It's very common to evaluate an expression dynamicly, so that's why we are here.

We use s-epxression syntax to parse and evaluate.

In computing, s-expressions, sexprs or sexps (for "symbolic expression") are a notation for nested list (tree-structured) data, invented for and popularized by the programming language Lisp, which uses them for source code as well as data.

For example, for expression in common way: ​ ( (gender = "female") and
((age % 2) != 0) ) it's coresponding format in s-expression is: ​ (and (= gender "female") (!= (% age 2) 0 ) )

• number
  For convenience, we treat float64, int64 and so on as type of number. For example, float 100.0 is equal to int 100, but not euqal to string "100"

• string
  character string quoted with `, ', or " are treated as type of string. You can convert type string to any other defined type you like by type convert functions which are mentioned later

• function or variable
  character string without quotes are regarded as type of function or variable which depends on whether this function exists. For example in expression (age birthdate), both age and birthdate is unquoted. age is type of function because we have registered a function named age, while birthdate is type of variable for not found. The program will come to errors if there is neither parameter nor function named birthdate when evaluating

You can evaluate directly:

params := evaluator.MapParams{
    "gender": "female",
}
res, err := evaluator.EvalBool(`(in gender ("female" "male"))`, params)
if err != nil {
    log.Fatal(err)
}
fmt.Println(res)
# true	

or you can reuse the Expression to evaluate multiple times:

params := evaluator.MapParams{
    "gender": "female",
}
exp, err := evaluator.New(`(in gender ("female" "male"))`)
if err != nil {
    log.Fatal(err)
}
res, err := exp.EvalBool(params)
if err != nil {
    log.Fatal(err)
}
fmt.Println(res)
# true

• (in gender ("male", "female"))

• (between now (td_time "2017-01-02 12:00:00") (td_time "2017-12-02 12:00:00"))

• (ne (mod (age birthdate) 7) 5)

• or multiple-line for clarity

  (and
  	(ne os "ios")
  	(eq gender "male")
  	(beteen version (t_version "2.7.1") (t_version "2.9.1"))
  )
  

p.s. either operand or function can be used in expression

Yes, you can write your own function by following thses steps:

1. implement your function
2. regist to functions
3. enjoy it

here is an example:

package main


import (
	"errors"
	"log"
	"time"

	"github.com/nullne/evaluator"
	"github.com/nullne/evaluator/function"
)

// define your own function and don't forget to register
func age(params ...interface{}) (interface{}, error) {
	if len(params) != 1 {
		return nil, errors.New("only one params accepted")
	}
	birth, ok := params[0].(string)
	if !ok {
		return nil, errors.New("birth format need to be string")
	}
	r, err := time.Parse("2006-01-02", birth)
	if err != nil {
		return nil, err
	}
	now := time.Now()
	a := r.Year() - now.Year()
	if r.Month() < now.Month() {
		a--
	} else if r.Month() == now.Month() {
		if r.Day() < now.Day() {
			a--
		}
	}
	return a, nil
}

func main() {
	if err := function.Regist("age", age); err != nil {
		log.Print(err)
	}

	exp := `(not (between (age birthdate) 18 20))`
	vvf := evaluator.MapParams{
		"birthdate": "1980-02-01",
	}
	e, err := evaluator.New(exp)
	if err != nil {
		log.Print(err)
	}
	r, err := e.Eval(vvf)
	if err != nil {
		log.Print(err)
	}
	log.Printf("expression: `%s`, wanna: %+v, got: %+v\r", exp, true, r)
}

• Params interface, which has a method named Get to get all params needed
• MapParams a simple implemented Params in map

BenchmarkEqualString-8   	 3000000	       473 ns/op
BenchmarkInString-8      	 2000000	       916 ns/op
BenchmarkBetweenInt-8    	 3000000	       467 ns/op
BenchmarkBetweenTime-8   	 1000000	      2089 ns/op
BenchmarkOverlapInt-8    	  500000	      2966 ns/op
BenchmarkTypeTime-8      	 2000000	       638 ns/op
BenchmarkTypeVersion-8   	 3000000	       539 ns/op

p.s. on MacBook Pro (Retina, 15-inch, Mid 2015), Memory: 16 GB 1600 MHz DDR3, Processor: 2.2 GHz Intel Core i7