use crate::*;
pub trait TryFromAtom<'a, T>: Sized {
    fn try_from_expr (_state: &'a T, _iter: TokenIter<'a>) -> Option<Self> { None }
    fn try_from_atom (state: &'a T, value: Value<'a>) -> Option<Self> {
        if let Exp(0, iter) = value { return Self::try_from_expr(state, iter) }
        None
    }
}
pub trait TryIntoAtom<T>: Sized {
    fn try_into_atom (&self) -> Option<Value>;
}
/// Map EDN tokens to parameters of a given type for a given context
pub trait Context<U>: Sized {
    fn get (&self, _atom: &Value) -> Option<U> {
        None
    }
    fn get_or_fail (&self, atom: &Value) -> U {
        self.get(atom).expect("no value")
    }
}
impl<T: Context<U>, U> Context<U> for &T {
    fn get (&self, atom: &Value) -> Option<U> {
        (*self).get(atom)
    }
    fn get_or_fail (&self, atom: &Value) -> U {
        (*self).get_or_fail(atom)
    }
}
impl<T: Context<U>, U> Context<U> for Option<T> {
    fn get (&self, atom: &Value) -> Option<U> {
        self.as_ref().map(|s|s.get(atom)).flatten()
    }
    fn get_or_fail (&self, atom: &Value) -> U {
        self.as_ref().map(|s|s.get_or_fail(atom)).expect("no provider")
    }
}
/// Implement `Context` for a context and type.
#[macro_export] macro_rules! provide {
    // Provide a value to the EDN template
    ($type:ty:|$self:ident:$State:ty|{ $($pat:pat => $expr:expr),* $(,)? }) => {
        impl Context<$type> for $State {
            #[allow(unreachable_code)]
            fn get (&$self, atom: &Value) -> Option<$type> {
                use Value::*;
                Some(match atom { $(Sym($pat) => $expr,)* _ => return None })
            }
        }
    };
    // Provide a value more generically
    ($lt:lifetime: $type:ty:|$self:ident:$State:ty|{ $($pat:pat => $expr:expr),* $(,)? }) => {
        impl<$lt> Context<$lt, $type> for $State {
            #[allow(unreachable_code)]
            fn get (&$lt $self, atom: &Value) -> Option<$type> {
                use Value::*;
                Some(match atom { $(Sym($pat) => $expr,)* _ => return None })
            }
        }
    };
}
/// Implement `Context` for a context and numeric type.
///
/// This enables support for numeric literals.
#[macro_export] macro_rules! provide_num {
    // Provide a value that may also be a numeric literal in the EDN, to a generic implementation.
    ($type:ty:|$self:ident:<$T:ident:$Trait:path>|{ $($pat:pat => $expr:expr),* $(,)? }) => {
        impl<$T: $Trait> Context<$type> for $T {
            fn get (&$self, atom: &Value) -> Option<$type> {
                use Value::*;
                Some(match atom { $(Sym($pat) => $expr,)* Num(n) => *n as $type, _ => return None })
            }
        }
    };
    // Provide a value that may also be a numeric literal in the EDN, to a concrete implementation.
    ($type:ty:|$self:ident:$State:ty|{ $($pat:pat => $expr:expr),* $(,)? }) => {
        impl Context<$type> for $State {
            fn get (&$self, atom: &Value) -> Option<$type> {
                use Value::*;
                Some(match atom { $(Sym($pat) => $expr,)* Num(n) => *n as $type, _ => return None })
            }
        }
    };
}
/// Implement `Context` for a context and the boolean type.
///
/// This enables support for boolean literals.
#[macro_export] macro_rules! provide_bool {
    // Provide a value that may also be a numeric literal in the EDN, to a generic implementation.
    ($type:ty:|$self:ident:<$T:ident:$Trait:path>|{ $($pat:pat => $expr:expr),* $(,)? }) => {
        impl<$T: $Trait> Context<$type> for $T {
            fn get (&$self, atom: &Value) -> Option<$type> {
                use Value::*;
                Some(match atom {
                    Num(n) => match *n { 0 => false, _ => true },
                    Sym(":false") | Sym(":f") => false,
                    Sym(":true")  | Sym(":t") => true,
                    $(Sym($pat) => $expr,)*
                    _ => return Context::get(self, atom)
                })
            }
        }
    };
    // Provide a value that may also be a numeric literal in the EDN, to a concrete implementation.
    ($type:ty:|$self:ident:$State:ty|{ $($pat:pat => $expr:expr),* $(,)? }) => {
        impl Context<$type> for $State {
            fn get (&$self, atom: &Value) -> Option<$type> {
                use Value::*;
                Some(match atom {
                    Num(n) => match *n { 0 => false, _ => true },
                    Sym(":false") | Sym(":f") => false,
                    Sym(":true")  | Sym(":t") => true,
                    $(Sym($pat) => $expr,)*
                    _ => return None
                })
            }
        }
    };
}