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goval

This library allows programs to evaluate arbitrary arithmetic/string/logic expressions. Accessing variables and calling custom functions is supported.

This project is considered stable and already used in production systems.
Please use the issue-tracker for any questions, feedback and bug-reports.

This project is licensed under the terms of the MIT license.

Demo

A small CLI demo that evaluates expressions can be found in the example folder:

go get -u github.com/maja42/goval
cd $GOPATH/src/github.com/maja42/goval/
go run example/main.go

Usage

Minimal example:

eval := goval.NewEvaluator()
result, err := eval.Evaluate(`42 > 21`, nil, nil) // Returns <true, nil>

Accessing variables:

eval := goval.NewEvaluator()
variables := map[string]interface{}{
    "uploaded": 146,
    "total":  400,
}
result, err := eval.Evaluate(`uploaded * 100 / total`, variables, nil)  // Returns <36, nil>

Calling functions:

// Implementing strlen()
eval := goval.NewEvaluator()
variables := map[string]interface{}{
    "os":   runtime.GOOS,
    "arch": runtime.GOARCH,
}

functions := make(map[string]goval.ExpressionFunction)
functions["strlen"] = func(args ...interface{}) (interface{}, error) {
    str := args[0].(string)
    return len(str), nil
}

result, err := eval.Evaluate(`strlen(arch[:2]) + strlen("text")`, variables, functions) // Returns <6, nil>

Custom functions allow the extension with arbitrary features like regex-matching:

// Implementing regular expressions (error handling omitted)
functions := make(map[string]goval.ExpressionFunction)
functions["matches"] = func(args ...interface{}) (interface{}, error) {
    str := args[0].(string)
    exp := args[1].(string)
    reg := regexp.MustCompile(exp)
    return reg.MatchString(str), nil
}

eval.Evaluate(`matches("text", "[a-z]+")`, nil, functions)  // Returns <true, nil>
eval.Evaluate(`matches("1234", "[a-z]+")`, nil, functions)  // Returns <false, nil>

Documentation

Types

This library fully supports the following types: nil, bool, int, float64, string, []interface{} (=arrays) and map[string]interface{} (=objects).

Within expressions, int and float64 both have the type number and are completely transparent.
If necessary, numerical values will be automatically converted between int and float64, as long as no precision is lost.

Arrays and Objects are untyped. They can store any other value ("mixed arrays").

Structs are note supported to keep the functionality clear and manageable. They would introduce too many edge cases and loose ends and are therefore out-of-scope.

Variables

It is possible to directly access custom-defined variables. Variables are read-only and cannot be modified from within expressions.

Examples:

var
var.field
var[0]
var["field"]
var[anotherVar]

var["fie" + "ld"].field[42 - var2][0]

Functions

It is possible to call custom-defined functions from within expressions.

Examples:

rand()
floor(42)
min(4, 3, 12, max(1, 3, 3))
len("te" + "xt")

Literals

Any literal can be defined within expressions. String literals can be put in double-quotes " or back-ticks `. Hex-literals start with the prefix 0x.

Examples:

nil
true
false
3
3.2
"Hello, 世界!\n"
"te\"xt"
`te"xt`
[0, 1, 2]
[]
[0, ["text", false], 4.2]
{}
{"a": 1, "b": {c: 3}}
{"key" + 42: "value"}
{"k" + "e" + "y": "value"}

0xA                 // 10
0x0A                // 10
0xFF                // 255 
0xFFFFFFFF          // 32bit appl.: -1  64bit appl.: 4294967295
0xFFFFFFFFFFFFFFFF  // 64bit appl.: -1  32bit appl.: error

It is possible to access elements of array and object literals:

Examples:

[1, 2, 3][1]                // 2
[1, [2, 3, 42][1][2]        // 42

{"a": 1}.a                  // 1
{"a": {"b": 42}}.a.b        // 42
{"a": {"b": 42}}["a"]["b"]  // 42

Precedence

Operator precedence strictly follows C/C++ rules.

Parenthesis () is used to control precedence.

Examples:

1 + 2 * 3    // 7
(1 + 2) * 3  // 9

Operators

Arithmetic

Arithmetic + - * /

If both sides are integers, the resulting value is also an integer. Otherwise, the result will be a floating point number.

Examples:

3 + 4               // 7
2 + 2 * 3           // 8
2 * 3 + 2.5         // 8.5
12 - 7 - 5          // 0
24 / 10             // 2
24.0 / 10           // 2.4

Power **

If both sides are integers, and the result can be represented as an integer, the resulting value is also an integer. Otherwise, the result will be a floating point number.

Examples:

2 ** 4              // 16
10 ** 3             // 1000
10 ** -6            // 0.000001
2.5 ** -2.5         // 0.10119...

Modulo %

If both sides are integers, the resulting value is also an integer. Otherwise, the result will be a floating point number.

Examples:

4 % 3       // 1
144 % 85    // -55
5.5 % 2     // 1.5
10 % 3.5    // 3.0

Negation - (unary minus)

Negates the number on the right.

Examples:

-4       // -4
5 + -4   // 1
-5 - -4  // -1
1 + --1  // syntax error
-(4+3)   // -7
-varName

Concatenation

String concatenation +

If either the left or right side of the + operator is a string, a string concatenation is performed. Supports strings, numbers, booleans and nil.

Examples:

"text" + 42     // "text42"
"text" + 4.2    // "text4.2"
42 + "text"     // "42text"
"text" + nil    // "textnil"
"text" + true   // "texttrue"

Array concatenation +

If both sides of the + operator are arrays, they are concatenated

Examples:

[0, 1] + [2, 3]          // [0, 1, 2, 3]
[0] + [1] + [[2]] + []   // [0, 1, [2]]

Object concatenation +

If both sides of the + operator are objects, their fields are combined into a new object. If both objects contain the same keys, the value of the right object will override those of the left.

Examples:

{"a": 1} + {"b": 2} + {"c": 3}         // {"a": 1, "b": 2, "c": 3}
{"a": 1, "b": 2} + {"b": 3, "c": 4}    // {"a": 1, "b": 3, "c": 4}
{"b": 3, "c": 4} + {"a": 1, "b": 2}    // {"a": 1, "b": 2, "c": 4}

Logic

Equals ==, NotEquals !=

Performs a deep-compare between the two operands. When comparing int and float64, the integer will be cast to a floating point number.

Comparisons <, >, <=, >=

Compares two numbers. If one side of the operator is an integer and the other is a floating point number, the integer number will be cast. This might lead to unexpected results for very big numbers which are rounded during that process.

Examples:

3 <-4        // false
45 > 3.4     // false
-4 <= -1     // true
3.5 >= 3.5   // true

And &&, Or ||

Examples:

true && true             // true
false || false           // false
true || false && false   // true
false && false || true   // true

Not !

Inverts the boolean on the right.

Examples:

!true       // false
!false      // true
!!true      // true
!varName

Ternary ? :

If the expression resolves to true, the operator resolves to the left operand.
If the expression resolves to false, the operator resolves to the right operand.

Examples:

true  ? 1 : 2                         // 1
false ? 1 : 2                         // 2
	
2 < 5  ? "a" : 1.5                    // "a"
9 > 12 ? "a" : [42]                   // [42]

false ? (true ? 1:2) : (true ? 3:4)   // 3

Note that all operands are resolved (no short-circuiting). In the following example, both functions are called (the return value of func2 is simply ignored):

true ? func1() : func2()

Bit Manipulation

Logical Or |, Logical And &, Logical XOr ^

If one side of the operator is a floating point number, the number is cast to an integer if possible. If decimal places would be lost during that process, it is considered a type error. The resulting number is always an integer.

Examples:

8 | 2          // 10
9 | 5          // 13
8 | 2.0        // 10
8 | 2.1        // type error

13 & 10        // 8
10 & 15.0 & 2  // 2

13 ^ 10        // 7
10 ^ 15 ^ 1    // 4

Bitwise Not ~

If performed on a floating point number, the number is cast to an integer if possible. If decimal places would be lost during that process, it is considered a type error. The resulting number is always an integer.

The results can differ between 32bit and 64bit architectures.

Examples:

~-1                   // 0
(~0xA55A) & 0xFFFF    // 0x5AA5
(~0x5AA5) & 0xFFFF    // 0xA55A

~0xFFFFFFFF           // 64bit appl.: 0xFFFFFFFF 00000000; 32bit appl.: 0x00
~0xFFFFFFFF FFFFFFFF  // 64bit appl.: 0x00; 32bit: error

Bit-Shift <<, >>

If one side of the operator is a floating point number, the number is cast to an integer if possible. If decimal places would be lost during that process, it is considered a type error. The resulting number is always an integer.

When shifting to the right, sign-extension is performed. The results can differ between 32bit and 64bit architectures.

Examples:

1 << 0    // 1
1 << 1    // 2
1 << 2    // 4
8 << -1   // 4
8 >> -1   // 16

1 << 31   // 0x00000000 80000000   64bit appl.: 2147483648; 32bit appl.: -2147483648
1 << 32   // 0x00000001 00000000   32bit appl.: 0 (overflow)

1 << 63   // 0x80000000 00000000   32bit appl.: 0 (overflow); 64bit appl.: -9223372036854775808
1 << 64   // 0x00000000 00000000   0 (overflow)

0x80000000 00000000 >> 63     // 0xFFFFFFFF FFFFFFFF   64bit: -1 (sign extension); 32bit: error (cannot parse number literal)
0x80000000 >> 31              // 64bit: 0x00000000 0000001; 32bit: 0xFFFFFFFF (-1, sign extension)

More

Array contains in

Returns true or false whether the array contains a specific element.

Examples:

"txt" in [nil, "hello", "txt", 42]   // true
true  in [nil, "hello", "txt", 42]   // false
nil   in [nil, "hello", "txt", 42]   // true
42.0  in [nil, "hello", "txt", 42]   // true
2         in [1, [2, 3], 4]          // false
[2, 3]    in [1, [2, 3], 4]          // true
[2, 3, 4] in [1, [2, 3], 4]          // false

Substrings [a:b]

Slices a string and returns the given substring. Strings are indexed byte-wise. Multi-byte characters need to be treated carefully.

The start-index indicates the first byte to be present in the substring.
The end-index indicates the last byte NOT to be present in the substring.
Hence, valid indices are in the range [0, len(str)].

Examples:

"abcdefg"[:]    // "abcdefg"
"abcdefg"[1:]   // "bcdefg"
"abcdefg"[:6]   // "abcdef"
"abcdefg"[2:5]  // "cde"
"abcdefg"[3:4]  // "d"

// The characters 世 and 界 both require 3 bytes:
"Hello, 世界"[7:13]    // "世界"
"Hello, 世界"[7:10]    // "世"
"Hello, 世界"[10:13]   // "界"

Array Slicing [a:b]

Slices an array and returns the given subarray.

The start-index indicates the first element to be present in the subarray.
The end-index indicates the last element NOT to be present in the subarray.
Hence, valid indices are in the range [0, len(arr)].

Examples:

// Assuming `arr := [0, 1, 2, 3, 4, 5, 6]`:
arr[:]    // [0, 1, 2, 3, 4, 5, 6]
arr[1:]   // [1, 2, 3, 4, 5, 6]
arr[:6]   // [0, 1, 2, 3, 4, 5]
arr[2:5]  // [2, 3, 4]
arr[3:4]  // [3]

Alternative Libraries

If you are looking for a generic evaluation library, you can also take a look at Knetic/govaluate. I used that library myself, but due to a several shortcomings I decided to create goval. The main differences are:

• More intuitive syntax
• No intermediate AST - evaluation and parsing happens in a single step
• Better type support:
  • Full support for arrays and objects.
  • Opaque differentiation between int and float64.
    The underlying type is automatically converted as long as no precision is lost.
  • Type-aware bit-operations (they only work with int-numbers).
  • No support for dates (strings are just strings, they don't have a special meaning, even if they look like dates).
    Support for dates and structs could be added if needed.
• More operators:
  • Accessing variables (maps) via . and [] syntax
  • Support for array- and object concatenation.
  • Slicing and substrings
• Hex-Literals (useful as soon as bit-operations are involved).
• Array literals with [] as well as object literals with {}
• Useful error messages.
• Highly optimized parser code by using go/scanner and goyacc.
  This leads to vastly reduced code size (and therefore little bug potential) and creates super-fast code.
• High test coverage (including lots of special cases).
  Also tested on 32 and 64bit architectures, where some (documented) operations like a bitwise-not can behave differently depending on the size of int.