Macro expr 

macro_rules! expr {
    ($template:expr) => { ... };
    ($template:expr, $($param:tt),*) => { ... };
}

Macro to create expressions with serde_json::Value as default type Syntax:

• expr!(“template”, arg) -> arg becomes Scalar
• expr!(“template”, (expr)) -> expr becomes Nested
• expr!(“template”, {deferred}) -> deferred becomes Deferred