When comparing F# vs Red, the Slant community recommends F# for most people. In the question“What is the best programming language to learn first?” F# is ranked 30th while Red is ranked 40th. The most important reason people chose F# is:
F#'s syntax tends to be terse while remaining very readable and easy to understand without being a chore to write.
Specs
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Pros
Pro Concise syntax
F#'s syntax tends to be terse while remaining very readable and easy to understand without being a chore to write.
Pro Easier transition from other paradigms
Since F# is not a purely functional language, it lends itself to being more easily picked up by programmers that have experience with other paradigms.
Pro .NET Interoperability
Since F# runs on the Common Language Runtime or CLR, it has access to the entire .NET Framework, as well as libraries written in other .NET languages such as C#, VB.NET, and C++/CLI.
Pro Multiplatform, it runs on .Net Core
Forget Xamarin and Mono. F# now runs on the multiplatform .Net Core!
Pro You can run F# in the browser
Thanks to a project like Fable, you can write and run your client app in F# and run it over JavaScript. It allows you to share code between your server and client.
Pro Natively supported by Visual Studio
This is a .Net language natively supported by Visual Studio. Though it is not as tooled up as C# the support is still substantial. In particular, C# deployment scenarios can be enabled for it with small C# wrapper projects. Integration with Visual Studio provides: IntelliSense, debugging, projects an other features.
Pro Fall into the pit of success
F# directs you into a workflow where the right way is the path of least resistance.
Coming from a C# background, its restrictions might feel arbitrary at first (e.g., what do you mean my code has to be in dependency order!? Arbitrary alphabetical or bust!), but you'll soon realize that your code is cohesive, concise and consistent in a way that it never was before - and you can compile and run with confidence!
Pro F# is supported by Xamarin
Thanks to Xamarin, F# is a functional language that you can use to build for iOS, Android and Windows.
Pro Simple toolchain
Other languages have complex, multi-step setups that beginners often get stuck on. Red has no installer, no setup, no dependencies*, just a single small (~1MB) command-line executable with both the compiler and repl. On Windows, you don't even have to launch executable from the command line--it has a GUI-console.
Pro Very simple syntax
Red syntax is a lot like Rebol. It's easier than most languages for beginners to pick up.
Pro Both low and high-level
Red has low enough access to do systems programming, but it's expressive enough for high-level scripting.
Pro Low cognitive load
Red has very simple syntax that's easy to learn. It gets out of your way and lets you think about the problem instead, enhancing productivity.
Cons
Con Has no ad-hoc polymorphism (á la Type Classes)
You have generics, you have interfaces, you have inheritance, you have a lot of things at your disposal but you don't have Type Clases. They can be emulated using some clever constructs but there's nothing like having the real thing.
Con Infested with OOP
Still relying on .NET OOP libraries.
Con Not production ready
Red is still under development and not considered stable.
Con Still in beta
It mostly works. It's good enough for building usable applications, but some planned features are missing.