When comparing Cor vs Opa, the Slant community recommends Opa for most people. In the question“What are the best languages that compile to JavaScript? ” Opa is ranked 30th while Cor is ranked 42nd. The most important reason people chose Opa is:
Opa uses type inference to decide what code runs on the client or the server, so you can organize code how you like, but still be sure that it runs where it needs to.
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Pros
Pro Easy to debug
Cor supports source-maps, allowing you the easy debugging in major browsers. However a line of Cor source is compiled to the exact line number in JavaScript for accurated debugging when developing in platforms that doesn't supports source-maps such as servers.
Pro Clean syntax
Cor enables you to write large applications by providing a clean syntax, classes and a modular architecture to keep organized code, enforcing the writing of readable source code based on conventions.
Pro Cross platform
Cor compiles to plain JavaScript, so, to run it in Node.js or in the browser is piece of cake.
Pro Support coroutines
Cor support coroutines which can be chained, stopped, and synchronized, fitting very well into the web asynchronous world.
Pro Hot reload
Cor provides a smooth front-end development by furnishing a builtin hot-loader which resolves dependences and compiles source code on the fly, with just reloading the Web page. You will only need to use CLI tools to deliver a production-ready version of the app.
Pro Concurrent and parallel
Cor allows to synchronize coroutines by passing messages through channels, and supports the execution of many tasks in parallel, all of that by writing sequential code.
Pro Type Inferred
Opa uses type inference to decide what code runs on the client or the server, so you can organize code how you like, but still be sure that it runs where it needs to.
Pro Familiar Javascript Syntax
Opa's syntax is very similar to JavaScript, making it an easy choice for developers who are used to developing in JavaScript.
Pro Client and Server in the same language
Cons
Con Work in progress
Cor is still a very much young project (as of November 2015) with just one contributor, few stars on GitHub and virtually no learning resources outside the official documentation.
Con Not popular
Opa is not as popular as other languages which compile to JavaScript. Making it harder to find learning resources or even answers if you get stuck somewhere.