Overview

Velo

Language version 0.1, distribution revision 2015.0103

"Jaws was never my scene, and I don't like Star Wars."
-- Queen, "Bicycle Race"

Velo is an object-oriented language, inspired somewhat by Ruby, and sharing somewhat Robin's "radically goo-oriented" approach.

(I don't really like the term "scripting language", but it seems to be fitting here, for whatever reason, so where I might normally say "program" I will instead say "script".)

We shall introduce Velo with a series of simple examples in Falderal format. The design of Velo is still somewhat inchoate, so do not expect all of these examples to be perfectly consistent or even necessarily feasible.

-> Tests for functionality "Interpret Velo Script"

First, the ubiquitous "Hello, world!":

| extend IO
| print {Hello, world!}
= Hello, world!

This example demonstrates what are likely the two most outstanding features of Velo:

  • Scripts are no different from object classes. Thus, extend is used in the very same manner as an "import" or "require" construct would be in some other language; it is not dissimilar to Ruby's include method.
  • Literal strings are delimited by curly braces. In fact, in Velo, strings are no different from code blocks.

Let those two ideas sink in for a moment, then we'll continue.

Strings as Blocks

Conditional statements are implemented by a method called if. This method is on the root object, called Object, from which all objects, including your script, descend.

| if ({X}.equals {X}), {IO.print {Yes}}, {IO.print {No}}
= Yes

| if ({X}.equals {Y}), {IO.print {Yes}}, {IO.print {No}}
= No

To try to hammer this block-is-a-string thing home, you can just as easily call this method with arguments that are string variables, rather than string literals (that look like blocks of code.)

| yes = {IO.print {Yes}}
| no = {IO.print {No}} 
| if ({X}.equals {Y}), yes, no 
= No

You may be thinking that this is basically an implicit and ubiquitous use of eval -- which would be accurate -- and that that means that execution of Velo code inevitably suffers in efficiency -- which is not accurate. When the string being evaled is a literal string, it can be transformed into the internal format for code as early as possible, making it no less efficient than any other code. It's only when you have to eval a string in a variable, where you can't predict what it will be until you evaluate the surrounding code, that you necessarily take a performance hit.

| p = {extend IO; print }
| yes = p.concat {{Yes}}
| no = p.concat {{No}}
| if ({X}.equals {X}), yes, no
= Yes

(The above example would be even more persuasive if yes and no were constructed from user input, or a random selection, but those are more difficult to present as readable, automated test cases.)

Scripts as Classes

Classes can be defined within a script:

| Jonkers = {
|   IO.print {What?}
| }.create new
= What?

Note that create is a method on strings, and it takes a parameter, which in this case is the result of calling the method new (to be explained later.) The class itself has a "body" of code which is run when the class is defined, not unlike the situation in Ruby. This code can be used to set up class-level attributes:

| Jonkers = {
|   name = {Ulysses}
| }.create new
| IO.print Jonkers.name
= Ulysses

Normally a class will also have some methods, but we'll cover that later.

The fact that classes can be defined in a script, and that scripts are no different from classes, means that classes can be defined within a class:

| Jonkers = {
|   Fordible = {
|     extend IO
|     print {Sure}
|   }.create new
| }.create new
= Sure

Our demonstrations above show that a class has a "body" of code which is run when it is defined with create.

The block used to define a class, of course, is just a string, and can be a string variable.

| a = {extend IO; print {What?}}
| Jonkers = a.create new
| Jonkers.new
= What?

| {extend IO; print {Yes!}}.create new
= Yes!

What's happening here is actually this.

The new method, inherited from Object, creates a new, almost featureless object; its only feature is that it inherits from Object.

| a = new
| a.IO.print {A new object inherits IO from Object.}
= A new object inherits IO from Object.

The create method, inherited from String, runs its receiver, as a script, on the object passed to it. This may seem an odd usage of the word "create", as in our examples above, it's actually new that creates the object. But in English, the word "create" does sometimes have this meaning; for example, a royal subject can be "created a knight". And, in Velo, there is nothing preventing you from passing an existing object to create.

| Jonkers = {foo = {123}}.create new
| {bar = {456}}.create Jonkers
| IO.print Jonkers.bar
= 456

Aside on Syntax

We've been so busy describing these remarkable qualities of Velo that we haven't said much about the basic properties of its syntax. Some may be obvious from the examples, but there are probably points worth clarifying here.

A Velo script is simply a list of expressions, seperated by end-of-line markers, with a few qualifications:

  • A sequence of linefeeds and carriage returns is an end-of-line marker.
  • The token ; is also considered an end-of-line marker.
  • A series of end-of-line markers, possibly with intervening whitespace, is considered a single end-of-line marker.
  • An end-of-line marker can optionally occur after the tokens (, =, and ,, without terminating the expression.
  • An end-of-line marker can optionally appear before any expression in a script, so that blank lines can appear at the start of a script.

Some examples of these properties follow.

| IO.print {Hi}; IO.print {there}
= Hi
= there

| 
| 
| IO.print {Hi}
| 
| 
| IO.print {there}
= Hi
= there

| IO.print (
|   {Hi there})
= Hi there

| a =
|   {Hi there}
| IO.print a
= Hi there

| if {true},
|   {IO.print {Yes}},
|   {IO.print {No}}
= Yes

A method call is followed by a list of arguments seperated by commas. (You saw this above with the if method.) Velo does not statically record the arity of a method, so you can pass any number of arguments that you want (but of course, the method may fail if it is not given the number it expects.) The parser tells when a method call ends by the fact that there are no more commas (it instead ran into a ) or an end-of-line marker or the end of the file.)

Method calls can be chained:

object.method.another.yetmore

Parentheses can be used to disambiguate when there are arguments in the method calls in the chain:

object.method a, object.method b, c
object.method a, (object.method b, c)
object.method a, (object.method b), c

Lines 1 and 2 above are equivalent, but line 3 is different.

Now, about those Methods

Typically, a class will define some methods. (For now, let's think of them as class methods.)

| Jonkers = {
|   announce = {
|     IO.print {This is }.concat {Maeve}
|   }.method
| }.create new
| Jonkers.announce
= This is Maeve

Which means a script can have methods.

| announce = {
|   IO.print {This is }.concat {Vern}
| }.method
| announce
= This is Vern

Note that, unlike Ruby, this method is actually defined on the script. (When a method is defined at the toplevel in Ruby, it is actually placed in Kernel, and placed on Object as a private method. There is probably some historical or byzantine architectural reason for this, but it struck me as quite bizarre when I learned about it.)

So, yeah, method is a method on strings too, just like create. It takes no arguments.

But, methods can have arguments, when called. In their definition, the first argument is referred to by #1, the second by #2, etc.

| announce = {
|   IO.print {This is }.concat #1
| }.method
| announce {Raina}
= This is Raina

The block used to define a method is, of course, just a string.

| a = {IO.print {This is }.concat #1}
| announce = a.method
| announce {Naoko}
= This is Naoko

A method may be recursive.

| count = {
|   temp = #1
|   if (temp.equals {XXXXXX}), { IO.print {Done!}}, {
|     IO.print temp
|     count temp.concat {X}
|   }
| }.method
| count {X}
= X
= XX
= XXX
= XXXX
= XXXXX
= Done!

Note, however, that a method is not a Velo object, at least not in this early version of Velo. The only operation that is defined on the result of calling the method method on a string, is assigning it to an attribute of an object, from whence it can be called. Trying to do anything else to it (pass it to another method, for example) is not defined.

Instantiation

Classes can be instantiated. The most straightforward way to do this is to use the new method we already discussed; in truth, it takes an optional argument, and if this is given, the new object extends the object passed to new.

| Jonkers = {
|   announce = {
|     IO.print {This is }.concat #1
|   }.method
| }.create new
| j = new Jonkers
| j.announce {Jamil}
| k = new Jonkers
| k.announce {Brian}
= This is Jamil
= This is Brian

This usage of new is just a shortcut, because you can always extend the new object yourself.

| Jonkers = {
|   announce = {
|     IO.print {This is }.concat #1
|   }.method
| }.create new
| j = new; j.extend Jonkers
| j.announce {Jamil}
= This is Jamil

Instances of classes have their own attributes, but obtain anything they might be missing, from the class.

| Jonkers = {
|   name = {Cheryl}
|   announce = {
|     IO.print {This is }.concat name
|   }.method
| }.create new
| 
| j = new Jonkers
| j.announce
| k = new Jonkers
| { name = {David} }.create k
| k.announce
= This is Cheryl
= This is David

We said { name = {David} }.create k above because, when the test was written, we couldn't say simply k.name = {David} yet. Now we can, so let's try that:

| Jonkers = {
|   name = {James}
|   announce = {
|     IO.print {This is }.concat name
|   }.method
| }.create new
| 
| j = new Jonkers
| j.announce
| k = new Jonkers
| k.name = {Joyce}
| k.announce
= This is James
= This is Joyce

Given what you see above, you might be wondering exactly the difference between classes and objects is. Well...

Delegation

We've been talking about classes as if they were a distinct language construct, but really, a class is just a relationship between objects.

Velo uses prototype-based object-orientation. Each object has a list of parent objects; these are its classes. But they're just objects.

When a method is called on an object, if that method is not defined on that object, its parent objects are checked for that method; if any are found, that method on the parent is called, but with the target object as "self".

| Jonkers = {
|   extend IO
|   announce = {
|     print {This is }.concat #1
|   }.method
| }.create new
| Jeepers = {
|   extend IO
|   greet = {
|     print {Hello, }.concat #1
|   }.method
| }.create new
| Jeepers.extend Jonkers
| 
| j = new Jeepers
| j.announce {Luke}
| j.greet {Luke}
= This is Luke
= Hello, Luke

(We had to say Jeepers.extend Jonkers in the above, instead of saying extend Jonkers in the definition of Jeepers, because inside that definition, Jonkers was not in scope. This would be a nice thing to fix...)

When you say extend, you are just adding another object to the list of parent objects for a class.

If a method is not found on an object, nor any of its parent objects, it is looked for on the built-in object Object. (If it is not there either, an exception is thrown.)

In fact, extend is itself a method on Object. When it is executed, it evaluates its string parameter to obtain an object, and adds that object to the list of parent objects of the current object.

Since scripts are no different from classes, a script can extend a class that it defines:

| Jonkers = {
|   extend IO
|   announce = {
|     print {This is }.concat #1
|   }.method
| }.create new
| extend Jonkers
| announce {Ike}
= This is Ike

The class doesn't even have to be given a name.

| extend {extend IO; p = {print #1}.method}.create new
| p {Hello!}
= Hello!

Multiple Inheritance

Because extend can be called as many times as you like on an object, an object can inherit from (delegate to) as many classes as you like.

For multiple inheritance, the method resolution order follows the source code order; the objects added as parent objects by more recently executed extends are searched before those added by earlier executed extends.

| Jonkers = {
|   foo = { IO.print {fourteen} }.method
| }.create new
| Jeepers = {
|   foo = { IO.print {twenty-nine} }.method
| }.create new
| 
| Jeskers = {
|   bar = { foo }.method
| }.create new
| Jeskers.extend Jonkers
| Jeskers.extend Jeepers
| 
| j = new Jeskers; j.bar
| 
| Jofters = {
|   bar = { foo }.method
| }.create new
| Jofters.extend Jeepers
| Jofters.extend Jonkers
| 
| j = new Jofters; j.bar
= twenty-nine
= fourteen

self

As you've noticed, Velo has an "implicit self" -- invoking a method just invokes it on the current self (which may be the script.) But sometimes you need to explicitly refer to self, for example, to pass it to some other method.

For this purpose, the Object object provides the method self which simply returns the object it is called on. Since all objects effectively "inherit" (read: delegate to, when all other options are exhausted) from Object, they can all use this "explicit self".

| a = {X}
| IO.print a.equals(a.self)
= true

| McTavish = {
|   bar = { a = #1; a.hey }.method
| }.create new
| Jeskers = {
|   bar = { a = #1; a.bar self }.method
|   hey = { IO.print {Hey!} }.method
| }.create new
| Jeskers.bar McTavish
= Hey!

Appendix

(This is all very slapdash right now.)

Summary of methods on Object

  • extend STRING
  • self
  • new ...
  • Object, String, IO, and all other predefined classes

Summary of methods on String

  • if STRING, STRING
  • while STRING
  • create
  • method STRING
  • concat STRING
  • eval STRING

Summary of methods on IO

  • print STRING
  • input

Grammar

Velo ::= {[EOL] Expr EOL}.
Expr ::= Name "=" [EOL] Expr
       | Expr {"." [EOL] Name} [Expr {"," [EOL] Expr}]
       | Name
       | "(" [EOL] Expr ")"
       | StringLiteral
       .

Future Work

  • Unify scripts and strings. (A script is just a string, after all.)
  • Unify methods and scripts. (A method is just a script, after all.)
  • Unify scripts and modules. (A module is just a script, after all.) Unfortunately, the Falderal format doesn't make this easy to illustrate; but there is no reason that we shouldn't be able to load external files as objects.