Skip to main content

Late Binding in Programming Languages

Late binding is key to enhanced productivity in programming languages. I believe that this is the single most important reason why so-called dynamic typed languages are so popular.

This note is part of an ongoing ‘language design’ series which aims to look at some key aspects of programming language design.

What do we mean by late binding?


Simply put, a programmer should not have to say more than they mean at any particular time.

To see what I mean, consider a function that computes a person's name from a first and last name. In Star, I can write this:

fullName(P) is P.firstName()++P.lastName()

This constitutes a complete definition of the function: there is no need to declare types; furthermore this function will work with any type that has a first and last name.

Contract this with a typical well-crafted Java solution:

boolean fullName(Person P){
return P.firstName()+P.lastName();
}

Not so different one might argue.

Except that we have had to define a type Person; at best this is an interface and not a class. The Java fullName method will only work with objects of type Person.

In designing the Java version we have to find and/or create a type Person. In addition, we must make sure that all classes that we want to compute the full name of implement the Person type.

This last aspect can be a real productivity killer. Suppose that we want to be able to compute the full name of many different kinds of objects; then we must arrange them all to implement the Person type. This may not even be possible if the classes in question are from a library that you don't have the source to.

Late Binding does not mean dynamic typing


One of the common perceptions is that you lose the safety of a type checker if you want to allow late binding. This is not true; at least, it is not true for Star.

Star is a statically typed language which supports a range of type constraints. In the case of the fullName function, the constraint is that the type of P has the firstName and lastName attributes. (The details of how this is done are too gory to go into here.)

These constraints can be checked; so, for example, every time the fullName function is used on an argument, the checker can verify that the type of the argument is consistent with having a first and last name. This check can be performed at compile-time.

It is even possible (necessary) to go one step further and allow generic functions to use the fullName function.

Other forms of Late Binding



Late binding shows up in other ways. For example, when specifying an imported library of some form or other, it should be possible to declare the requirement for a library in terms of what is needed, rather than the name of the library. I.e., instead of:

import java.util.List;
import java.util.ArrayList;

we should be able to say:

require List of integer;

or some such expression.

The difference is that the former says — in the source code — which package to import whereas the latter merely declares a contract requirement. How the contract is fulfilled is a separate step; one that a smart compiler system may even be able to automate.

(Some language do work this way. Typically LISP language systems organize their modules in terms of requires and provides.)

Take Away


It can be hard to know what features should go into a programming language. Having a few principles to guide us make the task of designing a language more tractable.

In this case, the message is that programmers should be able to program in terms of their requirements, not the compiler's. This need not come at the expense of safety or of performance.

Popular posts from this blog

Concept Oriented Markup

I have long been frustrated with all the different text mark up languages and word processors that I have used. There are many reasons for this; but the biggest issue is that markups (including very powerful ones like TeX) are not targeted at the kind of stuff I write.

Nowadays, it seems archaic to still be thinking in terms of sections and chapters. The world is linked and that applies to the kind of technical writing that I do.

I believe that the issue is fundamental. A concept like "section" is inherently about the structure of a document. But, what I want to focus on are concepts like "example", "definition", and "function type".

A second problem is that, in a complex environment, the range of documentation that is available to an individual reader is actually composed of multiple sources. Javadoc exemplifies this: an individual library may be documented using Javadoc into a single HTML tree. However, most programmers require access to multiple…

Robotic Wisdom

It seems to me that one of the basic questions that haunt AI researchers is 'what have we missed?' Assuming that the goal of AI is to create intelligence with similar performance to natural intelligence; what are the key ingredients to such a capability?

There is an old saw
It takes 10,000 hours to master a skill
There is a lot of truth to that; it effectively amounts to 10 years of more-or-less full-time focus. This has been demonstrated for many fields of activity from learning an instrument, learning a language or learning to program.

But it does not take 10,000 hours to figure out if it is raining outside, and to decide to carry an umbrella. What is the difference?

One informal way of distinguishing the two forms of learning is to categorize one as `muscle memory' and the other as 'declarative memory'. Typically, skills take a lot of practice to acquire, whereas declarative learning is instant. Skills are more permanent too: you tend not to forget a skill; but it is…

Comments Should be Meaningless

This is something of a counterintuitive idea:
Comments should be meaningless
What, I hear you ask, are you talking about? Comments should communicate to the reader! At least that is the received conventional wisdom handed does over the last few centuries (decades at least).

Well, certainly, if you are programming in Assembler, or C, then yes, comments should convey meaning
because the programming language cannot
So, conversely, as a comment on the programming language itself, anytime the programmer feels the imperative to write a meaningful comment it is because the language is not able to convey the intent of the programmer.

I have already noticed that I write far fewer comments in my Java programs than in my C programs.  That is because Java is able to capture more of my meaning and comments would be superfluous.

So, if a language were able to capture all of my intentions, I would never need to write a comment.

Hence the title of this blog.