When comparing Haste vs Haskell (via GHCJS), the Slant community recommends Haste for most people. In the question“What are the best languages that compile to JavaScript? ” Haste is ranked 19th while Haskell (via GHCJS) is ranked 28th. The most important reason people chose Haste is:
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.
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Pros
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
Pro Terse
Haskell is a very terse language, particularly due to its type inference. This means there's nothing to distract from the intent of the code, making it very readable. This is in sharp contrast to languages like Java, where skimming code requires learning which details can be ignored. Haskell's terseness also lends itself to very clear inline examples in textbooks, and makes it a pleasure to read through code even on a cellphone screen.
Pro Quick Feedback
It's often said that, in Haskell, if it compiles, it works. This short feedback loop can speed up learning process, by making it clear exactly when and where mistakes are made.
Pro All of Haskell, with the same tools you're used to
No need to learn new syntax or semantics, and no need to install and learn a bunch of new tools - it's just GHC.
Pro Provides seamless concurrency, without the need for promises or callbacks
Pro Built-in REPL which lets you interact with your GHCJS compiled web page
Cons
Con Lacks some minor Haskell functionality
Lacks support for Template Haskell.
Con The generated code can be difficult to debug
It should be mentioned that since the execution model of Haskell is very different from common imperative languages (lazy evaluation), the generated code doesn't resemble the source code at all, so it can be very difficult to debug.
Con Large runtime
GHCJS supports the entire Haskell runtime, the Javascript it outputs tends to be quite large. This is in contrast to options such as Fay, which save some overhead by not supporting some features such as multi-threading.
