Smalltalk is an object-oriented, dynamically typed, reflective programming language. It was designed and created in part for educational use.
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Pro Environment of live objects
You can modify the system as it's running. You're "swimming with the fish", instead of probing a black box by remote control.
Pro Inspector makes objects transparent
Programmers must make detailed mental models of the system they are developing. Bugs usually happen when the mental model does not match the actual system. This is one of the greatest difficulties beginners have because most systems are so opaque. It takes a lot of effort to see what's really going on. But in Smalltalk this is much easier, thanks to the powerful tools included in the environment, like the object inspector.
Pro Easy to learn and experiment
Pro Inspired many other languages' object systems
Pro Agile test-driven development
Smalltalk had the original (and still the best) unit test system that inspired it in many other languages (like Java's JUnit). This makes it easy to teach and learn the test-driven development style.
Pro Pure and easy object orientation
Smalltalk is one of few languages that are purely object oriented. This provides a solid and easy to understand base on which to learn object oriented programming, the most popular approach to programming.
Pro Graphical user interface
Beginners are usually stuck making command-line applications in other languages, because GUIs are too hard. Smalltalk GUIs are easy enough to start with.
Pro Internal source code and documentation
You can explore how everything works easily.
Pro Elegant syntax fits on a postcard
The syntax was designed to be easy enough for children to learn. Beginners can learn the language rules very quickly and then focus on programming without fighting the syntax at the same time. Things that have to be baked into the grammar in other languages are simple message sends with block arguments in Smalltalk.
Pro Powerful integrated debugger
You can edit code and swap it in while the your program is still running after an exception has already been signaled, or restart from anywhere in the call stack. You can inspect and modify the state of any object. Some Smalltalkers write unit tests and then program exclusively in the debugger.
Pro Save and restore virtual machine image
A Smalltalk environment can save its state and restore it later. This includes the internal state of live objects. This makes it easier for beginners to experiment without fear of breaking things.
Con Not common
Smalltalk and its implementations are no longer popular languages to use. Commercial development is scarce (but does exist), and online communities are relatively small. There is little to no modern books or resources made for the language, although older, very good books do exist.
Con Virtual machine in its own isolated world
Smalltalk wants to be the whole OS. While this has tremendous advantages internally, interacting with the world outside the VM is not so easy.