Getting started with Haskell

Question

For a few days I ve tried to wrap my head around the functional programming paradigm in Haskell  I ve done this by reading tutorials and watching screencasts  but nothing really seems to stick  Now  in learning various imperative OO languages  like C  Java  PHP   exercises have been a good way for me to go  But since I don t really know what Haskell is capable of and because there are many new concepts to utilize  I haven t known where to start   So  how did you learn Haskell  What made you really  break the ice   Also  any good ideas for beginning exercises

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To add on others  answers - there is one useful that will help you when coding  for example when solving project Euler problems   Hoogle   You can use either the command line interface or the web interface   Command Line  After you installed the Haskell platform be sure to cabal install hoogle  Hoogle usage example   You have a function f x   3   x   1 and you want to apply it on  5    Int   then apply it on the result and on that result and so on and get an infinite list of those values  You suspect there might already exist a function to assist you  not specifically for your f though    That function would be of type  a - gt  a  - gt  a - gt   a  if it takes f 5 or a - gt   a - gt  a  - gt   a  if it takes 5 f  we assume the function is for general types and not just Ints     hoogle  a - gt   a - gt  a  - gt   a   Prelude iterate     a - gt  a  - gt  a - gt   a    yep  the function you need already exists and it s called iterate  you use it by iterate func 5   Web interface  The result for the same example can be found here

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Try writing easy programs in it   You can find sample tasks in various textbooks  probably   I wouldn t recommend sticking to Haskell FP textbooks  just try to do simple things with it  calculations  string manipulations  file access   After I solved a dozen  I ve broke the ice     After that  read a lot on advanced concepts  Monads  Arrows  IO  recursive data structures   because haskell is infinite and there are a lot of them

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Graham Hutton s Programming in Haskell is concise  reasonably thorough  and his years of teaching Haskell really show  It s almost always what I recommend people start with  regardless of where you go from there   In particular  Chapter 8   Functional Parsers   provides the real groundwork you need to start dealing with monads  and I think is by far the best place to start  followed by All About Monads   With regard to that chapter  though  do note the errata from the web site  however  you can t use the do form without some special help  You might want to learn about typeclasses first and solve that problem on your own    This is rarely emphasized to Haskell beginners  but it s worth learning fairly early on not just about using monads  but about constructing your own  It s not hard  and customized ones can make a number of tasks rather more simple

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Some colleague of mine had good experience with Learn You a Haskell for Great Good       Tutorial aimed at people who have   experience in imperative programming   languages but haven t programmed in a   functional language before    And check the answers here too

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I m going to order this guide by the level of skill you have in Haskell  going from an absolute beginner right up to an expert  Note that this process will take many months  years    so it is rather long  Absolute Beginner Firstly  Haskell is capable of anything  with enough skill  It is very fast  behind only C and C   in my experience   and can be used for anything from simulations to servers  guis and web applications  However there are some problems that are easier to write for a beginner in Haskell than others  Mathematical problems and list process programs are good candidates for this  as they only require the most basic of Haskell knowledge to be able to write  Some good guides to learning the very basics of Haskell are the Happy Learn Haskell Tutorial and the first 6 chapters of Learn You a Haskell for Great Good  or its JupyterLab adaptation   While reading these  it is a very good idea to also be solving simple problems with what you know  Another two good resources are Haskell Programming from first principles  and Programming in Haskell  They both come with exercises for each chapter  so you have small simple problems matching what you learned on the last few pages  A good list of problems to try is the haskell 99 problems page  These start off very basic  and get more difficult as you go on  It is very good practice doing a lot of those  as they let you practice your skills in recursion and higher order functions  I would recommend skipping any problems that require randomness as that is a bit more difficult in Haskell  Check this SO question in case you want to test your solutions with QuickCheck  see Intermediate below   Once you have done a few of those  you could move on to doing a few of the Project Euler problems  These are sorted by how many people have completed them  which is a fairly good indication of difficulty  These test your logic and Haskell more than the previous problems  but you should still be able to do the first few  A big advantage Haskell has with these problems is Integers aren t limited in size  To complete some of these problems  it will be useful to have read chapters 7 and 8 of learn you a Haskell as well  Beginner After that you should have a fairly good handle on recursion and higher order functions  so it would be a good time to start doing some more real world problems  A very good place to start is Real World Haskell  online book  you can also purchase a hard copy    I found the first few chapters introduced too much too quickly for someone who has never done functional programming used recursion before  However with the practice you would have had from doing the previous problems you should find it perfectly understandable  Working through the problems in the book is a great way of learning how to manage abstractions and building reusable components in Haskell  This is vital for people used to object-orientated  oo  programming  as the normal oo abstraction methods  oo classes  don t appear in Haskell  Haskell has type classes  but they are very different to oo classes  more like oo interfaces   I don t think it is a good idea to skip chapters  as each introduces a lot new ideas that are used in later chapters  After a while you will get to chapter 14  the dreaded monads chapter  dum dum dummmm   Almost everyone who learns Haskell has trouble understanding monads  due to how abstract the concept is  I can t think of any concept in another language that is as abstract as monads are in functional programming  Monads allows many ideas  such as IO operations  computations that might fail  parsing      to be unified under one idea  So don t feel discouraged if after reading the monads chapter you don t really understand them  I found it useful to read many different explanations of monads  each one gives a new perspective on the problem  Here is a very good list of monad tutorials  I highly recommend the All About Monads  but the others are also good  Also  it takes a while for the concepts to truly sink in  This comes through use  but also through time  I find that sometimes sleeping on a problem helps more than anything else  Eventually  the idea will click  and you will wonder why you struggled to understand a concept that in reality is incredibly simple  It is awesome when this happens  and when it does  you might find Haskell to be your favorite imperative programming language    To make sure that you are understanding Haskell type system perfectly  you should try to solve 20 intermediate haskell exercises  Those exercises using fun names of functions like  quot furry quot  and  quot banana quot  and helps you to have a good understanding of some basic functional programming concepts if you don t have them already  Nice way to spend your evening with a bunch of papers covered with arrows  unicorns  sausages and furry bananas  Intermediate Once you understand Monads  I think you have made the transition from a beginner Haskell programmer to an intermediate haskeller  So where to go from here  The first thing I would recommend  if you haven t already learnt them from learning monads  is the various types of monads  such as Reader  Writer and State  Again  Real world Haskell and All about monads gives great coverage of this  To complete your monad training learning about monad transformers is a must  These let you combine different types of Monads  such as a Reader and State monad  into one  This may seem useless to begin with  but after using them for a while you will wonder how you lived without them  Now you can finish the real world Haskell book if you want  Skipping chapters now  doesn t really matter  as long as you have monads down pat  Just choose what you are interested in  With the knowledge you would have now  you should be able to use most of the packages on cabal  well the documented ones at least      as well as most of the libraries that come with Haskell  A list of interesting libraries to try would be   Parsec  for parsing programs and text  Much better than using regexps  Excellent documentation  also has a real world Haskell chapter   QuickCheck  A very cool testing program  What you do is write a predicate that should always be true  eg length  reverse lst     length lst   You then pass the predicate the QuickCheck  and it will generate a lot of random values  in this case lists  and test that the predicate is true for all results  See also the online manual   HUnit  Unit testing in Haskell   gtk2hs  The most popular gui framework for Haskell  lets you write gtk applications   happstack  A web development framework for Haskell  Doesn t use databases  instead a data type store  Pretty good docs  other popular frameworks would be snap and yesod     Also  there are many concepts  like the Monad concept  that you should eventually learn  This will be easier than learning Monads the first time  as your brain will be used to dealing with the level of abstraction involved  A very good overview for learning about these high level concepts and how they fit together is the Typeclassopedia   Applicative  An interface like Monads  but less powerful  Every Monad is Applicative  but not vice versa  This is useful as there are some types that are Applicative but are not Monads  Also  code written using the Applicative functions is often more composable than writing the equivalent code using the Monad functions  See Functors  Applicative Functors and Monoids from the learn you a haskell guide   Foldable Traversable  Typeclasses that abstract many of the operations of lists  so that the same functions can be applied to other container types  See also the haskell wiki explanation   Monoid  A Monoid is a type that has a zero  or mempty  value  and an operation  notated  lt  gt  that joins two Monoids together  such that x  lt  gt  mempty   mempty  lt  gt  x   x and x  lt  gt   y  lt  gt  z     x  lt  gt  y   lt  gt  z  These are called identity and associativity laws  Many types are Monoids  such as numbers  with mempty   0 and  lt  gt       This is useful in many situations   Arrows  Arrows are a way of representing computations that take an input and return an output  A function is the most basic type of arrow  but there are many other types  The library also has many very useful functions for manipulating arrows - they are very useful even if only used with plain old Haskell functions   Arrays  the various mutable immutable arrays in Haskell   ST Monad  lets you write code with a mutable state that runs very quickly  while still remaining pure outside the monad  See the link for more details   FRP  Functional Reactive Programming  a new  experimental way of writing code that handles events  triggers  inputs and outputs  such as a gui   I don t know much about this though  Paul Hudak s talk about yampa is a good start    There are a lot of new language features you should have a look at  I ll just list them  you can find lots of info about them from google  the haskell wikibook  the haskellwiki org site and ghc documentation   Multiparameter type classes functional dependencies Type families Existentially quantified types Phantom types GADTS others     A lot of Haskell is based around category theory  so you may want to look into that  A good starting point is Category Theory for Computer Scientist  If you don t want to buy the book  the author s related article is also excellent  Finally you will want to learn more about the various Haskell tools  These include   ghc  and all its features  cabal  the Haskell package system darcs  a distributed version control system written in Haskell  very popular for Haskell programs  haddock  a Haskell automatic documentation generator  While learning all these new libraries and concepts  it is very useful to be writing a moderate-sized project in Haskell  It can be anything  e g  a small game  data analyser  website  compiler   Working on this will allow you to apply many of the things you are now learning  You stay at this level for ages  this is where I m at   Expert It will take you years to get to this stage  hello from 2009    but from here I m guessing you start writing phd papers  new ghc extensions  and coming up with new abstractions  Getting Help Finally  while at any stage of learning  there are multiple places for getting information  These are   the  haskell irc channel the mailing lists  These are worth signing up for just to read the discussions that take place - some are very interesting  other places listed on the haskell org home page  Conclusion Well this turned out longer than I expected    Anyway  I think it is a very good idea to become proficient in Haskell  It takes a long time  but that is mainly because you are learning a completely new way of thinking by doing so  It is not like learning Ruby after learning Java  but like learning Java after learning C  Also  I am finding that my object-orientated programming skills have improved as a result of learning Haskell  as I am seeing many new ways of abstracting ideas

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These are my favorite  Haskell  Functional Programming with Types  Joeri van Eekelen  et al    Wikibooks        Published in 2012  597 pages   Real World Haskell     B  O Sullivan  J  Goerzen  D  Stewart   OReilly Media  Inc     Published in 2008  710 pages

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I suggest that you first start by reading BONUS  tutorial  And then reading Real World Haskell  online for free   Join the  Haskell IRC channel  on irc freenode com  and ask questions  These people are absolutely newbie friendly  and have helped me a lot over time  Also  right here on SO is a great place to get help with things you can t grasp  Try not to get discouraged  once it clicks  your mind will be blown   BONUS  tutorial will prime you up  and get you ready for the thrill ride that Real World Haskell brings  I wish you luck

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Don t try to read all the monad tutorials with funny metaphors  They will just get you mixed up even worse

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I do think that realizing Haskell s feature by examples is the best way to start above all   http   en wikipedia org wiki Haskell 98 features  Here is tricky typeclasses including monads and arrows  http   www haskell org haskellwiki Typeclassopedia  for real world problems and bigger project  remember these tags  GHC most used compiler   Hackage libraryDB   Cabal building system   darcs another building system     A integrated system can save your time  http   hackage haskell org platform   the package database for this system  http   hackage haskell org   GHC compiler s wiki  http   www haskell org haskellwiki GHC  After Haskell 98 features and Typeclassopedia  I think you already can find and read the documention about them yourself  By the way  you may want to test some GHC s languages extension which may be a part of haskell standard in the future   this is my best way for learning haskell  i hope it can help you

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If you only have experience with imperative OO languages  I suggest using a more conventional functional language as a stepping stone  Haskell is really different and you have to understand a lot of different concepts to get anywhere  I suggest tackling a ML-style language  like e g  F   first

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I d suggest joining the  haskell irc channel and asking questions there  That s how I learned Haskell  If you go through Real World Haskell as suggested above  real time answers to your questions will help greatly  Lots of smart people on  haskell write Haskell for fun and for profit  so you ll get lots of good input  Try it

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I enjoyed watching this 13 episode series on Functional Programming using Haskell   C9 Lectures  Dr  Erik Meijer - Functional Programming Fundamentals  http   channel9 msdn com shows Going Deep Lecture-Series-Erik-Meijer-Functional-Programming-Fundamentals-Chapter-1

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The first answer is a very good one  In order to get to the Expert level  you should do a PhD with some of the Experts themselves   I suggest you to visit the Haskell page  http   haskell org  There you have a lot of material  and a lot of references to the most up-to-date stuff in Haskell  approved by the Haskell community

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I can additionally recommend Yet Another Haskell Tutorial as an introduction  Another good learning resource  probably on the intermediate level   which has helped me a lot and hasn t been mentioned in the other answers as far as I can see  is Brent Yorgey s Typeclassopedia  which can be found in The Monad Reader  Issue 13  It is written in a very accessible style and contains  among many other things   the following introductory advice   There are two keys to an expert Haskell hacker   s wisdom   Understand the types   Gain a deep intuition for each type class and its relationship to other type classes  backed up by familiarity with many examples     The Monad Reader itself is an absolute treasure trove for functional programmers  not only Haskell programmers

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Here s a good book that you can read online  Real World Haskell  Most of the Haskell programs I ve done have been to solve Project Euler problems   Once piece of advice I read not too long ago was that you should have a standard set of simple problems you know how to solve  in theory  and then whenever you try to learn a new language you implement those problems in that language

[haskell] Getting started with Haskell

Command Line

Web interface

Examples related to haskell

Examples related to functional-programming