expressions 0.1.0 expressions: ^0.1.0 copied to clipboard
A library to parse and evaluate simple expressions.
example/expressions_example.dart
import 'package:expressions/expressions.dart';
import 'dart:math';
main() {
example_1();
example_2();
}
// Example 1: evaluate expression with default evaluator
example_1() {
// Parse expression:
Expression expression = Expression.parse("cos(x)*cos(x)+sin(x)*sin(x)==1");
// Create context containing all the variables and functions used in the expression
var context = {
"x": pi / 5,
"cos": cos,
"sin": sin
};
// Evaluate expression
final evaluator = const ExpressionEvaluator();
var r = evaluator.eval(expression, context);
print(r); // = true
}
// Example 2: evaluate expression with custom evaluator
example_2() {
// Parse expression:
Expression expression = Expression.parse("'Hello '+person.name");
// Create context containing all the variables and functions used in the expression
var context = {
"person": new Person("Jane")
};
// The default evaluator can not handle member expressions like `person.name`.
// When you want to use these kind of expressions, you'll need to create a
// custom evaluator that implements the `evalMemberExpression` to get property
// values of an object (e.g. with `dart:mirrors` or some other strategy).
final evaluator = const MyEvaluator();
var r = evaluator.eval(expression, context);
print(r); // = 'Hello Jane'
}
class Person {
final String name;
Person(this.name);
Map<String,dynamic> toJson() => {"name": name};
}
class MyEvaluator extends ExpressionEvaluator {
const MyEvaluator();
@override
evalMemberExpression(MemberExpression expression, Map<String, dynamic> context) {
var object = eval(expression.object, context).toJson();
return object[expression.property.name];
}
}