Macro zng::var::expr_var

macro_rules! expr_var {
    ($($expr:tt)+) => { ... };
}

New variable from an expression with interpolated vars.

Interpolation

Other variables can be interpolated by quoting the var with #{..}. When an expression contains other interpolated vars the expression var updates when any of the interpolated vars update.

Examples

let var_a = var(10);
let var_b = var(10);
let name = "var_eq";
let var_eq = expr_var! {
    let eq = #{var_a} == #{var_b};
    println!("{} updated: {} == {}: {}", name, #{var_a}, #{var_b}, eq);
    eq
};

In the example a var_eq var of type impl Var<bool> is created. When either var_a or var_b are set the value of var_eq is updated. Normal variables like name are moved in, like a closure capture.

Capture Mode

The expression operates like a closure that captures by move. Both the interpolated variables and any other let binding referenced from the scope are moved into the resulting variable.

Interpolation

Variable interpolation is done by quoting the variable with #{<var-expr>}, the braces are required.

The <var-expr> is evaluated before capturing starts so if you interpolate #{var_a.clone()} var_a will still be available after the expr_var call. Equal <var-expr> only evaluate once.

Expansion

The expression is transformed into different types of vars depending on the number of interpolated variables.

No Variables

An expression with no interpolation is simply evaluated into a var using IntoVar.

Single Variable

An expression with a single variable is transformed in a map operation, unless the expression is only the variable without any extra operation.

Multiple Variables

An expression with multiple variables is transformed into a merge_var! call.