pub(crate) use std::error::Error;

/// Standard result type.
pub type Usually<T> = Result<T, Box<dyn Error>>;

/// Standard optional result type.
pub type Perhaps<T> = Result<Option<T>, Box<dyn Error>>;

/// Implement the `From` trait.
#[macro_export] macro_rules! from {
    ($(<$($lt:lifetime),+>)?|$state:ident:$Source:ty|$Target:ty=$cb:expr) => {
        impl $(<$($lt),+>)? From<$Source> for $Target {
            fn from ($state:$Source) -> Self { $cb }
        }
    };
}

pub trait Has<T>: Send + Sync {
    fn get (&self) -> &T;
    fn get_mut (&mut self) -> &mut T;
}

#[macro_export] macro_rules! has {
    ($T:ty: |$self:ident : $S:ty| $x:expr) => {
        impl Has<$T> for $S {
            fn get (&$self) -> &$T { &$x }
            fn get_mut (&mut $self) -> &mut $T { &mut $x }
        }
    };
}

pub trait MaybeHas<T>: Send + Sync {
    fn get (&self) -> Option<&T>;
    fn get_mut (&mut self) -> Option<&mut T>;
}

#[macro_export] macro_rules! maybe_has {
    ($T:ty: |$self:ident : $S:ty| $x:block; $y:block $(;)?) => {
        impl MaybeHas<$T> for $S {
            fn get (&$self) -> Option<&$T> $x
            fn get_mut (&mut $self) -> Option<&mut $T> $y
        }
    };
}

/// May compute a `RetVal` from `Args`.
pub trait Eval<Args, RetVal> {
    /// A custom operation on [Args] that may return [Result::Err] or [Option::None].
    fn try_eval (&self, args: &Args) -> Perhaps<RetVal>;
    /// Invoke a custom operation, converting a `None` result to a custom `Box<dyn Error>`.
    fn eval <E: Into<Box<dyn std::error::Error>>> (&self, args: &Args, error: impl Fn()->E)
        -> Usually<RetVal>
    {
        match self.try_eval(args)? {
            Some(value) => Ok(value),
            _ => Result::Err(format!("Eval: {}", error().into()).into())
        }
    }
}

//impl<S: Eval<I, O>, I, O> Eval<I, O> for &S {
    //fn try_eval (&self, input: I) -> Perhaps<O> {
        //(*self).try_eval(input)
    //}
//}

//impl<S: Eval<I, O>, I: Ast, O: Dsl<S>> Eval<I, O> for S {
    //fn try_eval (&self, input: I) -> Perhaps<O> {
        //Dsl::try_provide(self, input)
    //}
//}