

        Ok(
            Value::Float(
                a_as_number.powi(2) + b_as_number.powi(2)
            )
        )
    },
    // Expect a vector as input, as we need multiple parameters.
    ValueType::VectorType
);

Generated code

use num_parser::{*, function::*};

fn hypotenuse(arguments: &Vec<Box<Expression>>, context: &Context, depth: u32) -> EvalResult<Value> {
    let unboxed = unbox_parameters(arguments, context, depth)?;
    type_wrapper(
        unboxed,
        FunctionType::Std,
        ValueType::VectorType,
        context,
        |v: Value| {
            read_vec_values!(v, a, b);

            let a_as_number = a.as_float()?;
            let b_as_number = b.as_float()?;
     
            Ok(
                Value::Float(
                    a_as_number.powi(2) + b_as_number.powi(2)
                )
            )
        }    
    )
}