# Created 2019-04-01 Mon 13:31
#+TITLE: Haskell for the working programmer
#+AUTHOR: Yann Esposito
#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
#+LANGUAGE: en
#+KEYWORDS: haskell
#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)

* TO-CLEAN Introduction

This is somehow a follow-up from Learn Haskell Fast and Hard.
Which was more about being able to /play/ with Haskell than to /work/ with it.

This is also an experiment.
I'm not sure if it will be as positive as I hope.
This book try to be a good resource to learn Haskell but to speed up the
learning in the first part I'll skip the explanation about why Haskell does
things the way it does.
As a consequence if you don't keep in mind that there is *very good* reasons to
make some things way more difficult in Haskell than in other languages you
might miss the real reason.

Also don't forget in the beginning you might only see what is more difficult or
harder to achieve in Haskell.
But for each thing harder, keep in mind that there are very difficult things in
other languages that are solved extremely easily in Haskell.
And I personnally believe the things Haskell make easier are essential to reach
the best balance between speed, elegance, safety and pragmatism with regards to
any programming language I ever used before.

So this book might be a bit raw.
And in fact not really "fun" unfortunately.
But it should be efficient.

This book is aimed to be one of the fastest way to learn how to be productive
with Haskell.

Know that there still will be a very long road ahead once this book will be
finished to master Haskell. That should be ok. Even with those basic knowledge,
you should already be more productive in Haskell than in most other programming.

Modern computing has unfortunately less to do with algorithmic than to create a
mashup of libs and external APIs.
So while learning all the details of Haskell can seems like an impossible challenge.
Learning the necessary skills to be productive shouldn't be that hard.

What does this book will talk about.

1. Having a clean and stable dev environment
2. Basic Introduction to the language
3. Professional Project developement workflow
4. Make command line program
5. Use external libraries
6. Handle the filesystem
7. Handle a few DBs
8. Make a basic REST API

What Haskell can do few other programming language can.

Ability to put strong constraingt on part of the code. For exemple you can have
confidence in 3rd party functions. You can be certain that there will be NO side
effect. Or you can also ensure that part of you code can only write logs and not
access the DB. We'll technique that will ensure that subpart of the code will
only access the User table in your DB etc...

Writting parallel and concurrent code because /very/ easy to write.
While this is generally a nightmare in most programming language.

** TO-CLEAN What does "working programmer" stand for?

Being able to:

- create a new working program from scratch,
- work with the filesystem (read/write files/directories),
- work with BDD (SQLite, PostgresSQL, MongoDB, etc...),
- work with network (send/receive HTTP request),
- make a REST API,
- use most libraries (OpenGL, ncurses, etc...)
- write test for your application,
- to deploy your application

This is more about being an user, consumer from the Haskell community than being
an active contributor. Hopefully the gap won't be hard to pass from user to
contributor. So I'll write a minimal chapter about how to write your own library
and publish it for other developpers.

** TO-CLEAN Prerequiste

The target audience I'm writting this book for is software developpers.

You should:

- be familiar with some programming language,
- be familiar with command line in a shell,
- know how to editing text files (I try to focus on generic editors like emacs,
  vim, etc...),
- know the basic usage of =git=

If you don't know that, your journey with this book might be a bit difficult but
I'll do my best to not make it impossible.

** TO-CLEAN Opinionated

Keep in mind that Haskell has a very active and open ecosystem.
And the language itself let you make very different choices to the fundamentals.

This book is very opinionated, because I wanted to be efficient in learning
fast for some specific kind of personalities.

It might not be for you.
One of my goal is to shortcircuit some classic learning detour.

For a lot of decisions I generally make only one choice. I'll try to talk about
the other choices and it will be your duty to explore other choices after you
completed this book to decide which is the one that has your preference.

Also note that this book was written in the past. And as I said Haskell
ecosystem evolve very fast. And some choices which are an evidence today might
be deprecated in a few months from now.

Typically there are many different and concurrent web frameworks, db libs, etc..

** TO-CLEAN A Word about Haskell philosophy

One Haskell main characteristic is that it tends to make the right/most secure
choice by default.

A very simple example is that it is generally harder to write unsafe code than
to write safe and pure code.

Also one of the reason I think Haskell is percieved as hard to learn by many
people is that you generally need to ingest a lot of concepts before being able
to be productive.
#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
#+LANGUAGE: en
#+KEYWORDS: haskell
#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)

** TO-CLEAN Install a dev environment (about 30 minutes)

 Installing a dev environment should hopefully be the most boring part of this
 book. But this is a necessary price to pay to really get why Haskell is
 considered so great by people using it.

**** TO-CLEAN Working environment

 A thing to note is the distinction between learning a language for personal
 interrest for some personal project and learning with the goal to achieve a
 "product" with some hard deadline.

 So for example, it can be nice to understand the language by playing inside a
 REPL. We won't use that much in this book as the goal is not to really gain a
 deeper knowledge but perhaps to be able to /use/ the language.

 The problem I try to solve in this book is to make you a professional /user/ of
 Haskell more than a /contributor/ to Haskell. While I encourage everybody to
 gain deeper understanding on the internals of Haskell this is not the primary
 goal of this book.

 What I mean by professional /user/ is that you should have the following
 features at your disposal:

 - DCVS
 - Generated documentation
 - Tests (Unit tests, Generative tests, Integration tests, etc...)
 - Benchmark
 - Continuous Integration
 - Continuous Deployment

 Choices:

 - Raw: get GHC and cabal exectuable and work with that. Too long and manual
 - Nix: this is really great because it's like a super make that can deal with
   external dependencies. Certainly the best tool in the long term.
 - Stack: fast to install focused on being user friendly. Has a lot of easy to
   use features like:
   - integration with docker that will make it easy to cross-compile, deploy.
   - integration with nix
   - easy to deal with many private repositories
   - good professional starting templates

**** TO-CLEAN Stack

 I recommend [[https://haskellstack.org][stack]]. But there are many different method to install Haskell.
 Stack should be simple and straight to the point.

 If thing haven't changed since the book was written it could be installed with:

 #+BEGIN_SRC shell
   curl -sSL https://get.haskellstack.org/ | sh
 #+END_SRC

**** TO-CLEAN git

 You should have =[[https://git-scm.com][git]]= installed.

**** TO-CLEAN Editor

 You should check any of the supported editor here:

 https://github.com/rainbyte/haskell-ide-chart#the-chart-with-a-link-to-each-plug-in

 I personnaly use [[http://spacemacs.org][spacemacs]] with the haskell layer because it comes with battery
 included. If you're not used to vim keybindings I believe it is easy to switch
 to more classical editor keybindings easily.

 Even if I don't have a strong opinion on the editor you should choose. It should
 at least be easy to support the Haskell tooling, like intero or ghc-mod. Because
 it's one of the best part of Haskell.

 For example without any configuration I have the following features:

 - I see errors, warn and even code hints while I'm typing my code.
 - very good code completion
 - auto styling of my source code and be able to change the style of my entire
   buffer
 - be able to get the type of the expression under my cursor
 - be able to add the type of a top level declaration
 - be able to launch a repl easily loading the current code of the file I'm
   currently editing

 And many other nice features.

 Note that in the past I had some problem with ghc-mod during upgrades while
 intero was mostly a no problem story.

 It is also useful to have hoogle and hayoo, which are search engine focused on
 Haskell.

***** TO-CLEAN Spacemacs

 So if you want to choose spacemacs:

 1. Install a recent emacs
 2. =git clone https://github.com/syl20bnr/spacemacs ~/.emacs.d=
 3. Launch emacs
 4. Edit your =~/.spacemacs= file to add to the layer list:

 #+BEGIN_SRC elisp
   haskell
   (auto-completion :variables
                    auto-completion-enable-help-tooltip t
                    auto-completion-enable-short-usage t)
 #+END_SRC

 If you're not used to vim keybinding and it is too much to handle for you.
 I think you can change the value of =dotspacemacs-editing-style= from ='vim=
 to ='hybrid= or ='emacs= in the =.spacemacs= file.

 It should be good now.
**** TO-CLEAN Conclusion

 First you can congratulate yourself to have installed all the prerequiste to
 have a great working development environment.

 I know it was already a lot of boring tasks to perform before being able to
 write any line of code. But I promise it will be worth it.

 By starting with this template, you won't use the classic prelude. It's quite a
 strong opinionated move. Because many classic function will be overwritten by
 safer/more generic one.

 So be prepared that the actual learning route is jumping other classical
 learning steps you can find in other learning resources. Don't worry I'll do my
 best to make the jump as natural as possible.
 #+TODO: TODO TO-CLEAN TO-REVIEW | DONE
 #+LANGUAGE: en
 #+KEYWORDS: haskell
 #+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)

* TODO Data Management

You can think of Haskell and its Prelude as a Linux installation.
You have a kernel and a bunch of executable.

So in order to manage your Unix env you need to know a few command line first.
Then, slowly with the need and experience, you'll learn more and more commands.

So we'll do the same thing here.
We'll start with only what we need to start a project.
And You'll be introduced with new notion as they are needed.

** Prelude/Protolude

So Haskell prelude is old and it is very easy to use more modern prelude.
So I'll tend to do just that.

*** Data Manipulation Functions
* TODO Effect Management

In that part of the book, we'll use simple examples. Thus instead of going
directly to a full project structure we'll focus on the language. That file can
be treated as a single executable strict.

For example:

#+BEGIN_SRC haskell
#!/usr/bin/env stack
{- stack script
   --resolver lts-13.13
   --install-ghc
   --package protolude
-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE OverloadedStrings #-}
import Protolude

main = putText "Hello World!"
#+END_SRC

The firsts line are simply here to set the correct execution environment.
The real program starts after them.
Once =stack= will be installed (see the /Install a dev environment/ section)
if you put that content in a file named =hello.hs= then you can launch it with:

#+BEGIN_SRC
> chmod +x hello.hs
> ./hello.hs
#+END_SRC

The first time it is launched can take a little bit of time because it will
download all dependencies. The advantage of this form of distribution is that it
is a quasi self-contained exectuable. That's a good one for minimal examples.

But after a short introduction we'll use full projects.

We'll start by example first and all notion will be introduced as they appear.
If you find confident you could feel free to skip some descriptions and
explanations.

** TODO Short Examples / Scripts
*** TO-CLEAN Guess a number
**** TO-CLEAN Print and read things

Now let's modify the code of =main= to print things.
First comment the import line for =Lib=.
Haskell comment are =--= till the end of the line or ={- .... -}=
for multiline comments.
Without this comment you'll get a warning that this import is unused.
And by default we compile using =-Werror= flag to GHC which tell that the
compilation should fail also on warnings as well as on errors.

The default template tend to be a professional environment and has more
restrictions in order to maximize confidence in quality.

#+BEGIN_SRC haskell
  #!/usr/bin/env stack
  {- stack script
     --resolver lts-11.6
     --install-ghc
     --package protolude
  -}
  {-# LANGUAGE NoImplicitPrelude #-}
  {-# LANGUAGE OverloadedStrings #-}
  import Protolude
  
  main = putText "Hello, world!"
#+END_SRC

Simple and natural.
Now let's ask your name.

#+BEGIN_SRC haskell
  #!/usr/bin/env stack
  {- stack script
     --resolver lts-11.6
     --install-ghc
     --package protolude
  -}
  {-# LANGUAGE NoImplicitPrelude #-}
  {-# LANGUAGE OverloadedStrings #-}
  import Protolude
  
  main = do
   putText "What is your name?"
   name <- getLine
   putText ("Hello " <> name <> "!")
#+END_SRC

We can try that in the REPL (GHCI). You should be able to start it from your
editor. For example in spacemacs I can load the current buffer (open file) in
the REPL with =SPC m s b=.

You could also start the repl in a terminal with =stack ghci=
And then load the module with =:l hello_name.hs=.
The =:l= is a shortcut for =:load=.

#+BEGIN_SRC
  > stack ghci
  
  Warning: No local targets specified, so ghci will not use any options from your package.yaml / *.cabal files.
  
           Potential ways to resolve this:
           ,* If you want to use the package.yaml / *.cabal package in the current directory, use stack init to create a new stack.yaml.
           ,* Add to the 'packages' field of ~/.stack/global-project/stack.yaml
  
  Configuring GHCi with the following packages:
  GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
  Loaded GHCi configuration from /private/var/folders/bp/_8thkcjd4k3g81mpxtkq44h80000gn/T/ghci70782/ghci-script
  Prelude> :l hello_name.hs
  [1 of 1] Compiling Main             ( hello_name.hs, interpreted ) [flags changed]
  Ok, one module loaded.
  ,*Main> main
  What is your name?
  Yann
  Hello Yann!
#+END_SRC

But you should also simply run it from command line:

#+BEGIN_SRC
  > ./hello_name.sh
  What is your name?
  Yann
  Hello Yann!
#+END_SRC

OK simple enough.

But let's take a moment to understand a bit more what's going on.

We started with the =do= keyword.
It's a syntactical sugar that helps in combining multiple lines easily.
Let's take a look at the type of each part.

#+BEGIN_SRC haskell
  putText :: Text -> IO ()
#+END_SRC

It means that =putText= is a function that take a =Text= as parameter and return
an =IO ()=.
Mainly =IO ()= simply means, it will return =()= (nothing) while doing some IO
or border effect.
The border effect here being, writing the text to the standard output.

#+BEGIN_SRC haskell
  putText "What is your name?" :: IO ()
#+END_SRC

So yes this line make an IO but returns nothing significant.

#+BEGIN_SRC haskell
  name <- getLine
#+END_SRC

The function =getLine= will read from standard input and provide the line read
and send the value as a =Text=. If you look at the type of =getLine= you have:

#+BEGIN_SRC haskell
  getLine :: IO Text
#+END_SRC

And that means that to be able to retrieve and manipulate the Text returned by
in an "IO context" you can use the =<-= notation.
So in the code the type of =name= is =Text=

More generally if =foo :: IO a= then when you write

#+BEGIN_SRC haskell
  do
    x <- foo :: IO a
#+END_SRC

Then the type of =x= is =a=.

Finally the last line:

#+BEGIN_SRC haskell
  putText ("Hello " <> name <> "!")
#+END_SRC

=putText= take a =Text= as argument so: =("Hello " <> name <> "!") :: Text=.

So =(<>)= is the infix operator equivalent to the function =mappend=.
Here are equivalent way to write the same thing:

#+BEGIN_SRC haskell
  "Hello" <> name <> "!"
  "Hello" `mappend` name `mappend` "!"
  
  mappend "Hello" (mappend name "!")
  (<>) "Hello" ((<>) name "!")
#+END_SRC

So in Haskell if your function contains chars it will be a prefix function.
If your function contains special chars then it is considered to be an infix
operator.

You can use your function as infix if you put "`" around it name.
And you can make your operator prefix if you put it inside parentheses.

So you should have remarqued a pattern here.
Which is really important. Each line of a =do= bloc has a type of =IO a=.

#+BEGIN_SRC haskell
  main = do
    putText "What is your name?"      :: IO ()
    name <- getLine                   :: IO Text
    putText ("Hello " <> name <> "!") :: IO ()
#+END_SRC

So whenever you have an error message try to think about the type of your
expression.

Another very important aspect to notice.
The type of ="Hello " <> name <> "!"= is =Text= not =IO Text=.
This is because this expression can be evaluated purely.
Without any side effect.

Here we see a clear distinction between a pure part of our code and the impure
part.

#+BEGIN_QUOTE


*☞ Pure vs Impure* (function vs procedure)

That is one of the major difference between Haskell and other languages.
Haskell provide a list of function that are considered to have border effects.
Those functions are given a type of the form =IO a=.

And the type system will restrict the way you can manipulate function with type
=IO a=.

So, first thing that might be counter intuitive.
If an expression has a type of =IO a= it means that we potentially perform a
side effect and we "return" something of type =a=.

And we don't want to ever perform a side effect while doing any pure evaluation.
This is why you can't write something like:

#+BEGIN_SRC haskell
  -- DOESN'T COMPILE
  main = do
     putText ("Hello " <> getLine <> "!")
#+END_SRC

Because you need to "traverse" the =IO= barrier to get back the value after the
evaluation.
This is why you NEED to use the =<-= notation.
Now knowing if a code is potentially making any side effect is /explicit/.
#+END_QUOTE

***** TO-CLEAN Strings in Haskell digression

Generally working with string is something you do at the beginning of learning a
programming language.
It is straightforward.
In Haskell you have many different choices when dealing with Strings depending
on the context.
But let just say that 95% of the time, you'll want to use Strict =Text=.

Here are all the possible choices:

- =String=: Just a list of =Char= very inefficient representation,
- =Text=: UTF-16 strings can be Lazy or Strict,
- =Bytestring=: Raw stream of =Char= and also =Lazy.Bytestring=.

That is already 5 different choices. But there is another package that provide other string choices.
In =Foundation= the strings are =UTF-8=.

Hmmm... so much choices.

A rule of thumbs is to never use =String= for anything serious.
Use =Text= most of the time because they support encoding.
Use =Bytestring= if you need efficient bytes arrays.

By using Protolude, we naturally don't use =String=.

**** TO-CLEAN Guess my age program

So far so good.
But the logic part of the code should be in a library in =src/= directory.
Because this part is easier to test.

The =src-exe/Main.hs= should be very minimalist, so now let's change its content
by:

#+BEGIN_SRC haskell
  #!/usr/bin/env stack
  {- stack script
     --resolver lts-11.6
     --install-ghc
     --package protolude
  -}
  {-# LANGUAGE NoImplicitPrelude #-}
  {-# LANGUAGE OverloadedStrings #-}
  import Protolude
  
  guess :: IO ()
  guess = undefined
  
  main :: IO ()
  main = do
    guess
    putText "Thanks for playing!"
#+END_SRC

We know that the type of guess must be =IO ()=.
We don't know yet what the code will be so I just used =undefined=.
This way the program will be able to typecheck.

The next step is to define the ~guess~ function.

#+BEGIN_SRC haskell
  #!/usr/bin/env stack
  {- stack script
     --resolver lts-11.6
     --install-ghc
     --package protolude
  -}
  {-# LANGUAGE NoImplicitPrelude #-}
  {-# LANGUAGE OverloadedStrings #-}
  import Protolude
  
  guess :: IO ()
  guess = guessBetween 0 120
  
  guessBetween :: Integer -> Integer -> IO ()
  guessBetween minAge maxAge = do
    let age = (maxAge + minAge) `div` 2
    if minAge == maxAge
      then putText ("You are " <> show minAge)
      else do
        putText ("Are you younger than " <> show age <> "?")
        answer <- getLine
        case answer of
          "y" -> guessBetween minAge (age - 1)
          _ ->  guessBetween (if age == minAge then age + 1 else age) maxAge
  
  main :: IO ()
  main = do
    guess
    putText "Thanks for playing!"
#+END_SRC

So going from there we declared the =guess= function to call the =guessBetween=
function with the two paramters 0 and 120 to guess an age between 0 and 120.

And the main function is a classic recursive function.
We ask for each age if the user is younger than some age.

the =let= keyword permit to introduce pure values in between =IO= ones.
so =age = (maxAge + minAge) `div` 2= is mostly straightforward.
Note that we manipulate =Integer= and so that mean =`div`= is the integer division.
so =3 `div` 2 == 1=.

We see that working in IO you can put print statements in the middle of your
code. First remark we used a recursive function. In most imperative programming
languages explicit loops are preferred to recursive functions for efficiency reasons.
That shouldn't be the case in Haskell.

In Haskell recursive functions are the natural way to program things.

Important Remarks to note:
- to test equality we use the =(==)= operator.
- Haskell is lazy, so the =age= value is only computed if needed. So if you are
  in the case where =minAge == maxAge=, =age= value is not evaluated.
- In Haskell =if .. then .. else ..= form always have an else body. There is no
  Implicit "no result" value in Haskell. Each expression need to return
  something explicitely. Even if it is the empty tuple =()=.

So now here we go:

#+BEGIN_SRC
  > stack build
  > stack exec -- guess-exe
  Are you younger than 60?
  y
  Are you younger than 29?
  n
  Are you younger than 44?
  y
  Are you younger than 36?
  n
  Are you younger than 39?
  n
  Are you younger than 41?
  y
  Are you younger than 39?
  n
  You are 40
  Bye!
#+END_SRC

We see we can still make the program better.
For example, the same question is asked twice in that example.
Still, it works.

*** TO-CLEAN Guess a random number

Let's write another slightly more complex example.
Instead of guessing the age of somebody.
This will be the role of the user to guess a random number choosen by the
program.

First we'll need to generate random numbers.
To that end we'll use a the =random= package as a new dependency.

You can get more information either on hackage or on stackage:

- https://hackage.haskell.org/package/random
- https://www.stackage.org/lts-11.7/package/random

Hackage is the official place where to put Haskell public libraries.
Stackage works in conjunction with =stack= and mainly it takes care of having a
list of packages version working together.
So that means that all packages in an LTS (Long Term Support) release can work
together without any build conflict.

Now let's use that package.
Notice the added =--package random= argument.

We'll start by writing a =guessNumber= function:

#+BEGIN_SRC haskell
  #!/usr/bin/env stack
  {- stack script
     --resolver lts-11.6
     --install-ghc
     --package protolude
     --package random
  -}
  {-# LANGUAGE NoImplicitPrelude #-}
  {-# LANGUAGE OverloadedStrings #-}
  
  import Protolude
  
  import System.Random (randomRIO)
  
  ...
  
  -- | Choose a random number and ask the user to find it.
  guessNumber :: IO ()
  guessNumber = do
    n <- randomRIO (0,100)
    putText "I've choosen a number bettween 0 and 100"
    putText "Can you guess which number it was?"
    guessNum 0 n
  
  -- | Given a number of try the user already made and the number to find
  -- ask the user to find it.
  guessNum :: Int -> Int -> IO ()
  guessNum nbTry nbToFound = undefined
#+END_SRC

So for now we just focus on how to get a random number:

#+BEGIN_SRC haskell
  do
    n <- randomRIO (0::Int,100)
    -- do stuff with n
#+END_SRC

You NEED to use the =<-= notation inside a =do= bloc.
If you try to use =let n = randomRIO (0,100)= it will fail because the
types won't match.

And that's it!

Now to write the =guessNum= function, we'll write a classical recursive function:

#+BEGIN_SRC haskell
  -- | Given a number of try the user already made and the number to find
  -- ask the user to find it.
  guessNum :: Int -> Int -> IO ()
  guessNum nbTry nbToFound = do
    putText "What is your guess?"
    answer <- getLine
    let guessedNumber = readMaybe (toS answer)
    case guessedNumber of
      Nothing -> putText "Please enter a number"
      Just n ->
        if n == nbToFound
          then putText ("You found it in " <> show (nbTry + 1) <> " tries.")
          else do
            if n < nbToFound
            then putText "Your answer is too low, try a higher number"
            else putText "Your answer is too high, try a lower number"
            guessNum (nbTry + 1) nbToFound
#+END_SRC

Let's read the program line by line:

- ~putText "What is your guess?"~ should be straightforward.
- ~answer <- getLine~ So the ~getLine~ read from standard input and returns the
  line entered by the user. The line will be put in the ~answer~ variable.
- ~let guessedNumber = readMaybe (toS answer)~: there are a few things to tell about this line.

If you open GHCI and ask the type for each interresting symbol here is what you get:

#+BEGIN_SRC
  λ :t getLine
  getLine :: IO Text
  
  λ :t toS
  toS :: StringConv a b => a -> b
  
  λ :t readMaybe
  readMaybe :: Read a => GHC.Base.String -> Maybe a
#+END_SRC

- ~answer~ comes from ~getLine :: IO Text~ so ~answer~ should have the type ~Text~.
- Now we want to read this ~Text~ and see if this is a number and compare it to another ~Int~.
- To transform the number we don't use a function ~textToInt~ we simply use a
  quite generic function ~readMaybe~ that take some ~String~ and try to
  transform that to some type. For our specific case, the compiler is able to
  figure out the type we want to transform the text into is ~Int~. Take the
  time to digest that: ~Int~ is specified in the type signature of the
  ~guessNum~ function so the compiler could discover that ~readMaybe~ should
  return a ~Maybe Int~. How does he do that? Let's follow:
  1. see a ~n == nbToFound~ so we can deduce ~n~ and ~nbToFound~ have the same type.
  2. Reading the type signature of the function it is clear ~nbToFound~ is of
     type ~Int~ (it's the second argument of a function with type ~Int -> Int -> IO ()~)
  3. Then ~n~ is generated from a pattern matching; the case ~Just n~ which
     could be the the result of the ~readMaybe~ function. So we can deduce
     that the ~a~ in the type signature of ~readMaybe~ is ~Int~ for this specific case.
- so ~guessedNumber :: Maybe Int~, if the user enter something that cannot be
  transformed in number from a string then ~guessedNumber~ would be equal
  to ~Nothing~ and we ask the user to enter a number. If the user entered a
  number the type will be ~Just n~ were ~n~ will be an ~Int~.
- We compare the ~guessedNumber~ to the number to found ~nbToFound~.
- If the user found the right number we stop here by displaying the number of try.
- If the user hasn't found the number, depending on its value we tell the user
  it's either too low or too high and we call the same function, this time, we
  increment the number of try.

The full program is then:

#+BEGIN_SRC haskell
  #!/usr/bin/env stack
  {- stack script
     --resolver lts-11.6
     --install-ghc
     --package protolude
     --package random
  -}
  {-# LANGUAGE NoImplicitPrelude #-}
  {-# LANGUAGE OverloadedStrings #-}
  
  import Protolude
  
  import System.Random (randomRIO)
  
  main :: IO ()
  main = guessNumber
  
  -- | Choose a random number and ask the user to find it.
  guessNumber :: IO ()
  guessNumber = do
    n <- randomRIO (0,100)
    putText "I've choosen a number bettween 0 and 100"
    putText "Can you guess which number it was?"
    guessNum 0 n
  
  -- | Given a number of try the user already made and the number to find
  -- ask the user to find it.
  guessNum :: Int -> Int -> IO ()
  guessNum nbTry nbToFound = do
    putText "What is your guess?"
    answer <- getLine
    let guessedNumber = readMaybe (toS answer)
    case guessedNumber of
      Nothing -> putText "Please enter a number"
      Just n ->
        if n == nbToFound
          then putText ("You found it in " <> show (nbTry + 1) <> " tries.")
          else do
            if n < nbToFound
            then putText "Your answer is too low, try a higher number"
            else putText "Your answer is too high, try a lower number"
            guessNum (nbTry + 1) nbToFound
#+END_SRC

which once executed:

#+BEGIN_SRC
  > ./guess_number.hs
  I've choosen a number bettween 0 and 100
  Can you guess which number it was?
  What is your guess?
  50
  Your answer is too low, try a higher number
  What is your guess?
  75
  Your answer is too low, try a higher number
  What is your guess?
  90
  Your answer is too high, try a lower number
  What is your guess?
  83
  Your answer is too low, try a higher number
  What is your guess?
  87
  You found it in 5 tries.
#+END_SRC

**** TO-CLEAN What did we learn so far?

So up until now, if you followed. You should be able to "reproduce" and make
minimal changes.
But I am certain than it still be difficult to make some changes.
It is time to learn some general principles.
I know it might be a bit repetitive but its important to be certain to ingest
those informations.

A generic function of type ~IO ()~ typically =main= should look like:

#+BEGIN_SRC haskell
  f :: IO a
  f = do
      α <- f1
      β <- f2
      γ <- f3
      δ <- f4
      f5
#+END_SRC

where each expression =fi= is of type =IO a= for some =a=.
You can use any value =α=, =β=, etc‥ as a parameter.
In order to be valid.
The last expression must have the same type as =f=.
so here =f5 :: IO a=.

Now if I give you the following functions:

- ~getLine :: IO Text~ that read a line from stdin.
- ~putText :: Text -> IO ()~ that read a line from stdin.

With that you have the ability to read stdin and print things.

- ~if τ then f1 else f2~ where =τ :: Bool= and the type of ~f1~ and ~f2~ must be the
  same. Generally this is denoted by: =:type f1 ~ :type f2= and that type
  will be the same as the entire ~if ‥ then ‥ else ‥~ expression.
- you can compare things that can be compared with ~<~, ~<=~, ~>~, ~>=~, ~==~, ~/=~ (different).
- you can concatenate things that could be concatenated (like Text) with ~<>~
- you can transform things as Text with ~show~ in particular numbers.

So that is a few number of component but they are all composable.
And so far we only needed that to write our first programs.

Haskell libs will provide you with a lot more base functions but also a lot more
composition functions.

*** TODO Command Line Application

Another thing you might want to achieve at first is to retrieve arguments for a
command line application.

**** TO-CLEAN Basic

The simplest way to retrieve parameters to a command line is to use the ~getArgs~ function.

#+BEGIN_SRC haskell
  getArgs :: IO [String]
#+END_SRC

Here is a minimal example.

#+BEGIN_SRC haskell
  #!/usr/bin/env stack
  -- stack --resolver lts-11.6 script
  {-# LANGUAGE OverloadedStrings #-}
  {-# LANGUAGE NoImplicitPrelude #-}
  import Protolude
  import System.Environment (getArgs)
  
  main :: IO ()
  main = do
    arguments <- getArgs
    case head arguments of
      Just filename -> die ("The first argument is: " <> toS filename)
      Nothing -> die "Please enter a filename"
#+END_SRC

#+BEGIN_SRC
  > ./cmdline-basic.sh foo
  The first argument is: foo
  > ./cmdline-basic.sh
  Please enter a filename
#+END_SRC

If you have a very basic command line it could be good enough.
But if you plan to have more things to configure you can consider
to use a library to parse options.

**** TODO Option Parsing

For that we will use the =optparse-generic= package.


#+BEGIN_SRC haskell
  #!/usr/bin/env stack
  {- stack script
     --resolver lts-11.6
     --install-ghc
     --package protolude
     --package optparse-generic
  -}
  {-# LANGUAGE NoImplicitPrelude #-}
  {-# LANGUAGE OverloadedStrings #-}
  import Protolude
  import System.Environment (getArgs)
  
  main :: IO ()
  main = do
    arguments <- getArgs
    case head arguments of
      Just filename -> die ("The first argument is: " <> toS filename)
      Nothing -> die "Please enter a filename"
#+END_SRC

*** TODO File Access
*** TODO Daemons & Logging
** TODO Intermediate
*** TO-CLEAN Stack template

☞ As a first projet a lot of new concept will be introduced. Don't be
discouraged by that.

Let's create a project with a sane and modern file organisation.

I made a stack templates largely inspired by =tasty-travis= template. It will
provide a bootstrap for organizing your application with tests, benchmarks and
continuous integration.

This template provide a file organisation for your projects.

Mainly do jump into programmin you could theoretically just download the binary
of the main Haskell compiler GHC to your compiler and compile each file with
=ghc myfile.hs=. But let's face it. It's not suitable for real project which
need more informations about it.

So let's start with a sane professional organisation for your files.

#+BEGIN_COMMENT
****** TODO modify the URL to use a better URL: torrent / IPFS
#+END_COMMENT

#+BEGIN_SRC shell
  stack new guess https://git.io/vbpej
#+END_SRC

After that, this should generate a new ~guess~ directory with the following
files:

#+BEGIN_SRC
  > tree
  .
  ├── CHANGELOG.md
  ├── LICENSE
  ├── README.md
  ├── Setup.hs
  ├── guess.cabal
  ├── package.yaml
  ├── src
  │   └── Lib.hs
  ├── src-benchmark
  │   └── Main.hs
  ├── src-doctest
  │   └── Main.hs
  ├── src-exe
  │   └── Main.hs
  ├── src-test
  │   └── Main.hs
  ├── stack.yaml
  └── tutorial.md
  
  5 directories, 13 files
#+END_SRC

Most of your source code should be in the =src= directory. Generally =src-exe=
should be a minimal code that could handle the =main= function to start your
application. We'll talk about other parts later in the book but most other file
should be quite straightforward.

Edit the file =src-exe/Main.hs=

The file contains:

#+BEGIN_SRC haskell
  import Protolude
  
  import Lib (inc)
  
  main :: IO ()
  main = print (inc 41)
#+END_SRC

To compile it do a

#+BEGIN_SRC
  > stack build
  > stack exec -- guess-exe
  42
#+END_SRC

So that program print 42 and stop.

*** TODO DB Access
**** NoSQL (Redis looks easy)
**** Stream DB (Kafka or NATS, etc...)
**** SQL (SQLite & Postgres)
Not sure about that part. Perhaps this should move in the Production section
*** TODO REST API
**** TODO Servant
**** TODO JSON manipulation
**** TODO Swagger-UI
** TODO Intermediate Conclusion

Congratulation for going this far. Now you should be able to work in Haskell at
least as well as in any other programming language.

Now there are different directions:

- learning more libraries
- learn to optimise code to make it as fast as C
- learn to understand details of the compilation and Haskell
- learn tips and tricks
- learn more about abstractions and type classes
- learn parallel and concurrent programming
- learn to deploy like a pro using nix

The order in which to learn all thoses things can be very different for everty need.
#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
#+LANGUAGE: en
#+KEYWORDS: haskell
#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)

* TODO Make Production Quality Products
** TODO Dhall /Maybe/
** TODO Error Handling

** TODO Unit testing / doctests
** TODO Generative Testing
** TODO Enhance reproductibility with docker
** TODO Enhance reproductibility with nix
** TODO How to deploy?

There are plenty of ways de deploy

*** TODO Trashy and easy

Compile in docker and copy the binary.

*** TODO With =nix= and =nixops=
#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
#+LANGUAGE: en
#+KEYWORDS: haskell
#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)

* TODO Enhance Code Quality
** TODO Type tricks


*** TODO New Types
*** TODO Phantom Types
** TODO Useful Abstractions
*** TODO Monoid
*** TODO Functors
*** TODO Applicative
*** TODO Monads
*** TODO Arrows
*** TODO Foldable
*** TODO Traversable
** TODO Useful Abstractions for Applications
*** TODO No organisation, everything in IO
*** TODO Handler Pattern

See the talk from Jasper Van Der Jeught

In my opinion as efficient as MTL, Free, Effects, but with more verbosity and
repetitions.

*** TODO Custom Monad
*** TODO MTL
*** TODO Free
*** TODO Effects
** TODO Lenses

This will only be an introduction for being an user of the library.

** TODO Generics and lens-generic
** TODO Code organisation
*** TODO No organisation, everything in IO
*** TODO Custom Monad
*** TODO Handler Pattern
*** TODO MTL
*** TODO Free
*** TODO Effects
** TODO Recognize some classical abstractions
*** TODO Algebra
*** TODO Catamorphisms
*** TODO Free & Interpreters
#+TODO: TODO TO-CLEAN TO-REVIEW | DONE
#+LANGUAGE: en
#+KEYWORDS: haskell
#+PROPERTY: header-args :output-dir HWP :mkdirp yes :tangle-mode (identity #o755)

* TODO Appendices

** TODO Papers You Should have read