When comparing Scala vs Haste, the Slant community recommends Scala for most people. In the question“What are the best Functional languages to learn for web-frontend development?” Scala is ranked 6th while Haste is ranked 11th. The most important reason people chose Scala is:
The immutable values make it perfect for working with concurrency
Specs
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Pros

Pro Immutable values
The immutable values make it perfect for working with concurrency
Pro Multiparadigm
Scala supports both Functional and Object Oriented styles of programming. Beginners can learn both paradigms without having to learn a new language, and experts can switch between the two according to what best suits their needs at the time.
Pro Type inference
Scala offers type inference, which, while giving the same safety as Java's type system, allows programmers to focus on the code itself, rather than on updating type annotations.
Pro Compiles to JVM bytecode
Aside from Java itself, Scala is by far the most popular of the many JVM languages. If you're developing for Android, or a similar JVM-only platform, or otherwise need out-of-the-box cross-platform compatibility, but the performance of a compiled language, Scala is the way to go.
Pro Very good online courses
On coursera you can find great introduction to Scala by Martin Odersky.
Pro Type inference leads to a simpler syntax
Pro Expressive functional programming abstraction for reusable and safe code
Pro Client and Server as the same application
Haste was designed to allow both the client and server to be written as parts of the same, type-safe application. This is in stark contrast to most other options, where the client and server are considered two separate entities, resulting in extra manual validation code and more chances for type errors.

Pro Almost full power of Haskell
Haste supports the Haskell 2010 standard except for Template Haskell as well as most GHC extensions.

Pro Automatic, type safe program slicing
Haste lets you write client and server as a single program, automatically generating code for the server as well as the client, giving you full type safety even across the Internet.
Pro Generates small, reasonably performant code
Cons

Con Can be intimidating for beginners
Scala is an industrial language. It brings functional programming to the JVM, but not with a "start small and grow the language" perspective, but rather a very powerful language for professional programmers.
Con Way too complex for beginners
Even for seasoned programmers it's a difficult language.
Con Static type system inherits cruft from Java
The type system is too complicated yet still less powerful than Haskell's.
Con Lacks some minor Haskell functionality
Lacks support for Template Haskell.
