Haskell is a purely functional programming language. In imperative languages you get things done by giving the computer a sequence of tasks and then it executes them. While executing them, it can change state. In purely functional programming you don’t tell the computer what to do as such but rather you tell it what stuff is. The factorial of a number is the product of all the numbers from 1 to that number, the sum of a list of numbers is the first number plus the sum of all the other numbers, and so on. You express that in the form of functions. In purely functional languages, a function has no side-effects. The only thing a function can do is calculate something and return it as a result. At first, this seems kind of limiting but it actually has some very nice consequences: if a function is called twice with the same parameters, it’s guaranteed to return the same result. That’s called referential transparency and not only does it allow the compiler to reason about the program’s behavior, but it also allows you to easily deduce (and even prove) that a function is correct and then build more complex functions by gluing simple functions together.
Haskell is lazy. That means that unless specifically told otherwise, Haskell won’t execute functions and calculate things until it’s really forced to show you a result. That goes well with referential transparency and it allows you to think of programs as a series of transformations on data. It also allows cool things such as infinite data structures. Say you have an immutable list of numbers xs = [1,2,3,4,5,6,7,8] and a function doubleMe which multiplies every element by 2 and then returns a new list. If we wanted to multiply our list by 8 in an imperative language and did doubleMe(doubleMe(doubleMe(xs))), it would probably pass through the list once and make a copy and then return it. Then it would pass through the list another two times and return the result. In a lazy language, calling doubleMe on a list without forcing it to show you the result ends up in the program sort of telling you “Yeah yeah, I’ll do it later!”. But once you want to see the result, the first doubleMe tells the second one it wants the result, now! The second one says that to the third one and the third one reluctantly gives back a doubled 1, which is a 2. The second one receives that and gives back 4 to the first one. The first one sees that and tells you the first element is 8. So it only does one pass through the list and only when you really need it. That way when you want something from a lazy language you can just take some initial data and efficiently transform and mend it so it resembles what you want at the end.
Haskell is statically typed. When you compile your program, the compiler knows which piece of code is a number, which is a string and so on. That means that a lot of possible errors are caught at compile time. If you try to add together a number and a string, the compiler will whine at you. Haskell uses a very good type system that has type inference. That means that you don’t have to explicitly label every piece of code with a type because the type system can intelligently figure out a lot about it. If you say a = 5 + 4, you don’t have to tell Haskell that a is a number, it can figure that out by itself. Type inference also allows your code to be more general. If a function you make takes two parameters and adds them together and you don’t explicitly state their type, the function will work on any two parameters that act like numbers.
Haskell is elegant and concise. Because it uses a lot of high level concepts, Haskell programs are usually shorter than their imperative equivalents. And shorter programs are easier to maintain than longer ones and have less bugs.