use crate::r#type::{Type, Type::*, util::*};

impl Type {
    /// Performs substitution on the type.
    ///
    /// Replaces all free occurrences of generics
    /// named `name` with the given type `value`
    /// in the type.
    /// ```rust
    /// use melisp::r#type::{Type::*, util::*};
    /// 
    /// assert_eq!(
    ///     List(vec![Integer, arr(Integer, VarType("A".to_string())), VarType("B".to_string())])
    ///         .subst("A", &Integer),
    ///     List(vec![Integer, arr(Integer, Integer), VarType("B".to_string())])
    /// );
    ///
    /// assert_eq!(
    ///     List(vec![Integer, arr(Integer, VarType("A".to_string())), VarType("B".to_string())])
    ///         .subst("B", &arr(Integer, Integer)),
    ///     List(vec![Integer, arr(Integer, VarType("A".to_string())), arr(Integer, Integer)])
    /// );
    /// ```
    pub fn subst(self, name: &str, value: &Type) -> Type {
        match self {

            Arrow(a, b) => arr((*a).subst(name, value), (*b).subst(name, value)),

            List(v) => List(
                    v.into_iter()
                     .map(|t| t.subst(name, value))
                     .collect()
                 ),

            VarType(s) if s == name => value.clone(),

            VarType(s) => VarType(s),

            Integer => Integer,

        }
    }
}