expression := `
{
    "comparator": "||",
    "rules": [
        {
        "comparator": "&&",
        "rules": [
            { "var": "a", "op": "<", "val": "2019-03-28T11:39:43+07:00" },
            { "var": "b", "op": "in", "val": [1, 2, 3] }
        ]
        },
        {
        "comparator": "&&",
        "rules": [
            { "var": "c", "op": "!=", "val": "string" },
            { "var": "d", "op": ">=", "val": 4 },
            { "var": "strlen(c)", "op": ">", "val": 0 }
        ]
        }
    ]
}
`

parameters := map[string]interface{}{
    "a": time.Now(),
    "b": 1,
    "c": "number",
    "d": 5,
}

functions := map[string]rules.Function{
    "strlen": func(args ...interface{}) (interface{}, error) {
        length := len(args[0].(string))
        return (float64)(length), nil
    },
}

expr, err := rules.ParseFromJSON([]byte(expression))
require.NoError(t, err)

res, err := rules.Evaluate(expr, parameters, functions)
require.NoError(t, err)
assert.True(t, res)

input := `a == "2006-01-02T15:04:05Z" && (b > 0 || c == "string") && d < 100 && strlen(c) > 0`

parameters := map[string]interface{}{
    "a": time.Now(),
    "b": 1,
    "c": "number",
    "d": 5,
}

functions := map[string]rules.Function{
    "strlen": func(args ...interface{}) (interface{}, error) {
        length := len(args[0].(string))
        return (float64)(length), nil
    },
}

expr, err := rules.ParseFromString(input)
require.NoError(t, err)
require.NotNil(t, expr)

res, err := rules.Evaluate(expr, parameters, functions)
require.NoError(t, err)
assert.False(t, res)