When comparing Vala vs ATS, the Slant community recommends ATS for most people. In the question“What are the best systems programming languages?” ATS is ranked 16th while Vala is ranked 17th. 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.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Rich GTK Support
Vala developed by Gnome Foundation.
Pro Fast
Underneath there is C. It makes some stuff easier to write than it would be with plain C code.
Pro Easy Syntax
Vala is syntactically similar to C# and includes several features such as: anonymous functions, signals, properties, generics, assisted memory management, exception handling, type inference, and foreach statements
Pro C compatible API (and ABI)
C programs can use libraries written in Vala, and vice versa.
Pro Good documentation
There is Valadoc and most of the libraries have Devhelp packages.
Pro Well-integrated with C language
Vala itself is compiled to C, therefore it can use the vast ecosystem of C language, with least effort.
Pro Stable ABI
You can write a library with ABI stability.
Pro A general purpose language
Vala is compiled to C and only requires GLib - or even nothing (posix profile)
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 Very few resources allocated to the language
Only a single person is working on Vala full time. Gnome Foundation isn't allocating sufficient resources to properly maintain Vala.
Con A very naïve approach to the real problem
Vala just adds more complexity to the game. No real benefits in using a 'programming language emulator' for UI code, you may think it adds to productivity but it's just a way to make the problem last longer, reinventing the wheel everytime and worse.
Con Very limited adoption
Vala isn't used in areas other than GTK development, and no major business has adopted the language.
Con Not a general purpose language
Can't use Vala in multiple software development areas.
Con Very limited tooling
Though using Gnome Builder, you can debug it easily.
Con Heavy reliance on GObject
Heavy reliance on GObject types, although Vala can be used without GObject. (posix profile)
Con Not well documented
It already has tons of documentation, even a book about Vala.https://leanpub.com/vala https://wiki.gnome.org/Projects/Vala/Documentation
Con No stable version
No stable version of Vala released for over a decade so far. Means that Vala isn't production ready.
Con No Windows version
But it does run on Cygwin.
