When comparing Crystal vs ATS, the Slant community recommends Crystal 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?” Crystal is ranked 28th while ATS is ranked 49th. The most important reason people chose Crystal is:
Native efficiency.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Compiles to native code
Native efficiency.
Pro Ruby-like syntax
Pro Very easy bindings to existing C libraries
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 compatible with Windows
How ridiculous is it that a general-purpose language doesn't support the most general PC operating system on the market?
Con Not many third party libraries available yet
Con No Windows version
But it does run on Cygwin.