ExpreTau

ExpreTau is a simple expression engine written in Java, of which the runtime codes are split from parsing and compiling codes. The classes in runtime are serializable so that they are suitable for runtime of distributed computing system, like Apache Flink.

ExpreTau is just "Expression" and "TAU". The idea of "TAU" is coming from The Tau Manifesto.

Getting Started

public class MyClass {
    public Object calc() {
        // The original expression string.
        String exprString = "(1 + 2) * (5 - (3 + 4))";
        // parse it into an Expr object.
        Expr expr = ExpretauCompiler.INS.parse(exprString);
        // Compile in a CompileContext (can be null without variables in the expression) and get an RtExpr object.
        RtExpr rtExpr = expr.compileIn(null);
        // Evaluate it in an EvalContext (can be null without variables in the expression).
        return rtExpr.eval(null);
    }
}

The RtExpr object can do eval multiple times in different EvalContext after generated by compileIn.

Module expretau_console can be simply used as a command line calculator, which is based on ExpreTau.

Dependencies

<?xml version="1.0" encoding="UTF-8"?>
<dependencies>
    <!-- Required if you want to do parsing and compiling -->
    <dependency>
        <groupId>io.github.datacanvasio.expretau</groupId>
        <artifactId>expretau-parser</artifactId>
        <version>1.0.0</version>
    </dependency>

    <!-- Required if you want to do evaluating -->
    <dependency>
        <groupId>io.github.datacanvasio.expretau</groupId>
        <artifactId>expretau-runtime</artifactId>
        <version>1.0.0</version>
    </dependency>
</dependencies>

Variables and Context

Variables can be used in expressions, but a CompileContext must be provided to define the types of variables.

A JSON Schema definition can be used as a source of CompileContext. For example (in YAML format for simplicity, but you can surely use JSON format)

type: object
properties:
    a:
        type: integer
    b:
        type: number
    c:
        type: boolean
    d:
        type: string
additionalProperties: false

where variables a, b, c, d are defined with specified types. The a RtExpr can be compiled as following,

public class MyClass {
    public RtExpr compile(String jsonSchemaInYamlFormat) {
        // jsonSchemaInYamlFormat can be a String/InputStream contains the JSON Schema definition.
        RtSchemaRoot schemaRoot = SchemaParser.YAML.parse(jsonSchemaInYamlFormat);
        Expr expr = ExpretauCompiler.INS.parse("a + b");
        return expr.compileIn(schemaRoot.getSchema());
    }
}

You can also create a parser with the RtSchemaRoot object to parse a JSON/YAML source into a EvalContext object.

public class MyClass {
    public Object calc(RtSchemaRoot schemaRoot, RtExpr rtExpr) {
        DataParser parser = DataParser.yaml().schema(schemaRoot);
        // RtData implements EvalContext
        RtData data = parser.parse("{a: 2, b: 3.0, c: true, d: foo}");
        // The result should be a Double 5.0
        return rtExpr.eval(data);
    }
}

Nested Context

In a JSON Schema definition, objects and arrays can be nested into each other, for example,

type: object
properties:
    a:
        type: object
        properties:
            b:
                type: number
            c:
                type: boolean
        additionalProperties: false
    d:
        type: array
        items:
            - type: integer
            - type: string
        additionalItems: false
additionalProperties: false

In this context, you can use a.b and a.c to access the number and the boolean variables. The syntax looks the same as map index, but they are really separate variables. On the contrary, a is not an existing variable. Also, you can use d[0] and d[1] to access the integer and the string variables and d is not an existing variable.

The additionalProperties and additionalItems are crucial. If they are set to true (which is default in JSON Schema Specification), a becomes a variable of Map type and d of List type, which can be accessed by the same syntax, but the operating is a runtime indexing, not a var identifying in compiling time.

The special variable $ can be used to access the whole context, so $.a is the same as a. $ is useful for a context with an array as root. The parser also looks on a.b as a['b'], so the syntax to access variables is much like JSONPath.

Operators

Category	Operator	Associativity
Parenthesis	( )
Function Call	( )	Left to right
Name Index	.	Left to right
Array Index	[ ]	Left to right
Unary	+ -	Right to left
Multiplicative	* /	Left to right
Additive	+ -	Left to right
Relational	< <= > >= == = != <>	Left to right
String	startsWith endsWith contains matches	Left to right
Logical NOT	! not	Left to right
Logical AND	&& and	Left to right
Logical OR	|| or	Left to right

Data Types

Type Name	JSON Schema Type	Hosting Java Type	Literal in Expression
Integer		java.lang.Integer
Long	integer	java.lang.Long	0 20 -375
Double	number	java.lang.Double	2.0 -6.28 3e-4
Boolean	boolean	java.lang.Boolean	true false
String	string	java.lang.String	"hello" 'world'
Decimal		java.math.BigDecimal
Time		java.util.Date
IntegerArray		java.lang.Integer[]
LongArray	array	java.lang.Long[]
DoubleArray	array	java.lang.Double[]
BooleanArray	array	java.lang.Boolean[]
StringArray	array	java.lang.String[]
DecimalArray		java.math.BigDecimal[]
ObjectArray	array	java.lang.Object[]
List	array	java.util.List
Map	object	java.util.Map
Object	object	java.lang.Object

For JSON Schema of type array, the final type is determined as in the following table.

Value of additionalItems	Value of items.type	Type Name
false		split into variables
true	integer	LongArray
true	number	DoubleArray
true	boolean	BooleanArray
true	string	StringArray
true	object	ObjectArray
true		List

For JSON Schema of type object, the final type is determined as in the following table.

Value of additionalProperties	Value of properties	Type Name
false		split into variables
true	not null	Map
true	null	Object

NOTE: Some types cannot be written literally in expressions, but they do exist in the engine. They can be got by pre-defined constants, variables or intermediate results.

Constants

Name	Value
TAU	6.283185307179586476925
E	2.7182818284590452354

There is not "3.14159265" but "TAU". 😄

Functions

Mathematical

See Math (Java Platform SE 8).

Function	Java function based on	Description
abs(x)	java.lang.Math.abs
sin(x)	java.lang.Math.sin
cos(x)	java.lang.Math.cos
tan(x)	java.lang.Math.tan
asin(x)	java.lang.Math.asin
acos(x)	java.lang.Math.acos
atan(x)	java.lang.Math.atan
cosh(x)	java.lang.Math.cosh
sinh(x)	java.lang.Math.sinh
tanh(x)	java.lang.Math.tanh
log(x)	java.lang.Math.log
exp(x)	java.lang.Math.exp

Type conversion

Function	Java function based on	Description
int(x)		Convert x to Integer
long(x)		Convert x to Long
double(x)		Convert x to Double
decimal(x)		Convert x to Decimal
string(x)		Convert x to String
string(x, fmt)		Convert x to String, x is a Time
time(x)		Convert x to Time
time(x, fmt)		Convert x to Time, x is a String

String

See String (Java Platform SE 8).

Function	Java function based on	Description
toLowerCase(x)	String::toLowerCase
toUpperCase(x)	String::toUpperCase
trim(x)	String::trim
replace(x, a, b)	String::replace
substring(x, s)	String::substring
substring(x, s, e)	String::substring

User defined functions

It is simple to add an user defined function to ExpreTau.

First, define a class like

public class HelloOp extends RtFun {
    private static final long serialVersionUID = -8060697833705004059L;

    protected HelloOp(@Nonnull RtExpr[] paras) {
        super(paras);
    }

    @Override
    protected Object fun(@Nonnull Object[] values) {
        return "Hello " + values[0];
    }

    @Override
    public int typeCode() {
        return TypeCode.STRING;
    }
}

Then register it to the FunFactory

public class TestUdf() {
    @Test
    public void test() {
        FunFactory.INS.registerUdf("hello", HelloOp::new);
        // Now you can use the `hello` function
        Expr expr = ExpretauCompiler.INS.parse("hello('world')");
        RtExpr rtExpr = expr.compileIn(null);
        System.out.println(rtExpr.eval(null));
    }
}

Modules

Module	Description	Documentation
expretau_annotations	An annotation processor to help generating some runtime code. This module is not required to using ExpreTau library.
expretau_console	An command line application to parse and evaluate expressions inputted from console.
expretau_parser	The ExpreTau parser, required to parse expression string.
expretau_runtime	The ExpreTau runtime, required to evaluate the compiled runtime object.