This library allows programs to evaluate arbitrary arithmetic/string/logic expressions. Custom extensions like variable access and function calls are supported.

This project is considered stable and will not receive any new significant features. If new (major) features are required, they are likely to be integrated in a separate project instead.

However, please use the issue-tracker for any questions, feedback and bug-reports.

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

A small demo that evaluates expressions given from stdin 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

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{}{
    "os": runtime.GOOS,
    "arch": runtime.GOARCH,
}
result, err := eval.Evaluate(`os + "/" + arch`, variables, nil) // Returns <"linux/amd64", nil>

Calling functions:

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>

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.

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]

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")

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

Operator precedence strictly follows C/C++ rules.

Parenthesis () is used to control precedence.

Examples:

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

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

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

Negates the number on the right.

Examples:

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

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"

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]]

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}

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

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

Examples:

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

Inverts the boolean on the right.

Examples:

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

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

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

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)

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

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"

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]

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 few shortcomings I decided to create goval. The main differences are:

• Full support for arrays and objects.
• Accessing variables (maps) via . and [] syntax
• Support for array- and object concatenation.
• Array literals with [] as well as object literals with {}
• 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).
• Hex-Literals (useful as soon as bit-operations are involved).
• 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.
• Useful error messages.
• Written with go/scanner and goyacc.
  This vastly reduces code size (and therefore vulnerabilities to bugs) 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.