When comparing Red vs ATS, the Slant community recommends ATS for most people. In the question“What are the best (productivity-enhancing, well-designed, and concise, rather than just popular or time-tested) programming languages?” ATS is ranked 49th while Red is ranked 57th. The most important reason people chose ATS is:
ATS has a theorem-proving type system powerful enough to prove that its functions meet their specifications. This happens at compile time with no performance impact at runtime. This can be used to prove that an ATS program doesn't have bugs commonly found in C++ programs, like "this function never leaks memory" or "this program never attempts to divide by zero" or "this buffer never overflows" or to verify pointer arithmetic, etc.
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Pros
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.
Pro Built-in formal specification
ATS has a theorem-proving type system powerful enough to prove that its functions meet their specifications. This happens at compile time with no performance impact at runtime. This can be used to prove that an ATS program doesn't have bugs commonly found in C++ programs, like "this function never leaks memory" or "this program never attempts to divide by zero" or "this buffer never overflows" or to verify pointer arithmetic, etc.
Pro Free and open-source compiler
The compiler (ATS/Postiats) is GPLv3.
Pro Functional programming
The syntax is ML-like with the usual functional language features like pattern matching and tail-call optimization.
Pro High-performance systems language
ATS works as a low-level systems language. ATS programs have performance and footprint comparable to programs written in C/C++.
Pro Good module system
Similar to Modula-3. This makes ATS a viable choice even for large-scale projects.
Pro Safe concurrency
ATS can prove its concurrent programs have no deadlocks or race conditions.
Cons
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.
Con No Windows version
But it does run on Cygwin.