Actions

Actions are an extension to the JavaScript object model tailored for a set of specific tasks.

To distinguish this from the native JavaScript elements we introduce new terminology, an action is an extended method while an action set is a mixin object (stateless, including only functionality) both usable stand-alone as well as mixed into other objects.

Here is a trivial use-case to illustrate the motivation for this tool set:

The problem:

var N = {
    times: function(n){
        this.value *= n
        return this
    }
}

var n = Object.create(N)

n.value = 3

n
    .times(3)
    .times(2)

To extend this object we'll need to:

n.times = function(n){
    console.log(this.value, 'times', n)

    var res = N.times.call(this, n)

    console.log('    ->', this.value)
    return res
}

Note that we are manually calling the super method and manually returning and re-returning this in each implementation of .times(..).

Another thing to note here is that the code above, though quite simple is not reusable, i.e.:

• we can't simply use the extending method for any other parent unless we either copy/rewrite it or complicate the code. ES5/ES6 only partially fixes this issue as super can only be used in some cases (literal methods) and not others (functions and functions as methods) which complicates things and makes them non-uniform.
• we can't use the extending method stand-alone, for example for testing

It is possible to go around these issues but not without introducing complicated and/or redundant code, Actions implements one approach to abstract this...

The solution:

var N = Actions({
    // Notice the brackets around the function...
    times: [function(n){
        this.value *= n
    }]
})

// Now we extend .times(..)
var ExtendedN = Actions({
    times: [function(n){
        console.log(this.value, 'times', n)

        return function(){
            console.log('    ->', this.value)
        }
    }]
})

And both objects can be used in the same way as before:

var n = mix(N, ExtendedN) // or Object.create(N) or Object.create(ExtendedN)...

n.value = 3

n
    .times(3)
    .times(2)

• this is returned automatically enabling us to chain calls to .times(..)
• the super method is resolved and called automatically
• both N and ExtendedN are independent of each other and reusable in different inheritance chains without any extra work needed.
• and more... (see below)

What we get:

• Call parent (extended) actions automatically
  All actions (methods) in a chain are guaranteed to get called if the action is called.
• Thread arguments up the call chain
  All actions in a chain will get the set of arguments passed to the action when called.
• Thread the return value down the call chain
  The return value will get passed through all the actions in a chain before returning to the action caller.
• Return this by default
• Organise and reuse actions
  Actions organized into action sets can be reused (mixed-in) in multiple inheritance chains without any extra work.
• Unified way to document actions
• Introspection and inspection API

Restrictions comparing to native JavaScript:

• No method shadowing
  The extending action can not "shadow" the extended action in a non destructive manner (e.g. via a throw), all actions in a chain are guaranteed to be called, unless a fatal error condition.
• No argument shadowing
  The extending action has access to all the arguments that the user passed but can not modify or reformat them before the extended action gets them.
• No return shadowing / Single return point
  The extending action can not replace the object returned by the extended action, though it can cooperatively update/modify it if needed. Only the root action can return a value, any other returns in chain are ignored
• No state transferred via mixin
  The only two things inherited from the object defining the actions via the mixin methods or mix function are properties and methods/actions, all data is discarded.

Notes:

• By design this tool-set promotes a cooperative model and makes it hard to change/modify existing signatures / contracts in extending code, hence the restrictions.
• mix(..) and the .mix*(..) (MetaActions/ActionSet) method family usually copy references to actions to the target object, this is done to go around the lack of multiple inheritance support in JavaScript and to enable clean action set reuse.
• mix(..) mixes actions in order, i.e. later overloads the former, this is not the same as the traditional multiple inheritance order in languages such as Python where the order is reversed.

Index

• Actions - The problem: - The solution:
  • What we get:
  • Restrictions comparing to native JavaScript:
  • Index
    • The main entities:
    • The action system main protocols:
      • 1. Documentation generation and introspection (MetaActions)
      • 2. Event-like callbacks for actions (MetaActions, Action)
      • 3. A mechanism to define and extend already defined actions
    • Secondary action protocols:
      • 1. A mechanism to manually call the pre/post stages of an action
      • 2. A mechanism to chain/wrap actions or an action and a function.
      • 3. .__actioncall__ action / handler
      • 4. Action attributes
      • 5. Pre-call testing if an action can be called
      • 6. Scheduling a call after the running action
      • 7. Calling action handlers sorted independently of the prototype chain
    • Alias protocols:
  • License

The main entities:

Action set

var empty_full = new ActionSet()

var minimal = Actions({
    // action and prop definitions...
})

var full = Actions(ActionSet(), {
    // ...
})

var inherited = Actions(full, {
    // ...
})

• an object containing a number of actions,
• optionally, directly or indirectly inherited from MetaActions and/or other action sets,
• the action handlers are bound relative to it (._action_handlers)

Action

Defined inside an action-set:

    // ...

    minimal: [function(){
        // ...
    }],

    full: ['Short info string',
        'Long documentation string, describing the action (optional)',
        function(){
            // pre code
            //    run before the parent action...
    
            return function(res){
                // post code
                //     run after the parent action or directly after 
                //     the pre-code of this is the root action...
            }
        }],

    // ...

The call diagram:

                        +  pre  +  pre  +       +  post +  post +
Action event handler:   o-------x                       o-------x
                                v                       ^
Actions:                        o-------x       o-------x
                                        v       ^
Root Action:                            o---|---x

• Action(..) creates a method (an action),
• an action calls all the shadowed/overloaded actions in the inheritance chain in sequence implicitly,
  Notes:
  • there is no way to prevent an action in the chain from running, this is by design, i.e. no way to fully shadow.
• top actions in the inheritance chain are called base or root actions.
• an action returns the action set (this) by default (for call chaining),
• the base/root action can return any value.
  Notes:
  • if undefined is returned, it will be replaced by the action context/action set,
  • if a function is returned it is treated as a post phase action,
  • to return a reserved value (undefined, function) wrap it in actions.ASIS(..)
  • any other return value is returned as-is,
  • there is no distinction between root and other actions other than that root action's return values are not ignored.
• if the root action returns a Promise, the post phase is run when that promise is resolved or rejected. This can be disabled by setting the 'await' action attribute to false (default: true).
• an action can consist of two parts: the first is called before the next action in chain (pre-callback) and the second after (post-callback).
• post-callback has access to the return value and can modify it but not replace it.
• an action can be bound to, a-la an event, calling the handlers when it is called (see below),

Action (event) handler

When actionSet object is inherited from a ActionSet object or from MetaActions:

actionSet.on('action_name', function(){
    // post code...
})

actionSet.on('action_name.post', function(){
    // post code...
})


actionSet.on('action_name.pre', function(){
    // pre code...
})

• a handler is a function,
• it can be bound to run before and/or after the action itself,
• it is local to an action set it was bound via,
• when an action is triggered from an action set, all the pre handlers in its inheritance chain will be called before the respective actions they are bound to and all the post handlers are called directly after.
• pre handlers are passed the same arguments the original actions got when it was called.
• post action handlers will get the root action result as first argument succeeded by the action arguments.

Alias

    fullAlias: ['Alias to .full(..) action...',
        `This alias will call the .full(..) action and pass it a couple of
        arguments`,
        // the alias code...
        'full: "argument" 1'],

• an action created by Alias(..),
• identical to an action with one key difference: instead of a function Alias(..) expects a string/code,
• code syntax is configurable, defaulting to the defined by parseActionCall(..),
• aliases are designed to be defined and managed in runtime while actions are mainly load-time entities.

The action system main protocols:

By default Actions(..) defines no additional methods. Most of the API methods are defined in MetaActions and can be optionally inherited from an instance of ActionSet. In general this includes all ActionSet / object level methods while anything accessible from the action is build-in.

1. Documentation generation and introspection (MetaActions)

<action>.toString()
    -> code of original action function

<action-set>.getDoc()
<action-set>.getDoc(<action-name>[, ..])
    -> dict of action-name, doc

<action-set>.getHandlerDocStr(<action-name>)
    -> formated string of action handlers

<action-set>.actions
    -> list of action names

2. Event-like callbacks for actions (MetaActions, Action)

<action-set>.on('action', function(){ ... })
<action-set>.on('action.post', function(){ ... })

<action-set>.on('action.pre', function(){ ... })

3. A mechanism to define and extend already defined actions

This replaces / complements the standard JavaScript overloading mechanisms (Action, Actions)

// Actions...
var X = Actions({
    m: [function(){ console.log('m') }]
})
var O = Actions(X, {
    m: [function(){
        console.log('pre')
        return function(res){
            console.log('post')
        }
    }]
})

Notes:

• what is done here is similar to calling O.__proto__.m.call(..) but is implicit, and not dependant on the original containing object name/reference (O), thus enabling an action to be referenced and called from any object and still chain correctly.

Secondary action protocols:

1. A mechanism to manually call the pre/post stages of an action

Pre phase...

<action>.pre(<context>)
<action>.pre(<context>, [<arg>, ..])
    -> <call-data>

Post phase...

<action>.post(<context>, <call-data>)
    -> <result>

This is internally used to implement the action call as well as the chaining callbacks (see below).

All action protocol details apply.

Notes:

• there is no reliable way to call the post phase without first calling the pre phase due to how the pre phase is defined (i.e. pre phase functions can return post phase functions).

2. A mechanism to chain/wrap actions or an action and a function.

This enables us to call a callback or another action (inner) between the root action's (outer) pre and post stages.

Outer action                o-------x       o-------x
                                    v       ^
Inner action/callback               o---|---x

A trivial example:

actionSet.someAction.chainApply(actionsSet, 
    function(){
        // this gets run between someAction's pre and post 
        // stages...
    }, 
    args)

This is intended to implement protocols where a single action is intended to act as a hook point (outer) and multiple different implementations (inner) within a single action set can be used as entry points.

    // Protocol root action (outer) definition...
    protocolAction: [function(){}],

    // Implementation actions (inner)...
    implementationAction1: [function(){
        return this.protocolAction.chainApply(this, function(){
            ...
        }, ..)
    }],

    implementationAction2: [function(){
        return this.protocolAction.chainApply(this, function(){
            ...
        }, ..)
    }],

Now calling any of the 'implementation' actions will execute code in the following order:

1. pre phase of protocol action (outer)
2. implementation action (inner)
3. post phase of protocol action (outer)

Notes:

• this will not affect to protocol/signature of the outer action in any way.
• both the inner and outer actions will get passed the same arguments.
• another use-case is testing/debugging actions.
• this is effectively the inside-out of normal action overloading.
• there is intentionally no shorthand for this feature, to avoid confusion and to discourage the use of this feature unless really necessary.

3. .__actioncall__ action / handler

This action if defined is called for every action called. It behaves like any other action but with a fixed signature, it always receives the action name as first argument and a list of action arguments as the second arguments, and as normal a result on the post phase.

Notes:

• it is not necessary to define the actual action, binding to a handler will also work.
• one should not call actions directly from within a actioncall handler as that will result in infinite recursion.
• one should use this with extreme care as this will introduce an overhead on all the actions if not done carefully.

4. Action attributes

Setting action attributes:

    someAction: [
        {attr: 'value', .. },
        function(){
            ...
        }],

Attribute access:

<action-set>.getActionAttr('action', 'attr')
    -> <value>

<action-set>.getActionAttrAliased('action', 'attr')
    -> <value>

<action-set>.getRootActionAttr('action', 'attr')
    -> <value>

5. Pre-call testing if an action can be called

A pre call test is called before the action's pre handlers are called and if it returns anything truthy the action is not called and that return value is returned instead.

To return a falsey value wrap it in actions.ASIS(..)

Only the top-most pre call test is called.

Defining a pre call test:

    someAction: [
        {precall: actions.debounce(200, true)},
        function(){
            ...
        }],

The test is called in the context of the <action-set>

<pre-call-test>(<action>, ...)
    -> undefined
    -> <value>

6. Scheduling a call after the running action

This enables the action code to schedule a call after the current action level or the root action is done.

<action-set>.afterAction(func)
<action-set>.afterAction('top', func)
    -> this

<action-set>.afterAction('local', func)
    -> this

Example:

    someAction: [
        function(){
            ...

            // the function passed will get called after the root action 
            // and all it's handlers are done.
            this.afterAction(function(){ ... })

            ...
        }],

Notes:

• The functions are executed in order of registration.
• This is pointless outside of an action call, thus an exception will be thrown.

7. Calling action handlers sorted independently of the prototype chain

This sorts action handlers by priority .sortedActionPriority then order and calls them.

This protocol enables us to call actions in a deterministic order independent of the order the handlers are defined in the prototype chain.

<action-set>.callSortedAction(name, ...args)
	-> this

Example action:

    someAction: [
        { sortedActionPriority: 'high' },
        function(){
            ...
        }],

sortedActionPriority can take the following values:

• number
• 'high' (equivalent to 50)
• 'normal' (equivalent to 0)
• 'low' (equivalent to -50)

The greater the priority the earlier the handler is called. Handlers with prioorities greater than 0 will always precede the unprioretized (i.e. .sortedActionPriority unset, null or 0) handlers; Handlers with prioorities less than 0 will always follow the unprioretized handlers. Unprioretized handlers keep their relative order.

Notes:

• .callSortedAction(..) ignores handler return values by design. This is done to prevent actions competing to return a value.
• if action name does not exist this will do nothing and return normally (without error)...

Alias protocols:

1. Defining aliases in runtime (MetaActions)

An alias is a mechanism to call an action (or alias) passing it a fixed set of arguments.

<action-set>.alias('alias', 'action: args')
<action-set>.alias('alias', .., 'action: args')
    -> <action-set>

Aliases can be defined inline:

someAction: [
	'action: arg'],

Alias code syntax:

ALIAS ::= 
    <action-name>
    | <action-name>: <args>
    | <action-name>: <args> <comment>
<args> ::=
    <arg>
    | <arg> <args>
<arg> ::=
    Number|String|Array|Object
    | IDENTIFIER
    | ...
    | '$[0-9]'
<comment> ::=
    '--.*$'
  		

Special arguments:

• IDENTIFIER
  expanded to context[IDENTIFIER]
• $N
  expanded to an instance of parseStringAction.Argument
• ...
  expanded to parseStringAction.ALLARGS (singleton)

Example:

go: [
    function(direction, ...opts){
        // ...
    }],

// aliases to go...
north: ['go: "north" -- Go north...'], 
south: ['go: "south" -- Go south...'], 
east: ['go: "east" -- Go east...'], 
west: ['go: "west" -- Go west...'], 

Notes:

• .alias(..) is signature compatible to Action(..) / Alias(..), supporting all the documentation and attribute definition.
• To enable extending in runtime .alias(..) itself is implemented as an action, thus all action protocols also apply.

2. Deleting aliases in runtime (MetaActions)

<action-set>.alias('alias', null)
<action-set>.alias('alias', false)
    -> <action-set>

Notes:

• only own aliases can be deleted via .alias(.., null|false)

3. Documentation generation and introspection (MetaActions, Alias)

Alias code...

<alias>.alias
<alias>.toString()
    -> <code>

List own aliases...

<action-set>.aliases
    -> <action-set>

License

BSD 3-Clause License

Copyright (c) 2018, Alex A. Naanou, All rights reserved.