How difficult is it to learn functional programming languages? [closed]

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Given that a persona has extensive experience in different imperative programming languages right from C to JAVA. Whats the learning curve for functional programming languages such a Haskell ?

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Before taking on Haskell you may find it easier to learn something that isn't purely functional, like a Lisp or ML. Given your Java experience, Clojure, a Lisp that runs on the JVM, might be a good place to start.

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Like the OP, I came from an OOP/imperative background before jumping to functional programming around 2006ish.

I had lots of C# and Java under my belt, and wanted to learn a new language. I started on Python, picked up on Perl, and cut my teeth on C++. It was all fine and dandy, but there wasn't anything new to learn, I was basically re-learning the same language over and over again with a little different syntax. By accident, I stumbled on Paul Graham's classic article Beating the Averages, where he talks about Lisp and functional programming.

I thought it'd be fun to learn one of these "obscure" functional programming languages. So, between Erlang, Lisp, Haskell, and OCaml, I picked up OCaml for no particular reason except that camels are wicked cuteness (totally!). Lots and lots of new stuff to struggle with here:

• Graaaaah, why I have to fight the compiler to get type-inference to work right?
• Arrrragh, why aren't there any decent IDEs for this language? Why doesn't emacs have a GUI editor? Where's my intellisense!
• Currying? Is that just a gimmick, or is it supposed to be useful?
• Waaaaah, how can I do anything practical with algebraic data types?
• [] -> ... | x::xs -> ..., WTF does that mean?

For me, moving from Java to Python was easy: similar architectural idioms, similar programming style, etc. The hardest part was really learning a new library, which takes all of 6 weeks to go from n00b to pro.

Moving from Java to OCaml was really really hard, because there's nothing build on, its like starting from step 0. It took about 6 months of waffling around with the language before I stopped saying to myself "now how would I do this in OCaml?"

In retrospect, I probably wouldn't have struggled as much had I started with Haskell. Optional mutability + OOP makes OCaml seem superficially like familiar territory, and monads are still a mystery to me.

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Taken from link

Users of a procedural or object-oriented language like C, Java, or Python should beware, however: You'll have to forget most of what you already know about programming. Haskell is completely different from those languages, and requires a different way of thinking about programming. It's best to go into this tutorial with a blank slate and try not to compare Haskell to imperative languages, because many concepts in them (classes, functions, return) have a significantly different meaning in Haskell.

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I haven't developed anything practical in functional programming languages, but I did not have very much trouble adapting to it.

Before starting, I'd suggest improving your thinking recursively, because that's what you're going to need to do, whether in a familiar or unfamiliar language.

Also, you'll need to adapt to the idea that functions are objects, you can pass them to functions, and functions can return them.

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Haskell's learning curve is somewhat higher than other functional languages (like OCaml), though it is my personal favorite.

The key, IMO, is to think in terms of composition and inputs and outputs (if you come from OO, you probably think almost entirely in terms of inheritance -- this can hamper your learning).

Try to separate pure (no side effects) from impure (side effect -- example IO, state, etc).

Remember that functions are first-class citizens.

And learn the trick of partial function application. It is quite handy once you get used to it.