soupi / Haskell Study Plan

• Haskell Study Plan ** About This Guide This guide is an opinionated list of resources for learning Haskell.

It is aimed at more experienced programmers that would like a denser Haskell tutorial.

If you prefer a gentler introduction, try one of these resources:

• [[https://www.seas.upenn.edu/~cis194/spring13/lectures.html][cis194 course (2013)]] or [[https://haskell-via-sokoban.nomeata.de/][Haskell via Sokoban]]
• [[https://en.wikibooks.org/wiki/Haskell][Haskell Wikibook]]
• [[http://haskellbook.com/][Haskell Programming From First Principles]]
• [[http://www.cs.nott.ac.uk/~pszgmh/pih.html][Programming in Haskell]]

And/or this resource to get started quickly: [[https://kowainik.github.io/projects/learn4haskell][Learn4Haskell]].

** Table of Contents :TOC_3:

• [[#haskell-study-plan][Haskell Study Plan]]
  • [[#about-this-guide][About This Guide]]
  • [[#beginning][Beginning]]
  • [[#more-basics-drill-down][More Basics Drill Down]]
  • [[#useful-packages][Useful Packages]]
  • [[#tools][Tools]]
  • [[#exercises][Exercises]]
    • [[#lists][Lists]]
    • [[#sort-a-list][Sort a List]]
    • [[#dict][Dict]]
    • [[#ppm][PPM]]
    • [[#rpn-calculator][RPN Calculator]]
  • [[#lambda-calculus][Lambda Calculus]]
    • [[#overview][Overview]]
    • [[#exercises-1][Exercises]]
  • [[#kinds][Kinds]]
    • [[#overview-1][Overview]]
    • [[#exercise][Exercise]]
  • [[#what-is-io][What is IO?]]
    • [[#overview-2][Overview]]
    • [[#do-notation][Do notation]]
    • [[#exercises-2][Exercises]]
  • [[#type-classes][Type classes]]
    • [[#overview-3][Overview]]
    • [[#more-material][More Material]]
    • [[#exercise-1][Exercise]]
  • [[#monoids-functors-applicative-monads-and-more][Monoids, Functors, Applicative, Monads and More]]
    • [[#overview-4][Overview]]
    • [[#instances][Instances]]
    • [[#exercises-3][Exercises]]
    • [[#bonus-counterexamples-of-type-classes][Bonus: Counterexamples of Type Classes]]
    • [[#more][More]]
  • [[#error-handling][Error Handling]]
    • [[#using-either-for-errors][Using Either for errors]]
    • [[#exceptions][Exceptions]]
    • [[#exercises-4][Exercises]]
  • [[#debugging][Debugging]]
  • [[#laziness][Laziness]]
  • [[#performance][Performance]]
    • [[#resources][Resources]]
    • [[#data-structures][Data Structures]]
  • [[#monad-transformers][Monad Transformers]]
    • [[#overview-5][Overview]]
    • [[#exercises-5][Exercises]]
  • [[#ghc-language-extensions][GHC Language Extensions]]
  • [[#functional-patterns][Functional Patterns]]
    • [[#effectful-outer-layer-uneffectful-core][Effectful outer layer, Uneffectful core]]
    • [[#compose-smaller-things-to-bigger-things][Compose Smaller Things to Bigger Things]]
    • [[#type-classes-patterns][Type Classes Patterns]]
    • [[#parse-dont-validate][Parse, Don't Validate]]
    • [[#more-1][More]]
  • [[#more-2][More]]
    • [[#more-resources-tutorials-and-project-ideas][More resources, tutorials and project ideas]]
    • [[#some-advanced-topics][Some Advanced Topics]]
    • [[#references][References]]
    • [[#news-aggregators][News Aggregators]]
    • [[#simple-example-programs][Simple Example Programs]]
    • [[#a-few-cool-open-source-applications][A Few Cool Open-Source Applications]]

** Beginning

1. [[https://github.com/kowainik/learn4haskell#installing-haskell][Install GHC+cabal-install]] (or [[https://github.com/kakkun61/ghcups][ghcups]] for windows powershell) and [[https://haskell.fpcomplete.com/get-started][Get Started]] for Stack
2. [[https://soupi.github.io/rfc/reading_simple_haskell][Reading Simple Haskell]]
3. [[https://soupi.github.io/rfc/writing_simple_haskell][Writing Simple Haskell]]
4. [[https://en.wikibooks.org/wiki/Haskell/Indentation][Indentation]]
5. [[https://www.seas.upenn.edu/~cis194/spring13/lectures/01-intro.html][Haskell intro (cis194)]]
6. [[http://www.scs.stanford.edu/16wi-cs240h/slides/basics.html][Haskell Basics (cs240h)]]
7. [[https://gilmi.me/blog/post/2020/10/01/substitution-and-equational-reasoning][Substitution and Equational Reasoning]]
8. [[https://github.com/Gabriel439/slides/blob/master/bigtechday/slides.md][Haskell and proving things]]
   • Read until "Everything is a Monoid" (right after "Chaining proofs")

The basics are important, each resource here brings it's own view on it which will help solidify this material. If there are exercises to do, do them!

Key ideas:

• In Haskell we use a different computation model

  • Instead of "telling the machine what to do in order to change the state of the machine" we "transform data until we have the result we want"

• Referential Transparency enables equational reasoning

• Types help prevent errors and help model programs ** More Basics Drill Down

• Wikibook

  • [[https://en.wikibooks.org/wiki/Haskell/Recursion][Recursion]]
  • [[https://en.wikibooks.org/wiki/Haskell/Lists_and_tuples][Lists and tuples]]
  • [[https://en.wikibooks.org/wiki/Haskell/Type_basics][Type basics]]
  • [[https://en.wikibooks.org/wiki/Haskell/More_on_datatypes#Named_Fields_(Record_Syntax)][Records]]
  • [[https://en.wikibooks.org/wiki/Haskell/Higher-order_functions][Higher Order Functions]]
  • [[https://en.wikibooks.org/wiki/Haskell/Modules][Modules]] and [[https://en.wikibooks.org/wiki/Haskell/Standalone_programs][Standalone Programs]] ** Useful Packages Here are a few useful packages you might want to use when building software with Haskell:

• [[https://hackage.haskell.org/package/base][base]] - Haskell standard library. Contains large collection of useful libraries ranging from data structures to parsing combinators and debugging utilities.

• [[https://hackage.haskell.org/package/containers][containers]] - Contains efficient general-purpose implementations of various immutable container types including sets, maps, sequences, trees, and graphs.

• [[http://hackage.haskell.org/package/vector][vector]] - Efficient arrays.

• [[https://hackage.haskell.org/package/text][text]] - An efficient unicode text type. It is much more efficient than the built in String type.

• [[https://hackage.haskell.org/package/bytestring][bytestring]] - An efficient vector of byte type.

• [[http://hackage.haskell.org/package/async][async]] - API for running IO operations asynchronously.

• [[http://hackage.haskell.org/package/network][network]] - Low-level networking interface.

• [[http://hackage.haskell.org/package/random][random]] - random number library.

[[https://hackage.haskell.org/][And more]]. ** Tools

• [[https://en.wikibooks.org/wiki/Haskell/Using_GHCi_effectively][Using GHCi Effectively]]
• [[https://www.haskell.org/hoogle/][Hoogle]]
• [[https://github.com/ndmitchell/ghcid#readme][GHCid]]
• Editor Integration
  • [[https://marketplace.visualstudio.com/items?itemName=haskell.haskell][VSCode / VSCodium]] (* Recommended. Just install the haskell extension in vscode/vscodium.)
  • [[https://github.com/soupi/minimal-haskell-emacs][Emacs]]
    • [[https://github.com/soupi/minimal-haskell-emacs/tree/evil][+ vim bindings]]
  • [[https://www.reddit.com/r/haskell/comments/9bxbwp/which_ide_are_you_using_for_hakell/][More Options]]
• [[https://www.ahri.net/practical-haskell-programs-from-scratch/][Practical Haskell programs from scratch - a quick and easy guide]]
• [[https://sakshamsharma.com/2018/03/haskell-proj-struct/][Structuring your first Haskell project with Stack]] ** Exercises *** Lists
• [[https://wiki.haskell.org/99_questions/1_to_10][1-10 Haskell Problems]]
• [[https://wiki.haskell.org/99_questions/11_to_20][11-20 Haskell Problems]] *** Sort a List Sort a list of ints by inserting all its elements into a binary search tree.

1. Define a data type of a binary search tree
2. Write the type signatures of the functions relevant to the task (sort, insertElementToTree, listToTree, flatten, display, etc.)
3. Implement these functions

Think of scenarios and test your functions. *** Dict Compress and decompress a file using dict compression.

Dict compression takes text, splits it by words, and creates two things:

1. A mapping from each word in the text to a number
2. the original text where each word is replaced by it's map's number

Your task is to create an application that can either compress or decompress a text file.

There are two commands: compress and decompress, they both get a text file.

• To compress: > dict compress file.txt
• To decompress: > dict decompress file.txt

For the compress command, the output should be the compressed items ((1) and (2)). For the decompress command, the output should be the original text.

Note: You can use the functions read and show to convert from/to some types and String. *** PPM Create a program that will output a [[https://en.wikipedia.org/wiki/Netpbm_format#PPM_example][PPM file]].

1. The size of each "pixel" should be controlled by a parameter
2. Your input should be a list of list of colors
3. If a row is not long enough fill the rest of it with the color white
4. Bonus: Choose a pallete of 8 or 16 basic colors and read a file containing numbers from 0 to 7 (or 15) separated by spaces and newlines, and output it's image *** RPN Calculator Create a program that calculates an arithmetic expression written in [[https://en.wikipedia.org/wiki/Reverse_Polish_notation][reverse polish notation]].

Implement the following operations:

literal integers, +, -, *, /, negate

Example execution:

#+BEGIN_SRC $ rpn-calc 5 7 2 negate + * 25 #+END_SRC ** Lambda Calculus *** Overview The lambda calculus is a minimalistic language that is in the core of functional programming.

It presents a minimalistic framework to learn about many common features in functional languages.

• [[http://www.inf.fu-berlin.de/lehre/WS03/alpi/lambda.pdf][A tutorial on the lambda calculus]]
• [[https://gitlab.cecs.anu.edu.au/pages/2019-S1/courses/comp1100/lectures/lambda.pdf][Slides on the lambda calculus]]
• [[https://en.wikipedia.org/wiki/Lambda_calculus][Wikipedia article on the Lambda Calculus]]

*** Exercises

1. Reduce the following expressions to normal form using pen and paper
   1. λx. x
   2. (λx. x) y
   3. (λx. x x) (λy. y)
   4. (λw. λx. λz. x w z) a (λb. λc. c b) (λd. d)
2. Use eta conversion on the following expression
   1. λx. f x
   2. λf. λy. (λx. f x) y
3. Write the expression 2 + 3 in the lambda calculus and evaluate it using pen and paper
4. Write the expression factorial 5 in the lambda calculus and evaluate it using pen and paper

Use this [[http://cdparks.github.io/lambda-machine/][Lambda Machine]] to check your answers

** Kinds *** Overview Every expression has a concrete type.

Kinds are the types of types.

This is a simplified view of how kinds are represented in GHC:

#+BEGIN_SRC haskell data Kind = Type -- can also be written as: * | KArr Kind Kind -- KArr in Haskell this is written as: -> #+END_SRC

Think of Type being the kind of concrete (or inhabited) types, and KArr is a function from Kind to Kind.

If a type is parametarized (when defining the ADT you pass it parameters) then in order for it to be concrete you have to supply it with all the types it expects to get.

Example:

#+BEGIN_SRC haskell

data Bool = True | False

data Maybe a = Just a | Nothing

#+END_SRC

Bool is not parametarized so it is a concrete type (which means it's kind is Type) and has the Values True and False.

Maybe is not a concrete type, it need to be supplied with a type for a. (It has the kind Type -> Type).

Maybe Bool is a concrete type because all of the paramters for Maybe have been supplied.

An expression can only have a type with the kind Type.

Examples:

| Value | Type | Kind | Comments | |-----------+------------------------+--------------------------------+--------------------------------------| | True | Bool | Type (also written *) | a value | | 'c' | Char | Type | | | "Hello" | String | Type | | | not True | Bool | Type | function application | | Just True | Maybe Bool | Type | | | ["Hello"] | [String] | Type | | | Nothing | Maybe a | Type | polymorphic | | id | a -> a | Type | a function | | map | (a -> b) -> [a] -> [b] | Type | | | map not | [Bool] -> [Bool] | Type | partially applied function | | getLine | IO String | Type | | | putStrLn | String -> IO () | Type | | | | Void | Type | a concrete types with no values | | | Maybe | Type -> Type | isn't fully supplied with parameters | | | IO | Type -> Type | | | | Either | Type -> Type -> Type | | | | Either a | Type -> Type | partially supplied with parameters | | | Free | (Type -> Type) -> Type -> Type | the first argument is of higher kind |

You can use ghci to query the kind of a type using :kind

Why do we care about Kinds? It let us generalize things and create abstractions.

Let's take a look at a data type that uses higher kinds:

#+BEGIN_SRC haskell data Rec f a = Rec a (f (Rec f a)) #+END_SRC

• This data type has two type parameters, f and a. From their use in the right side of the = we can see that a has the kind Type because it is placed as a field without type arguments. We can also see that f has kind Type -> Type because it is placed as a field with one type argument (which in this case, is the same data type we defined). This makes Rec kind to be (Type -> Type) -> Type -> Type.

Why is this data type interesting? Let's try to plug some types and see. We need some a which as kind Type so let's just choose Int for now, and let's use Maybe for f. Let's look at some values of our new type Rec Maybe Int.

• x1 = Rec 1 Nothing
• x2 = Rec 1 (Just (Rec 2 Nothing))
• x3 = Rec 1 (Just (Rec 2 (Just (Rec 3 Nothing))))

See a pattern here? it seems like this is an encoding of a non-empty list:

• You always have at least one value
• Nothing is similar to Nil
• Just is similar to Cons

Let's take a look at another example with this type:

#+BEGIN_SRC haskell data Identity a = Identity a #+END_SRC

Identity basically just holds a value of type a. Nothing interesting here.

Let's try to plug it in Rec (and get Rec Identity Int) and see what kind of value we can have:

• y1 = Rec 1 (Identity (Rec 2 (Identity (Rec 3 (Identity ...)))))
• y2 = Rec 0 y2

As you can see we basically need to keep providing new values with no way of bailing out. So we got an infinite list of values (or a stream).

We can write all kinds of generic algorithms on this data type and reuse them for different scenarios and needs simply by pluging in a different f!

We'll see more of those after we talk about type classes.

There is more to Haskell's kinds system, and a really good article about it is linked later on the tutorial.

And by the way, the real name of Rec is [[https://hackage.haskell.org/package/free-5.1/docs/Control-Comonad-Cofree.html][Cofree]].

*** Exercise Try to plug into our Rec a different type of kind Type -> Type that you know and see what happens! ** What is IO? *** Overview It is a parametarized type constructor (it has the kind Type -> Type).

IO a represents a description of a program (or subroutine) that when executed will produce some value of type a and may do some I/O effects while at it.

Evaluating an IO a is pure - the evaluation will always reduce to the same description of a program.

In an executable, you need to define main :: IO () - a description of a program to run. The Haskell runtime will execute this.

You can combine subroutine descriptions to create bigger subroutine descriptions:

1. pure :: a -> IO a

   Produces a value without doing any I/O.

   • Example: pure True

   Which has the type IO Bool, will not do any I/O and when executed will produce a value of type Bool, specifically True.

2. fmap :: (a -> b) -> IO a -> IO b

   Similar to map on lists, it will apply a function on the parameter of IO.

   • Example: fmap not (pure True)

   Which has the type IO Bool will not do any I/O and when executed will produce a value of type Bool by first applying the function not on the result of pure True, and so will produce the value False.

3. (>>) :: IO a -> IO b -> IO b

   Run this first thing, discard the result, and then run the second thing.

   • Example: #+BEGIN_SRC haskell putStrLn "Hello" >> putStrLn "World" #+END_SRC

   Which has the type IO (), when executed, will print the string Hello and then will print the string World and will produce a value of type (), specifically () (in this case the value has the same name as the type).

4. (>>=) :: IO a -> (a -> IO b) -> IO b

   Run this first thing, take its result, pass it to the function which is the second argument, and then execute that.

   • Example: getLine >>= putStrLn

   Which has the type IO () will read a String from the user, apply that String to putStrLn and then execute it, thus printing the same string it got from the user. Then it will produce a value of type (), specifically ().

   Note: You can implement (>>) using (>>=) like this:

   #+BEGIN_SRC haskell (>>) prog1 prog2 = prog1 >>= _ -> prog2 #+END_SRC

5. join :: IO (IO a) -> IO a

Takes a description of a program that produces a description of a program that produces a value of type a and converts it to a descrption of a program that will produce a value of type a by executing the first, and then executing the result.

• Example: join (fmap putStrLn getLine)

Which is the same as getLine >>= putStrLn. As you can see we can implement >>= using fmap and join

#+BEGIN_SRC haskell
(>>=) prog func = join (fmap func prog)
#+END_SRC

There are many more functions and combinators that return IO a. You can view some of them in the module [[http://hackage.haskell.org/package/base-4.11.1.0/docs/System-IO.html#t:IO][System.IO]]. *** Do notation

do notation is syntactic sugar around >> and >>=.

Example:

#+BEGIN_SRC haskell main = do putStrLn "Tell me your name." let greet name = "Hello, " ++ name ++ "!" name <- getLine putStrLn (greet name) #+END_SRC

Will be desugared to:

#+BEGIN_SRC haskell main = putStrLn "Tell me your name." >> let greet name = "Hello, " ++ name ++ "!" in getLine >>= \name -> putStrLn (greet name) #+END_SRC

1. A regular line that does not create a binding will be sequenced to the next using >>
2. A new definition can be created using let, it will be translated to let in
3. A line that creates a binding with <- will use >>= to pass the result and the lambda (\name ->) is used to bind the variable to the result
4. The last line will remain the same - no desugar needed

This is basically CPS (continuation passing style).

| code | operator | type of the left side | type of the right side | comments | |-------------------------+----------+-----------------------+------------------------+---------------------------------------------------------------------------------------------| | let gretting = "hello" | = | String | String | = means both side are interchangeable (they both mean exactly the same thing) | | let mygetline = getLine | = | IO String | IO String | Here we just create a new name that is identical to getLine. We are not running anything | | name <- getLine | <- | String | IO String | <- is syntactic sugar for >>= where we bind the result of the computation to the name |

IO's API fits a pattern that can be seen in more types in Haskell, which is why the type signatures of the functions presented here are more general. We'll discuss that later. *** Exercises

• Implement a number guessing game
  • Generate a random number between 1 and 100, the user should try to guess what it is.
    • If the user guess is too high, say it's too high.
    • If the user guess is too low, say it's too low.
    • Hint: you can use [[https://hackage.haskell.org/package/random-1.1/docs/System-Random.html#v:randomRIO][randomRIO]] to generate a random number
  • Bonus: Remember the amount of times the user guesses and print that at the end of the game.
    • Hint: In pure functional programming we use recursion to emulate state
  • Bonus: Remember the user's guesses and tell them if they already tried that guess.
• Implement a [[https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop][REPL]] interface to your [[#rpn-calculator][RPN Calculator]]
  • Create an interactive interface that lets the user repeatedly write calculations and return the evaluations for them ** Type classes *** Overview We use type classes to describe groups of types that all behave in a similar way and refer to them generically.

A good type class will have operations on the type and laws attached to it - similar to abstract algebra.

Laws cannot be enforced by the compiler - a good convention in Haskell is not to define lawless type classes and not implement unlawful instances.

We define a type class like this:

#+BEGIN_SRC haskell class Eq (a :: *) where (==) :: a -> a -> Bool #+END_SRC

We define a class of types that can implement the operation (==).

We implement an instance of a type class for a given type like this:

#+BEGIN_SRC haskell -- In this case we place Bool in place of a everywhere instance Eq Bool where (==) b1 b2 = case (b1, b2) of (True, True) -> True (False, False) -> True _ -> False #+END_SRC

Now we can implement polymorphic functions that will work on a subset of all types - all types that fill the constraint - have instances of a type class.

#+BEGIN_SRC haskell (/=) :: Eq a => a -> a -> Bool (/=) x y = not (x == y) #+END_SRC

class instances should be defined in the same place as the type class definition or at the same place as the type definitions. Failing to do that may cause [[https://wiki.haskell.org/Orphan_instance][Orphan Instances]].

| Abstraction | definition | different substitutions | comments | |-------------------------+-------------------------------------+-------------------------------------------------------------+---------------------------------------------------------------------------------| | No polymorphism | func1 :: Int -> Int -> Int | none | we know exactly which types are used and can do all kinds of operations on them | | Parametric polymorphism | func2 :: a -> a -> a | a can be any type | We don't know which type a is and can't do any type related operations on it | | Type classes (ad-hoc) | func3 :: Ord a => a -> a -> a | a can be any type that can be ordered (Bool, Int, String) | anything to the left of => is a constraint on the type |

*** More Material

• [[https://www.youtube.com/watch?v=6COvD8oynmI][Adventure with Types in Haskell - SPJ]]
• [[https://en.wikibooks.org/wiki/Haskell/Classes_and_types][Haskell Wikibook Chapter on Classes and Types]]
• [[https://en.wikibooks.org/wiki/Haskell/Type_basics_II][Numbers type classes]]

*** Exercise

• Read about a few common type classes:
  • Show
  • Read
  • Eq
  • Ord
  • Num
  • Integral
  • Floating
• Go back to [[#sort-a-list][Sort a List]] exercise and change it to work on more types than just Int

Note: We can create instances for higher kinded types (for example: Type -> Type). We will see some of those next. ** Monoids, Functors, Applicative, Monads and More *** Overview Key idea:

These are abstract algebraic structures

They define operations and laws on them such as identity and associativity.

Many patterns fit these structures, making them useful as abstractions!

Type classes you should care about (at the moment):

• Semigroup

• Monoid

• Functor

• Applicative

• Monad

• Foldable

• Traversable

Read about them in the [[https://wiki.haskell.org/Typeclassopedia][typeclassopedia]] in this order.

After that: read [[http://dev.stephendiehl.com/hask/#monads][The monads section in wiwik]] to meet some useful monad instances.

• [[https://github.com/Gabriel439/slides/blob/master/bigtechday/slides.md][Haskell and proving things]]
  • Read from "Everything is a Monoid" (right after "Chaining proofs") or from the beginning if you want to review it again

*** Instances Make sure to meet:

• Maybe
• Either
• List
• -> (Functions)
• IO
• Reader
• State
• Writer

And understand why and how they work! *** Exercises

• Implement some instances to a few types you like.
• Implement Functor, Foldable and Traversable instances for the Tree data type you defined at [[#sort-a-list][Sort a list]] and revised in [[#type-classes][Type Classes]]
• Implement a Foldable instance for the Rec data type we defined in the section on Kinds.
  • Test your solution by using Sum, Product, Any or All from Data.Monoid.
• Implement a Functor instance for the Rec data type we defined in the section on Kinds.
  • Test your solution by mapping and then folding *** Bonus: [[https://blog.functorial.com/posts/2015-12-06-Counterexamples.html][Counterexamples of Type Classes]] *** More
• [[https://en.wikibooks.org/wiki/Haskell][Haskell wikibook section on Monads]] ** Error Handling *** Using Either for errors There are quite a few ways to indicate and handle errors in Haskell. We are going to look at one solution: using the type [[https://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Either.html][Either]]. Either is defined like this:

#+BEGIN_SRC haskell data Either a b = Left a | Right b #+END_SRC

Simply put, a value of type Either a b can contain either a value of type a, or a value of type b. Well can tell them apart from the contructor used.

#+BEGIN_SRC haskell Left True :: Either Bool b Right 'a' :: Either a Char #+END_SRC

Using this type, we can represent computations that may fail by using Either with one type to represent error values and the other type to represent the values we want if the computation succeeds.

For example, let's say that we want to parse a String as a decimal digit to an Int. We have two possible failures:

1. The string contains more than one character
2. The string is empty
3. The character is not one of 0,1,2,3,4,5,6,7,8,9

We can represent this as a type

#+BEGIN_SRC haskell data ParseDigitError = EmptyString | StringIsTooLong | NotADigit Char deriving Show #+END_SRC

And our function can have the type

#+BEGIN_SRC haskell parseDigit :: String -> Either ParseDigitError Integer #+END_SRC

Now when we check our string we can return Left on error and Right on successful parsing.

#+BEGIN_SRC haskell parseDigit :: String -> Either ParseDigitError Integer parseDigit str = case str of -- empty string [] -> Left EmptyString -- more than one character _ : _ : _ -> Left StringIsTooLong [c] -> if elem c "0123456789" then Right (read [c]) else Left (NotADigit c) #+END_SRC

Either a is also an instance of Functor, Applicative, and Monad, so we have some combinators to work with if we want to combine these kind of computations.

For example, we can use our function to parse an integer by trying to parse each character (using traverse) and then use a function to sum them all together by applying it to the Int value using fmap.

#+BEGIN_SRC haskell parseInteger :: String -> Either ParseDigitError Integer parseInteger str = do if null str then Left EmptyString else -- We use (:[]) first because each element of a String is a Char and our functions works on String. -- This also means that in this case only NotADigit error can be return, which is still fine. let digits = traverse (parseDigit . (:[])) str in fmap ( foldr (+) 0 . zipWith (\e n -> 10 ^ e * n) [0..] . reverse ) digits #+END_SRC

Try it!

Note that since Either has kind Type -> Type -> Type and Functor, Applicative and Monad expect something of kind Type -> Type, we can only create instances for Either a and not Either.

This means that when we use, for example, <*> which has the type

#+BEGIN_SRC haskell (<*>) :: Applicative f => f (a -> b) -> f a -> f b #+END_SRC

we replace f with Either a and not Either:

#+BEGIN_SRC haskell -- We'll use e for the left type of the either instead of a here because a is already taken (<*>) :: Either e (a -> b) -> Either e a -> Either e b #+END_SRC

This means that e must be the same. If you want, for example, to use two different error types, two approaches you can use are:

1. Replace them with one big ADT that contain both errors
2. Make one ADT that combines both types just like Either does with a and b and use the function first from [[https://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Bifunctor.html][Data.Bifunctor]] to convert from one error type to the other. (first is like fmap but for the first type variable in Either)

*** Exceptions

• [[https://www.oreilly.com/library/view/parallel-and-concurrent/9781449335939/ch08.html#sec_exceptions][Exceptions]] *** Exercises
• Revise your [[#rpn-calculator][RPN Calculator]] to use Either to terminate early due to errors. ** Debugging
• [[https://en.wikibooks.org/wiki/Haskell/Debugging][Using Traces]] ** Laziness
• [[http://blog.ezyang.com/2011/04/the-haskell-heap/][The Haskell Heap]]
• [[https://www.oreilly.com/library/view/parallel-and-concurrent/9781449335939/ch02.html#sec_par-eval-whnf][Lazy Evaluation and Weak Head Normal Form]]
• [[https://two-wrongs.com/how-laziness-works][How laziness works - a tour through Haskell IRs]] ** Performance Haskell can be fast and have a low memory foot-print in many scenarios even when you use immutable data structures and uneffectful code.

It is a good idea to keep your code idiomatic and measure before you decide to use mutation and other fancier methods. You may not need it! *** Resources **** General

• [[https://en.wikibooks.org/wiki/Haskell/Performance_introduction][Introduction]]
• [[http://slides.com/fp-ctd/lecture-10#/][Haskell ITMO course at CTD - Lecture 10]] **** Profiling
• [[https://stackoverflow.com/questions/32123475/profiling-builds-with-stack][Profiling Builds with Stack]]
• [[http://book.realworldhaskell.org/read/profiling-and-optimization.html][Profiling and Optimization]]
• [[https://slides.com/sumith1896/space-leaks-in-haskell][Space Leaks in Haskell]] and [[http://neilmitchell.blogspot.com/2015/09/detecting-space-leaks.html][Detecting Space Leaks]]
• [[https://github.com/mpickering/eventlog2html][eventlog2html]] **** Case Studies
• [[https://chrisdone.com/posts/fast-haskell-c-parsing-xml/][Fast Haskell: Competing with C at parsing XML]]
• [[https://markkarpov.com/post/migrating-text-metrics.html][Migrating text metrics to pure Haskell]]
• [[https://two-wrongs.com/on-competing-with-c-using-haskell.html][On Competing With C Using Haskell]]
• [[https://github.com/ChrisPenner/wc][wc - Counting Words With Haskell]]
• [[https://www.joachim-breitner.de/blog/758-Winter_is_coming_even_more_quickly][Winter is coming even more quickly]] *** Data Structures

The choice of a data structure is determined by the properties of your data and the algorithms used.

Single-linked lists are a fairly ubiquious data structure in Haskell. Due to their simplicity and syntactic sugar, they're used all over the place - often when they're not a good choice.

Lists are good for:

1. You only need to add or take the beginning of the list (consing), which is O(1)
2. You use map, filter, zip and folds, which are O(N) anyway and are subject to operation fusion (aka. map f . map g = map (f . g)
3. Your list is really small and is not expected to grow
4. Your list is infinite

Lists are not good if:

1. You use lookup - use Map
2. You want the elements to be unique - use Set
3. You expect the list to have at least one argument, use NonEmpty
4. You use append or concat, use DList or Seq
5. You use sort with non-unique values, use Seq

• [[http://dev.stephendiehl.com/hask/#data-structures][More Information]] ** Monad Transformers *** Overview Functors and applicative interfaces [[https://hackage.haskell.org/package/transformers-0.3.0.0/docs/Data-Functor-Compose.html][can be composed easily]], but monads cannot.

Monad transformers are a way to compose the capabilities of multiple type's monadic interface to one type.

• [[http://slides.com/fp-ctd/lecture-9#][Haskell ITMO course at CTD - Lecture 9]]

• [[https://two-wrongs.com/a-gentle-introduction-to-monad-transformers][A Gentle Introduction to Monad Transformers]]

• [[https://www.schoolofhaskell.com/user/commercial/content/monad-transformers][School of Haskell - Monad Transformers]]

• [[https://blog.jle.im/entry/mtl-is-not-a-monad-transformer-library.html][mtl is Not a Monad Transformer Library]] *** Exercises

• To your [[#rpn-calculator][RPN Calculator]] REPL:

  • Use Either to terminate an evaluation of an expression early when encountering errors
  • Add the Reader interface to thread through the evaluation the build-in operations
  • Add the ability for the user to define new words (with the syntax: : ) ** GHC Language Extensions Haskell is a standartized programming language. The last standard is [[https://www.haskell.org/onlinereport/haskell2010/][Haskell 2010]]. GHC, the most popular Haskell compiler, contains more features than what's available in Haskell 2010. To use those features, we must tell the compiler that we want to use them. We do this by invoking a compiler flag or adding a LANGUAGE pragma at the top of the source file.

• [[https://impurepics.com/posts/2019-08-01-haskell-extensions.html][Haskell Extensions in Pictures]]

• [[https://limperg.de/ghc-extensions/][A Guide to GHC's Extensions]] ** Functional Patterns *** Effectful outer layer, Uneffectful core Code that does no effects is easier to test, debug and reason about.

Keeping most of our program's logic uneffectful makes it more flexible.

But programs still need to interact with the outside world.

For that, we can create an outer layer that is responsible for interacting with the user and dispatching the right logic functions.

Notice this pattern in these [[http://www.haskellforall.com/2015/10/basic-haskell-examples.html][Basic Haskell Examples]]. *** Compose Smaller Things to Bigger Things

• [[https://wiki.haskell.org/Combinator_pattern][Combinator Pattern]] *** Type Classes Patterns Type Classes such as Monoid, Functor, Applicative and Monad can be thought of as patterns. They are all around us and are at the core API of many libraries.

You can find them when doing web development, streaming, IO, concurrency, parsing, error handling, testing, build systems and more.

Examples:

• [[https://kseo.github.io/posts/2014-01-16-applicative-parsing.html][Applicative Parsing]]

• [[https://hackage.haskell.org/package/lucid-2.9.10/docs/Lucid.html][Lucid - a DSL for writing HTML]]

• [[https://www.oreilly.com/library/view/parallel-and-concurrent/9781449335939/ch10.html][Software Transactional Memory]] *** Parse, Don't Validate

• [[https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/][Parse, don't validate]] *** More

• [[https://www.reddit.com/r/haskell/comments/5r271m/haskell_design_patterns/][Discussion on Reddit]]

• [[https://gumroad.com/l/CLyzT][William Yao - Abstractions in Context (book)]] ** More *** More resources, tutorials and project ideas

• [[http://gilmi.me/blog/post/2015/02/25/after-lyah][In this blog post]] *** Some Advanced Topics These may not be as useful for your everyday programming tasks, but it's nice to know they exist when you need them

• [[https://en.wikibooks.org/wiki/Haskell/FFI][Foreign Function Interface]]

• [[https://chrisdone.com/posts/data-typeable][Generic Programming]]

• [[https://markkarpov.com/tutorial/th.html][Meta Programming with Template Haskell]]

• [[https://diogocastro.com/blog/2018/10/17/haskells-kind-system-a-primer/][Haskell's kind system - a primer]] and [[https://www.parsonsmatt.org/2017/04/26/basic_type_level_programming_in_haskell.html][Basic Type Level Programming]]

• [[https://blog.sumtypeofway.com/an-introduction-to-recursion-schemes/][Recursion Schemes]]

• [[https://skillsmatter.com/skillscasts/4251-lenses-compositional-data-access-and-manipulation][Lenses]] *** References

• [[https://haskell.fpcomplete.com/tutorial/operators][Operators Glossary]]

• [[http://dev.stephendiehl.com/hask/][What I Wish I Knew Learning Haskell]] *** News Aggregators

• [[https://haskellweekly.news/][Haskell Weekly News]]

• [[https://haskell.pl-a.net/][Haskell Planetarium]] *** Simple Example Programs

• [[https://anardil.net/tag/coreutils.html][Unix core utilities in Haskell]]

• [[https://gist.github.com/soupi/199a16be6e2071c3b724][Simple File Reader]]

• [[https://gitlab.com/gilmi/sdl2-snake][Snake Game]]

• [[https://gitlab.com/gilmi/hen][Static Blog Generator]]

• [[https://gitlab.com/gilmi/sod-cmus][Simplified Web Interface to cmus]]

• [[https://gitlab.com/gilmi/imgs][Image Server]]

• [[https://gitlab.com/gilmi/plaste][paste-bin]]

• [[https://github.com/jackoe/discourse-tui][A terminal UI for discourse]] *** A Few Cool Open-Source Applications Here are a few cool open source applications written in Haskell that might accept contributions if you're interested.

• [[https://github.com/aurapm/aura/][Aura]] - A package manager for Arch Linux and its AUR.

• [[https://github.com/google/codeworld][CodeWorld]] - CodeWorld is an educational environment using Haskell.

• [[https://lettier.github.io/gifcurry/][gifcurry]] - Your open source video to GIF maker built with Haskell.

• [[https://github.com/therewillbecode/haskell-poker][Haskell-Poker]] - A poker site built with Haskell.

• [[http://hledger.org/][hledger]] - Friendly, robust, plain text accounting.

• [[https://owickstrom.github.io/komposition][Komposition]] - The video editor built for screencasters.

• [[https://github.com/matterhorn-chat/matterhorn][Matterhorn]] - A terminal client for the Mattermost chat system.

• [[https://github.com/lettier/movie-monad][Movie-Monad]] - A free and simple to use video player made with Haskell.

• [[https://neuron.zettel.page/][neuron]] - A future-proof command-line app for managing your plain-text Zettelkasten notes.

• [[https://github.com/jaspervdj/patat][patat]] - Terminal-based presentations using Pandoc.

• [[https://github.com/begriffs/postgrest][postgrest]] - REST API for any Postgres database.

• [[https://github.com/purescript/purescript][PureScript]] - A strongly-typed language that compiles to Javascript.

• [[https://github.com/agentm/project-m36][Project:m36]] - A relational algebra engine as inspired by the writings of Chris Date.

• [[https://taskell.app/][Taskell]] - Command-line Kanban board/task management.

• [[https://github.com/cdepillabout/termonad][termonad]] - A terminal emulator configurable in Haskell.

• [[https://github.com/tidalcycles/Tidal][Tidal]] - Language for live coding of pattern.