When comparing Haxe vs Haskell, the Slant community recommends Haskell for most people. In the question“What is the best programming language to learn first?” Haskell is ranked 26th while Haxe is ranked 32nd. The most important reason people chose Haskell is:
Haskell's referential transparency, consistency, mathematics-oriented culture, and heavy amount of abstraction encourage problem solving at a very high level. The fact that this is all built upon little other than function application means that not only is the thought process, but even concrete solutions are very transferable to any other language. In fact, in Haskell, [it's quite common for a solution to simply be written as an interpreter that can then generate code in some other language](http://programmers.stackexchange.com/questions/242795/what-is-the-free-monad-interpreter-pattern). Many other languages employ language-specific features, or work around a lack of features with heavy-handed design patterns that discourage abstraction, meaning that a lot of what is learned, and a lot of code that is needed to solve a particular problem just isn't very applicable to any other language's ecosystem.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Compiles to multiple platforms and languages
Haxe allows you to develop for Web, iOS, Android, Blackberry, Windows, OSX, Linux and others, all at once, without the need to switch languages and maintain separate code bases.
This is possible because Haxe can compile to JavaScript, ActionScript, Flash AVM bytecode, C++, Neko, PHP, C# (.NET) and Java.
Support for even more platforms and languages is under development.
Pro Powerfully expressive but easy to learn
The language was designed to be very expressive with the smallest possible amount of syntactic sugar. There are actually fewer keywords than other languages with similar power.
Pro Extremely fast compilation
Haxe can easily compile over 100,000 lines of code to JS in seconds on a mid-spec computer.
Pro Similar to JavaScript and ActionScript 3
The language is very easy to learn for those with background in JavaScript or ActionScript 3.

Pro Large library support. From servers to games.
Haxelib (common library repo) and other sources contain large codebases for anything from cryptography to communications. A lot of these are fully cross platform and work with the JavaScript target.
The JavaScript target can be used for everything from node.js server applications (with code completion) to games using either the Flash-like OpenFL library or direct canvas or WebGL programming.
Pro Established project
Haxe has been around for more than 10 years (since 2005) and whilst not the most popular project, has had continuous growth.
Highly unlikely to disappear or for support to stop.

Pro Friendly community
Friendly community
Pro Pick up errors at compile time
One big advantage over pure javascript, (or some other languages listed here) is that Haxe will pick up a whole range of errors when you compile, saving you the pain of having to try and debug them later. This includes everything from syntax errors ("Unexpected ;") to type errors ("Class user has no field username. Suggestion: username").
Pro First class code completion
Code completion is built into the compiler and available to the IDE allowing for much smarter code completion that can actually parse and understand the syntax tree.
Pro Small, readable output
The output that is generated can be trimmed using "dead code elimination" to only include those functions and libraries that are strictly necessary. All code is very readable with only minimal extras for specific functionality.
Small output is good for frontend development as file size is a major concern.
Pro Powerful type inference with strong typing
After a type is inferred from its context, it cannot be changed to a new type, and type safety is done at compile time so it stays safe without the extra maintenance required for static typing.

Pro Syntactic macros
Syntactic macros allow you to extend compiler features at the syntax tree step. Macros come into play after code is parsed into the abstract syntax tree, and macros allow you to transform it before the rest of the compilation completes.
This provides for immense power, while at the same time scoping the extensibility at a level that is powerful, but well constrained.
Pro Code reuse server side and client side
You can use the same classes on the server as you do on the client where applicable. This saves a lot of time.

Pro Ability to use existing JS libraries
Haxe has the ability to use "externs". These are haxe files which describe the usage of existing JS libraries. Get code completion and compile-time-checking for everything from jQuery to Node.js.
Even without externs, native JS code can still be used through untyped code.
Pro Can create complex applications without needing webpack, npm or other crutches
Haxe has the power and expression to not need the npm dependancy hell that is common in js and typescript, bit it's still simple.
Pro Algebraic data types and pattern matching.
Pro Offload execution to the server with remoting
Using a remoting proxy you can get type safe server to client communications, allowing for remote method execution on the server as if they were part of the client side code.
Pro Package management like Java
Package tree is just directory tree, it's wonderful!
Pro Builtin conditional compilation support
Haxe supports conditional compilation, so depending on compiler flags Haxe will include or exlcude sections of your code. Making it easy to have debug and release builds.
Pro Abstract enums allow constants with exhaustiveness check
You can define constants in an abstract enum and when used in a switch/case statement Haxe checks for exhaustiveness, making sure every constant is covered - with no runtime implication.
Pro Type safety for exísting JS libraries
Haxe compiler will check types when using externs for existing libraries.

Pro Available in NPM
Pro Ability to skip type checking when calling non Haxe code
You should use externs when calling non Haxe code, but if you just need to call one or two external JS functions, you can skip type checking by calling untyped code.
Pro Create without needing to be limited to a language, target, or commercial ecosystem
Pro Abstracts allows me to create more intative api's without runtime overhead
Pro Highly transferable concepts
Haskell's referential transparency, consistency, mathematics-oriented culture, and heavy amount of abstraction encourage problem solving at a very high level. The fact that this is all built upon little other than function application means that not only is the thought process, but even concrete solutions are very transferable to any other language. In fact, in Haskell, it's quite common for a solution to simply be written as an interpreter that can then generate code in some other language. Many other languages employ language-specific features, or work around a lack of features with heavy-handed design patterns that discourage abstraction, meaning that a lot of what is learned, and a lot of code that is needed to solve a particular problem just isn't very applicable to any other language's ecosystem.
Pro Forces you to learn pure functional programming
It is pure and does not mix other programming paradigms into the language. This forces you to learn functional programming in its most pure form. You avoid falling back on old habits and learn an entirely new way to program.
Pro Open source
All Haskell implementations are completely free and open source.
Pro Mathematical consistency
As Haskell lends itself exceedingly well to abstraction, and borrows heavily from the culture of pure mathematics, it means that a lot more code conforms to very high-level abstractions. You can expect code from vastly different libraries to follow the same rules, and to be incredibly self-consistent. It's not uncommon to find that a parser library works the same way as a string library, which works the same way as a window manager library. This often means that getting familiar and productive with new libraries is often much easier than in other languages.
Pro Referentially transparent
Haskell's Purely Functional approach means that code is referentially transparent. This means that to read a function, one only needs to know its arguments. Code works the same way that expressions work in Algebra class. There's no need to read the whole source code to determine if there's some subtle reference to some mutable state, and no worries about someone writing a "getter" that also mutates the object it's called on. Functions are all directly testable in the REPL, and there's no need to remember to call methods in a certain order to properly initialize an object. No breakage of encapsulation, and no leaky abstractions.

Pro Hand-writeable concise syntax
Conciseness of Haskell lets us to write the expression on the whiteboard or paper and discuss with others easily. This is a strong benefit to learn FP over other languages.
Pro Very few language constructs
The base language relies primarily on function application, with a very small amount of special-case syntax. Once you know the rules for function application, you know most of the language.
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 Functions curry automatically
Every function that expects more than one arguments is basically a function that returns a partially applied function. This is well-suited to function composition, elegance, and concision.
Pro Easy to read
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 Popular in teaching
Haskell is really popular in universities and academia as a tool to teach programming. A lot of books for people who don't know programming are written around Haskell. This means that there are a lot of resources for beginners in programming with which to learn Haskell and functional programming concepts.
Pro Easy syntax for people with a STEM degree
Since the basic syntax is very similar to mathematics, Haskell syntax should be easy for people who have taken higher math courses since they would be used to the symbols used in maths.
Pro Powerful categorical abstractions
Makes categorical higher order abstractions easy to use and natural to the language.
Cons
Con Bad support in some popular IDEs
While it has great support in Visual Studio Code and Vim for example, it still lacks support in some IDEs such as IntelliJ.
Con You need to code interfaces to work with existing JavaScript code
Some popular libs like JQuery have maintained externs, for any specific code or lib already in JS you have to write the externs to use it in your haxe application.
Con No Qt support
There is currently no support for Qt.
Con Full programs only
You can create small utility functions with Haxe, but generally it is a lot more work than with other JS compilers. Haxe is best used when you have a larger project.
Con It's not easy to convince people it's as good as it really is unless you can get them to really try it
Con Language extensions lead to unfamiliar code
Haskell's language extensions, while making the language incredibly flexible for experienced users, makes a lot of code incredibly unfamiliar for beginners. Some pragmas, like NoMonomorphismRestriction, have effects that seem completely transparent in code, leading beginners to wonder why it's there. Others, like ViewPatterns, and particularly TemplateHaskell, create completely new syntax rules that render code incomprehensible to beginners expecting vanilla function application.
Con Difficult learning curve
Haskell lends itself well to powerful abstractions - the result is that even basic, commonly used libraries, while easy to use, are implemened using a vocabularly that requires a lot of backround in abstract mathematics to understand. Even a concept as simple as "combine A and B" is often, both in code and in tutorials, described in terms of confusing and discouraging terms like "monad", "magma", "monoid", "groupoid", and "ring". This also occasionally bears its ugly head in the form of complicated error messages from type inference.
Con Package manager is unstable & lacking features
Cabal (There are other choices but this is the most popular) can not uninstall a package. Also working at a few locations it is difficult to have the same environment for each one be the same.

Con You have to learn more than just FP
Haskell is not only a functional language but also a lazy, and statically typed one. Not only that but it's almost necessary to learn about monads before you can do anything useful.
Con Symbols everywhere
Haskell allows users to define their own infix operators, even with their own precedence. The result is that some code is filled with foreign looking operators that are assumed to be special-case syntax. Even for programmers who know they're just functions, operators that change infix precedence can potentially break expectations of how an expression is evaluated, if not used with care.
Con Obscure ugly notation
0 = 1
Using "=" like this: <code>
-- Using recursion (with pattern matching)
factorial 0 = 1
factorial n = n * factorial (n - 1) </code> Example from https://en.wikipedia.org/wiki/Haskell_(programming_language)
is quite simply annoying aesthetics.
Con Documentation for most packages is short and lacking
A few Haskell packages are well documented but this is the exception, not the rule.
Most of the time a list of function signatures is what passes for documentation.
Con Too academic, hard to find "real world" code examples
Con You need some time to start seeing results
Haskell's static typing, while helpful when building a project, can be positively frustrating for beginners. Quick feedback for errors means delaying the dopamine hit of code actually running. While in some languages, a beginner's first experience may be their code printing "Hello World" and then crashing, in Haskell, similar code would more likely be met with an incomprehensible type error.
Con Lazily evaluated
Haskell's lazy evaluation implies a level of indirection - you're not passing a value, you're passing a thunk. This is often difficult to grasp not just for beginners, but for experienced programmers coming from strictly evaluated languages. This also means that, since for many, strict evaluation is their first instinct, initial expectations of a function's performance and complexity are often broken.
Con Only pure functional programming
Not proper functional programming but a subset of the style called pure functional programming.
Con Curried type signatures obfuscate what were the in and out types originally
