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// usage: cargo run --release --example advanced-vars
fn main() -> Result<(), fasteval::Error> {
let mut cb = |name:&str, args:Vec<f64>| -> Option<f64> {
let mydata : [f64; 3] = [11.1, 22.2, 33.3];
match name {
// Custom constants/variables:
"x" => Some(3.0),
"y" => Some(4.0),
// Custom function:
"sum" => Some(args.into_iter().fold(0.0, |s,f| s+f)),
// Custom array-like objects:
// The `args.get...` code is the same as:
// mydata[args[0] as usize]
// ...but it won't panic if either index is out-of-bounds.
"data" => args.get(0).and_then(|f| mydata.get(*f as usize).copied()),
// A wildcard to handle all undefined names:
_ => None,
}
};
let val = fasteval::ez_eval("sum(x^2, y^2)^0.5 + data[0]", &mut cb)?;
// | | |
// | | square-brackets act like parenthesis
// | variables are like custom functions with zero args
// custom function
assert_eq!(val, 16.1);
// Let's explore some of the hidden complexities of variables:
//
// * There's really no difference between a variable and a custom function.
// Therefore, variables can receive arguments too,
// which will probably be ignored.
// Therefore, these two expressions evaluate to the same thing:
// eval("x + y") == eval("x(1,2,3) + y(x, y, sum(x,y))")
// ^^^^^ ^^^^^^^^^^^^^^
// All this stuff is ignored.
//
// * Built-in functions take precedence WHEN CALLED AS FUNCTIONS.
// This design was chosen so that builtin functions do not pollute
// the variable namespace, which is important for some applications.
// Here are some examples:
// pi -- Uses the custom 'pi' variable, NOT the builtin 'pi' function.
// pi() -- Uses the builtin 'pi' function even if a custom variable is defined.
// pi(1,2,3) -- Uses the builtin 'pi' function, and produces a WrongArgs error
// during parse because the builtin does not expect any arguments.
// x -- Uses the custom 'x' variable.
// x() -- Uses the custom 'x' variable because there is no 'x' builtin.
// x(1,2,3) -- Uses the custom 'x' variable. The args are ignored.
// sum -- Uses the custom 'sum' function with no arguments.
// sum() -- Uses the custom 'sum' function with no arguments.
// sum(1,2) -- Uses the custom 'sum' function with two arguments.
Ok(())
}