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use crate::types::DataType;
use crate::value::Value;

/// Standard function accept array of values and return single [`Value`]
pub type Function = fn(&[Value]) -> Value;

/// Aggregation function accept a selected row values for each row in group and return single [`Value`]
///
/// [`Vec<Vec<Value>>`] represent the selected values from each row in group
///
/// For Example if we have three rows in group and select name and email from each one
///
/// [[name, email], [name, email], [name, email]]
///
/// This implementation allow aggregation function to accept more than one parameter,
/// and also accept any Expression not only field name
///
pub type Aggregation = fn(Vec<Vec<Value>>) -> Value;

/// Signature struct is a representation of function type
///
/// Function type in GitQL used to track parameters and return type for now
/// but later can track parameter names to allow pass parameter by name and improve error messages
///
/// GitQL Function Signature has some rules to follow
///
/// Rules:
/// - Parameters must contains 0 or 1 [`DataType::Varargs`] parameter type only.
/// - [`DataType::Varargs`] must be the last parameter.
/// - You can add 0 or more [`DataType::Optional`] parameters.
/// - [`DataType::Optional`] parameters must be at the end but also before [`DataType::Varargs`] if exists.
///
/// The return type can be a static [`DataType`] such as Int, Flow or Dynamic
/// so you can return a dynamic type depending on parameters.
#[derive(Clone)]
pub struct Signature {
    pub parameters: Vec<DataType>,
    pub return_type: DataType,
}