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The evalfilter package provides an embeddable evaluation-engine, which allows simple logic which might otherwise be hardwired into your golang application to be delegated to (user-written) script(s).

There is no shortage of embeddable languages which are available to the golang world, this library is intended to be less complex, allowing only simple tests to be made against structures/objects. That said the flexibility present means that if you don't need full scripting support this might just be sufficient for your needs.

To give you a quick feel for how things look you could consult:

  • example_test.go.
    • This filters a list of people by their age.
  • example_function_test.go.
    • This exports a function from the golang-host application to the script.
    • Then uses that to filter a list of people.
  • Some other simple examples are available beneath the _examples/ directory.

API Stability

The API will remain as is for any 1.x.x release.

Sample Use

You might have a chat-bot which listens to incoming messages and runs "something interesting" when specific messages are seen. You don't necessarily need to have a full-scripting language, you just need to allow a user to specify whether the interesting-action should occur, on a per-message basis.

  • Create an instance of the evalfilter.
  • Load the user's script.
  • For each incoming message run the script against it.
    • If it returns true you know you should carry out your interesting activity.
    • Otherwise you will not.

Assume you have a structure describing your incoming messages which looks something like this:

type Message struct {
    Author  string
    Channel string
    Message string
    Sent    time.Time

The user could now write following script to let you know that the incoming message was interesting:

// You can see that comments are prefixed with "//".
// This script is invoked by your Golang application as a filter,
// the intent is that the user's script will terminate with either:
//   return false;
// or
//   return true;
// Your host application will then carry out the interesting operation
// when it receives a `true` result.

// If we have a message from Steve it is "interesting"!
if ( Author == "Steve" ) { return true; }

// A bug is being discussed?  Awesome.
if ( Message ~=  "panic" ) { return true; }

// OK the message is uninteresting, and will be discarded, or
// otherwise ignored.
return false;

You'll notice that we don't define the object here, because it is implied that the script operates upon a single instance of a particular structure, whatever that might be. That means Author is implicitly the author-field of the message object, which the Run method was invoked with.

Scripting Facilities

The engine supports scripts which:

  • Perform comparisons of strings and numbers:
    • equality:
      • "if ( Message == "test" ) { return true; }"
    • inequality:
      • "if ( Count != 3 ) { return true; }"
    • size (<, <=, >, >=):
      • "if ( Count >= 10 ) { return false; }"
      • "if ( Hour >= 8 && Hour <= 17 ) { return false; }"
    • String contains:
      • "if ( Content ~= "needle" )"
    • Does not contain:
      • "if ( Content !~ "some text we dont want" )"
  • You can also add new primitives to the engine.
    • By implementing them in your golang host application.
  • Your host-application can set variables which are accessible to the user-script.
    • if ( time == "Steve" ) { print("You set 'time' to the value 'Steve'\n"); }
  • Finally there is a print primitive to allow you to see what is happening, if you need to.
    • This is just one of the built-in functions, but perhaps the most useful.

You'll note that you're referring to structure-fields by name, they are found dynamically via reflection.

if conditions can be nested as the following sample shows, and we also support an else clause.

 if ( Count > 10 ) {
     print("Count is > 10\n");

     if ( Count > 50 ) {
          print("The count is super-big!\n");
     } else {
          print("The count is somewhat high!\n");

Function Invocation

In addition to operating upon the fields of an object/structure literally you can also call functions with them.

For example you might have a list of people, which you wish to filter by the length of their names:

// People have "name" + "age" attributes
type Person struct {
  Name string
  Age  int

// Now here is a list of people-objects.
people := []Person{
    {"Bob", 31},
    {"John", 42},
    {"Michael", 17},
    {"Jenny", 26},

You can filter the list based upon the length of their name via a script such as this:

// Example filter - we only care about people with "long" names.
if ( len(Name) > 4 ) { return true ; }

// Since we return false the caller will know to ignore people here.
return false;

This example is contained in example_function_test.go if you wish to see the complete code.

Built-In Functions

The following functions are built-in, and available by default:

  • len(field | value)
    • Returns the length of the given value, or the contents of the given field.
  • lower(field | value)
    • Return the lower-case version of the given input.
  • match(field | str, regexp)
    • Returns true if the specified string matches the supplied regular expression.
    • You can make this case-insensitive using (?i), for example:
      • if ( match( "Steve" , "(?i)^steve$" ) ) { ...
  • trim(field | string)
    • Returns the given string, or the contents of the given field, with leading/trailing whitespace removed.
  • type(field | value)
    • Returns the type of the given field, as a string.
      • For example string, integer, float, boolean, or null.
  • upper(field | value)
    • Return the upper-case version of the given input.


Your host application can register variables which are accessible to your scripting environment via the SetVariable method. The variables can have their values updated at any time before the call to Eval is made.

For example the following example sets the contents of the variable time, and then outputs it. Every second the output will change, because the value has been updated:

eval := evalfilter.New(`
            print("The time is ", time, "\n");
            return false;

for {

    // Set the variable `time` to be the seconds past the epoch.
    eval.SetVariable("time", &object.Integer{Value: time.Now().Unix()})

    // Run the script.
    ret, err := eval.Run(nil)

    // If there are errors - abort
    if err != nil {

    // Show the result
    fmt.Printf("Script gave result %v\n", ret)

    // Update every second.

    time.Sleep(1 * time.Second)

Standalone Use

If you wish to experiment with script-syntax you can install the standalone driver:

go get

This driver allows you to supply:

  1. A JSON object.
  2. A script to run against that object.

You can run those interactively to see what happens, for example in the cmd/evalfilter directory:

 ./evalfilter run -json on-call.json on-call.script

This driver an also be used to reproduce any problems identified via fuzz-testing.


If this solution doesn't quite fit your needs you might investigate:

Github Setup

This repository is configured to run tests upon every commit, and when pull-requests are created/updated. The testing is carried out via .github/ which is used by the github-action-tester action.


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