Gilad Bracha - The Dart Programming Language

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Gilad Bracha

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Library of Congress Control Number: 2015953614

Copyright 2016 Pearson Education, Inc.

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ISBN-13: 978-0-321-92770-5
ISBN-10: 0-321-92770-2
Text printed in the United States on recycled paper at RR Donnelley in Crawfordsville, Indiana.
First printing, December 2015

To my mother, Shoshana,
who taught me to be picky.

In the early spring of 2006, I wrote a short blog post called Gilad is Right where, as a recovering typaholic, I admitted that Gilads idea of optional and layered type systems, where static types cannot change the runtime behavior of the program and do not prevent an otherwise legal program from compiling or executing, was a necessary design trade-off for programming languages aimed at millions of developers. At that time I was working on Visual Basic, which already supported a form of optional typing by means of the Option Strict Off statement, but that feature was under heavy fire from static typing proponents. Type systems are often highly non-linear and after a certain point their complexity explodes while adding very little value to the developer and making life miserable for the language implementors. Optional and layered type systems enable a much more gradual approach by allowing strong static typing to coexist peacefully with dynamic typing. Now nearly a decade later, the vision Gilad pioneered has become mainstream under the name gradual typing. Many programming languages that have been created in the last few years, such as Hack, TypeScript, Flow, Racket, and of course Dart, are gradually typed. Even academics have embraced the idea and write papers about it with frivolous titles that include words such as threesomes and blame.

Another pragmatic aspect of Dart, but one that language purists have not yet accepted, is the fact that the Dart type system is deliberately unsound. In normal English this means that the Dart type checker will not flag certain type errors at compile time, but relies on runtime checking instead to ensure type safety. The main source of type unsoundness in Dart is covariant generics. To explain what variance is, lets first look at a vending machine from which we can only take drinks. If a cafeteria requires a vending machine with soda pop, we can legally install a vending machine that dispenses root beer since root beer is a type of soda pop (but it is illegal to install a vending machine for soda pop where a vending machine for root beer is required). In programming language speak we say that vending machines are covariant. Next lets take a look at garbage cans into which we can throw only garbage. If a cafeteria requires a garbage can for recyclables, we can legally install a garbage can for trash since recyclable garbage is a type of trash (but it is illegal to install a garbage can for recyclables where a garbage can for trash is required). In programming language speak we say that garbage cans are contravariant. If you are a little puzzled about contravariance you are not the only one, and you will appreciate Darts decision to make all generic types covariant. The consequence of that choice is that if you need a garbage can for trash, you can legally install a garbage can for recyclables, but that garbage can will reject all non-recyclable trash that people are trying to dump in it. While theoretically unsound, unsafe variance actually feels rather natural for most developers, and I applaud the choice the Dart designers made here. As anyone that has struggled with ? super and ? extends can attest, languages that have chosen in favor of static type safety for generics do so at the expense of their users.

The Dart language designers made additional pragmatic choices that make coding in Dart a smooth experience. For example Dart has no interfaces, abstract base classes, or normal classes. Instead Dart only has classes that can be used as interfaces by implementing them, or used as base classes by extending them, or have their implementation reused by mixing them in. Every type in Dart is an object, so there is no difference between primitive (e.g., numeric) types and regular object types. Even though everything in Dart is an object, it is possible to define top-level functions and variables, so one no longer needs the dreaded public static void main incantation inside a top-level class to get going. Dart allows user-defined arithmetic operators, but does not support type-based overloading of methods. This significantly simplifies the language. In other programming languages that do support type-based overloading, the exact semantics of that feature often take up an unjustifiably large fraction of the language specification. Null-aware operators (even