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Promise: Defining Types and Classes

Once you get into TDD with statically typed languages you realize you almost always want to deal with interfaces not classes, because you are almost always looking at two implementations of the same contract: the real one and the Mock. If you are a ReSharper junkie like myself, this duplication happens without giving it much thought, but it still is a tedious verbosity of code. Add to that that prototyping and simple projects carry with them syntactic burden of static tyypes (again, a lot less so for us ReSharper afflicted) and you can understand people loving dynamic languages. Now, I personally don't want to use one language for prototyping and smaller projects and then rewrite it in another when the codebase gets complex enough that keeping the contract requirements in your head becomes a hinderance to stability.

Promise tries to address these problems by being typed only when the programmer feels it's productive and by having classes automatically generate Types to match them without tying objects that want to be cast to the type of be instantiated by the class. This means that as soon as you create a class called Song, you have access to a Type called Song whose contract mirrors all non-wildcard methods from the class. Since the Type Song can be provided by any instance that services the contract, just taking an object, attaching a wildcard method handler to it creates a mock Song.

A diversion about what would generally be called static methods

The class model of Promise is a prototype object system inspired by Ruby/Smalltalk/javascript et al. Unlike class based languages, a class is actually a singleton instance. This means that a "static" method call is a dispatch against an instance method on that class singleton, so it would be better described as a Class Method.

But even that's not quite accurate. Another difference I eluded to in my Lambda post: Whenever you see the name Song in code and it's not used to change the definition of the class, it's actually the Type Song. So what looks like a call to the Class Method is a dispatch to the singleton instance of the class that is currently registered to resolve when a new instance for the Type Song is created. So, yes, it's a Class Method but not on the Class Song, but on the class that is the default provider for the Type Song.

If you've dealt with mocking in static languages, you are constantly trying to remove statics from your code, because they're not covered by interfaces and therefore harder to mock. In Promise, Class Methods are instance methods on the special singleton instance attached to an object. And since calling that methods isn't dispatched via the class but the implicit or explicit type, Class methods are definable in a Type.

Type definition

Type definitions are basically named slots followed by the left-hand-side of a lambda:

type Jukebox {
  Add:(Song song);
  ^FindJukeboxWithSong:(Song song|Jukebox);

One of the design goals I have with Promise is that I want keep Promise syntax fairly terse. That means that i want to keep a low number of keywords and anything that can be constructed using the existing syntax should be constructed by that syntax. That in turn means that I'm striving to keep the syntax flexible allow most DSL-like syntax addition. The end result, I hope is a syntax that facilitates the straddling of the dynamic/static line to support both tool heavy IDE development and quick emacs/vi coding. Here, instead of creating a keyword static, I am using the caret (^) to identify Class methods. The colon (:), while superfluous in Type definitions, is used to disambiguate method invocation from definition, when method assignment happens outside the scope of Type or Class definitions.

Attaching Methods to Instances

You don't have do define a class to have methods. You can simply grab a new instance and start attaching methods to it:

// add a method to a class
Object.Ping:() { println "Ping!"; };

// create blank instance
var instance =;

// attach method to instance
instance.Say:(str) {
  println "instance said '{str}'";

instance.Say("hello"); // => instance said 'hello'
instance.Ping(); // => Ping!

A couple of things of note here:

First, the use of the special base class Object from which everything derives. Attaching methods to Object makes them available to any instance of any class, which means that any object created now can use Ping().

Second, there is the special method .new, which creates a new instance. .new is a special method on any Type that invokes the languate level IoC to build up a new instance. It can be called with the JSON call convention, which on a Object will initialize that object with the provided fields. If an instance from a regular class is instantiated with the JSON call convention, then only matching fields in the json document are initialized in the class, all others are ignored. I'll cover how JSON is a first class construct in Promise and used for DTOs as well as the default serialization format in another post.

Last, the method Say(str) is only available on instance, not on other instances created from Object. You can, however call to get another instance with the same methods as the prior instance.

Defining a Class

Another opinionated style from Ruby that I like is the use prefixes to separate local variables from fields from class fields. Ruby uses no prefix for local, @ for fields (instance variables) and @@ for class fields. Having spent a lot of time in perl, @ still makes me think of arrays, so it's not my favorite choice of symbol, but I'd prefer it over the name collision and disambiguation of Java/C#.

Having used a leading underscore ( _ ) for fields in C# for a while, I've opted to use it as the identifying prefix for fields in Promise. In addition, we already have the leading caret as the prefix for Class Methods, so we can use it for Class Fields as well.

class Song {

  // Class Field
  Num ^songCount;

  // Class Method
  TotalSongs:(|Num) { ^songCount; };

  // Fields
  Stream _stream;

  // public Method
  Play:() {

  // protected Method
  _CheckStream:() { ... };

Just as the Caret is used both for marking Class Fields and Methods, underscore does the same for Methods and Fields: While Fields can only be protected, methods can be public or protected -- either way underscore marks the member as protected.

The method definition syntax is one of assigning a lambda to a named slot, such that :. The aspect of this behavior that is different from attaching functions to fields in JSON constructs in javascript is that these slots are polymorphic. In reality the slots are HashSets that can take many differeny lambda's as long as the signature is different. If you assign multiple lambdas with same signature to a single slot, the last one wins. This means that not only can you attach new methods to a class after definition, you can also overwrite them one signature at a time.

More on instance construction

Although .new is special, it is a Class Method, so if a more complex constructor is needed or the constructor should do some other initialization work, an override can easily be defined. Since Class Methods are reflected by types, this means that custom constructors can be part of the contract. The idea is that if the construction requirements are stringent enough for the class, they should be part of the Type so that alternative implemenentations can satisfy the same contract.

Song.^new:(name) {
  var this =;
  this._name = name;
  return this;

An override to .new is just a Class method not a constructor. That means that has to be called to create an instance, but since the method is in the scope of the Class definition, the instance's fields are accessible to override. There is no this in Promise, so the use of this in the above example is just a variable used by convention, similar to the perl programmer's usage of $self inside of perl instance subs.

But wait, there is more!

There are a number of special method declarations beyond simple alphanumerically named slot, such as C# style setters, operators, wildcards, etc. But there is enough detail in those cases, that I'll save that for the next post.

More about Promise

This is a post in an ongoing series of posts about designing a language. It may stay theoretical, it may become a prototype in implementation or it might become a full language. You can get a list of all posts about Promise, via the Promise category link at the top.

Promise: Lambdas

Lambdas in Promise, like other languages, are anonymous functions and first-class values. They act as closures over the scope they are defined in and can capture the free variables in their current lexical scope. Promise uses lambdas for all function definitions, going further even than javascript which has both named and anonymous functions. In Promise there are no named functions. Just slots that lambdas get assigned to for convenient access.

Straddling the statically/dynamically typed divide by allowing arguments and return values to optionally declare a Type, Promise mimicks C# lambda syntax more than say, LISP, javascript, etc. A simple lambda example looks like this:

var i = 0;
var incrementor = { ++i; };
print incrementor(); // => 1

This declaration doesn't have any input arguments to declare, so it uses a shortform of simply assigning a block to a variable. The standard form uses the lambda operator =>, so the above lambda could just as well be written as:

var incrementor = () => { ++i; };

I'm currently debating whether I really need the =>. It's mostly that i'm familiar with the form from C#. But given that there are no named functions, parentheses followed by a block can't occur otherwise, so there is no ambiguity. So, i'm still deciding whether or not to drop it:

var x = (x,y) => { x + y };
// vs.
var x = (x,y) { x + y };


The signature definition of lambdas borrows heavily from C#, using a left-hand side in parantheses for the signature, followed by the lambda operator. Input arguments can be typed, untyped or mixed:

var untypedAdd = (x,y) => { x + y; };
var typedAdd = (Num x, Num y) => { x + y; };
var mixedtypeAdd = (Num x, y) => { x + y; };


In dynamic languages, lambda definitions do not need a way to express whether they return a value--there is no type declaration so whether or not to expect a value is convention and documentatio driven. In C# on the other hand, a lambda can either not return a value, a void method, which uses one of the Action delegates, or return a value and declare it as the last type in the declaration using the Func delegates. In Promise all lambdas return a value, even if that value is nil (more about the special singleton instance nil later). Values can be returned by explicitly using the return keyword, otherwise it defaults simply to the value of the last statement executed before exiting the closure. Since return values can be typed, we need a way to declare that Type. Unlike C#, our lambdas aren't a delegate signature, so instead of reserving the last argument Type as the return Type, which would be ambiguous, Promise uses the pipe '|' character to optionally declare a return type:

var returnsUntyped = (x,y) => { x + y; };
var returnsTyped = (x,y|Num) => { x + y; };
var explicitReturn = (|Num) => { returnsTyped(1,2); };

Default values

Lambdas can also declare default values for arguments, which can be simple values or expressions:

var simple = (x=2,y=3) => { x + y; };
var complex= (x=simple()) => { x; };

Calling Conventions

Promise supports three different method calling styles. The first is the standard parentheses style as shown above. In this style, optional values can only be used by leaving out trailing arguments like this:

var f = (x=1,y=2,z=3) => { x + y +z; };
print f(2,2,2); // => 6
print f(2,2);   // => 7
print f(4);     // => 9
print f();      // => 6

If you want to omit a leading argument, you have to use the named calling style, using curly brackets, which was inspired by DekiScript. The curly bracket style uses json formatting, and since json is a first-class construct in Promise, calling the function by hand with {} or providing a json value behaves the same, allowing for simple dynamic argument construction:

print f{y: 1};       // => 5
print f{z: 1, y: 1}; // => 3
var args = {z: 5};
print f args;        // => 8

Finally there is the whitespace style, which replaces parentheses and commas with whitespace. This style exists primarily to make DSL creation more flexible:

print f 2 2 2; // => 6 print f 2 2; // => 7 print f 4; // => 9 print f; // => 6

Note the final call simply uses the bare variable f. This is possible because in Promise a lambda requiring no arguments can take the place of a value and accessing the variable executes the lambda. Sometimes it's desirable to access or pass a reference to a lambda, not execute it, in which case the reference notation '&' is needed. Using reference notation on a value is harmless (at least that's my current thinking), since Promise has no value types, so the reference of a value is the value:

var x = 2;
var y = () => { x+10; };
var x2 = &x;
var y2 = &y;
var y3 = y;
print x2; // => 3;
print y2; // => 13;
print y3; // => 12;

The output of y3 is only 12, because assignment of y3 evaluated y, capturing x before it was incremented.

Closures and Scope

As mentioned above, Lambdas can capture variables from their current scope. Scopes are nested, so a lambda can capture variables from any of the parent scopes

var x = 1;
var l1 = (y) => {
  return () => { x++; y++; x + y; };
print l1(2); // => 5
print l1(2); // => 6
print x;     // => 3

Similar to javascript, a block is not a new scope. This is done primarily for scoping simplicity, even if it introduces some side-effects:

() => {
  var x = 5;
  if( x ) {
    var x = 5; // illegal
    var y = 10;
  return y; // legal since the variable declaration was in the same scope

Using anonymous functions

As I've said that lambdas are the basic building block, meaning there is no other type of function definition. You can use them as lazily evaluated values, you can pass them as blocks to be invoked by other blocks and as I will discuss next time, Methods are basically polymorphic, named slots defined inside the closure of the class (i.e. capturing the class definition's scope), which is why there is no need for explicitly named functions.

More about Promise

This is a post in an ongoing series of posts about designing a language. It may stay theoretical, it may become a prototype in implementation or it might become a full language. You can get a list of all posts about Promise, via the Promise category link at the top.

Promise: Classes aren't Types

I wanted to start with the basic unit of construction in Promise, the lambda or closure. However, since Promise's lambda's borrow from C#, allowing Type definition in the declaration, I really need to cover the concepts of the promissory type system before i can get into that.

Classes are prototypes

Borrowing from javascript and Ruby and by extension Smalltalk, Promise classes are prototypes that are used to instantiate objects with the characteristics of the class. But those instances are untyped and while they can be inspected to see the originating class, the instances can also be changed with new characteristics making them diverge from that class.

Types are contracts

Similar to Interfaces in C#, Java, etc., Types in Promise describe the characteristics of some object. Unlike those languages, classes do not implement an interface. Rather, they are similar to Interfaces in Go in that any instance that has the methods promised by the Type can be used as that Type. Go, however, is still a typed language where an instance has the Type of its class but can be cast to an Interface, while Promise doesn't require any Type.

Compile time checking of Types

This part I still need to play with syntax to figure out useful and intuitive behavior. Classes aren't static and neither are instances, so checking the Type contract against the instantiation Class' capabilities isn't always the final word, Furthermore, if the object ever is passed without a Type annotation, the compiler can't determine the class to inspect. Finally, classes can have wildcard methods to catch missing method calls. In order to have some kind of compile time checking on the promissory declarations, there will have to be some way to mark variables as promising a Type cast that the compiler will trust, so that Type annotated and pure dynamic instances can work together.

This last part is where the dynamic keyword in C# fails for me, since you cannot cross from dynamic into static without a way of statically declaring the capabilities of an instance. That means that in C# you cannot take a dynamic object and use it anywhere that expects a typed class, regardless of its capabilities. I wrote Duckpond for C# to only address casting classes to interfaces that they satisfy, but I should extend it to proxy dynamic objects as well.

Implicit Types and the Type/Class naming overlap

In order to avoid needless Type declarations, every Class definition generates a matching Type with the same name (based only on its non-wildcard methods) at compile time. If you create a class called Song and a lambda with the signature (Song song) => { ... };, it might look like the Class is the Type. What's really going on though is that the class definition generated a Type called Song that expects an instance with the capabilities of the compile time definition of the Song class.

The shadowing of Classes by Types is possible because Class and Type names do not collide. A class name is only used for Class definition and modification, while Type names are used for signature and variable type. And since the shadowing is implicit, it's also possible to create a Song Type by hand to use instead of the implicit one. This is particularly useful when the Song class has several similar methods handled by a single wildcard method, but those signatures should be captured in the Type.

A final effect of the naming overlap, that I won't get into until I talk about the language level IoC, is that is not a call on the class, but on the Type -- this is also an artifact of how Promise treats Class/static methods and how the IoC container resolves instance creation. Can you see how this could be useful for mocking?

Enough with the theory already

Sorry for another dry, codeless post, but I couldn't see getting into the syntax that will use Types and Classes without explaining how Classes and Types interact in Promise first.

Next time, lambdas, lambdas, lambdas.

More about Promise

This is a post in an ongoing series of posts about designing a language. It may stay theoretical, it may become a prototype in implementation or it might become a full language. You can get a list of all posts about Promise, via the Promise category link at the top.

I made this half-pony half-monkey monster to please you

I made this half-pony half-monkey monster to please you But I get the feeling that you don't like it What's with all the screaming? You like monkeys, you like ponies Maybe you don't like monsters so much Maybe I used too many monkeys Isn't it enough to know that I ruined a pony making a gift for you?

Jonathan Coulton - Skullcrusher Mountain

I'm primarily a C# developer these days, but tinker in various other languages to see what others are doing better or worse. I am clearly biased towards static languages, despite lots of features I do like in various dynamic languages. This isn't some fear of the unknown, it's preference from extensive experience -- before switching to C# by way of Java, I was a perl developer for about 8 years. But I'm not trying to start yet another static vs. dynamic flame war here.

Playing with lots of languages got me thinking about what i'd ideally like to see in a programming language. So over the next couple of posts are going to be largely a thought experiment in designing that language. I do this not because i think there aren't enough languages, but to gain getter insights into how I work, how language design works and what goes into good usability design.

The conclusion of these posts may be that I find a language that already offers what I am looking for or that I start prototyping the new language in the DLR, since that's another skill i want to expand on.


The primary feature of this language, which I'm calling Promise for now, is that it's a dynamic language with a duck-typing system to allow declaration of contracts both for discoverability and compile time (AOT or JIT) verification, but without forcing strict class inheritance hierarchies on the programmer. Its mantra is "I promise that you will get an instance that does what you expect it to do".

Below are the top-level features I find desirable:

Optional Type Annotation

Some things can be completely dynamic, but I find it a lot more useful if a method can express the type of instance it expects, rather than having that information out-of-band in the documentation. As I said, I want it to be pure duck-typing: Classes aren't types and Types aren't classes, a class can be cast to a type if it satisfies the contract. This attaching the Types at the consumption rather than the declaration side I've previously written about and it's one thing i really like about Go. I want types to aid in discovery and debugging, not be the yoke it can become in purely statically typed languages

Runs in a virtual machine

I like virtual machines that can host many languages. It allows you to write in your favorite language but still take advantage of features and libraries written in other languages without complicated and platform specific interop stories. It took a while for this promise to come to fruit but both the JVM and CLR are turning into pretty cool polyglot ecosystems. So the language must run in one of those two environments (pending the emergence of another VM that has as much adoption).

Just-in-Time compilation

While I am a big fan of compiling and deploying bytecode, for rapid development and experimentation, I really want the language to be able to run either as bytecode or as loose files that are compiled just in time.


I will continue to structure logical units as work as classes with state and methods and pass around complex data in some kind of entity for a fair amount of the work I do. So organizing code in classes that are instantiated is a fundamental building block I rely oh.


But just as much as object-orientation is useful for organization of responsibilities, defining anonymous functions with closures for callbacks, continuations, etc. are another essential to way I code and a fundamental building block for asynchronous programming.


While C# 3.5 extension methods are a decent way of extending existing classes and attaching functionality to interfaces, I would prefer the mix-in style of attaching functionality to a class or instance without going down the multiple inheritance path.

Language level Inversion of Control

Having to know how to construct dependency hierarchies by hand or knowing about and managing the lifetime scopes of instances is, imho, an imperative programming concept that just needlessly complicates things. I just want to get an instance that I can work with and not have to deal with a myriad of constructors or have to know how to initialize a fresh instance otherwise. Nor do I want to deal with knowing when to get my own instance vs. a shared one. All that is plumbing that is common and fundamental enough that it should move into the language itself, rather than having to build an IoC framework that takes over constructors for you, or using Service Location to self-initialize.

With the 10k foot overview out of the way...

From these qualities, I have started to define the syntax for Promise and next time I'll go into some detailed specifications and start working through various syntax examples.

If you want to skip ahead and see my brainstorm notes, they can be found here.

If you think there already is a language that satisfies most if not even all my requirements, I'd love to hear about it as well.

More about Promise

This is a post in an ongoing series of posts about designing a language. It may stay theoretical, it may become a prototype in implementation or it might become a full language. You can get a list of all posts about Promise, via the Promise category link at the top.