Pharo is an innovative, open, integrated and highly productive eco-system comprising language (forked from Squeak Smalltalk), modern libraries (extensive), very rich and integrated IDE, meta programming tools, cross platform virtual machine(s) and a small but growing and thriving community including academics and industry partners. Now at Release 5 (stable, March 2016) with Release 6 well under way in development community.
Pharo can be used for server and client programming. It is open with easily used networking, parsing, generating and persistence frameworks.
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Pro Live updates
The nature of Pharo being a "live" environment allows you to perform live updates to your system without requiring to restart it. You can upgrade/modify classes while serving requests at the same time.
The framework for developing sophisticated web applications in Smalltalk is developed in Pharo. Seaside lets you build highly interactive web applications quickly, reusably and maintainably.
In Pharo everything is an object. Compiler - object, network - object, method - also an object. And objects communicate with messages. No operators, no control-flow statements. Just objects and messages. Few things to learn, but you can learn OOP well.
Pro Highly productive
Pro Can run headless for production
Pro Easily learnt
There is good, free documentation including several books written by experts with extensive examples. There is an online MOOC. There are many tutorial videos. Supportive conferences and community. Even a professional support option if desired.
Pro Remote debugging
Pro Advanced code analysis tools
MOOSE environment provides extensive, easily leveraged and class leading tools for code analysis and improvement.
Pro Glamorous toolkit & GTInspector
Most languages are still copying the Smalltalk tools of yesterday - GTInspector (written in Glamorous) takes live exploration of code/running objects to a new level. It's really slick, and better yet, you can easily write your own inspectors in 10 lines of code.
Pro Beautiful coding patterns in your IDE
No need to search google for compact beautiful examples of how to do things, your live environment source is available and you can easily live search, see how it works and copy how the masters would do it (examples most languages still copy too).
Pro Really simple networking and REST with Zinc
Pro Graphics, graphing and visualisation framework - Roassal
Roassal and Mondrian provide fantastic and easily used frameworks for graphics, graphing and advanced visualisations (comparable to D3.js) but with much less code. Visualisations can be rendered into web friendly graphics (SVG, .png etc.) without additional work.
Pro Code can be run on rock solid GemStone environment
Con Small community
But they are very friendly and supportive. Best help comes through the mailing lists so not always easily googlable. There is also a Slack community where help is nearly instantaneous.
Con No 64 bit VM
Although 64 bit support is on its way, there is no official VM to work in a 64 bit system, requiring you to use 32 bit libraries.
Available in Pharo 5.