PutziSan/haskell-notes.md

## haskell-notes.md

      
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    haskell

Notes for learning haskell
ToDo

weitermachen bei http://learnyouahaskell.com/recursion
basics

notes from the help-cmd-tool (haskell landing-page) + learnyouahaskell.com

E-V-E-R-Y-T-H-I-N-G is a function

list


[1, '2'] => ERROR : a list can contain only one single type
[2,4,8,10] !! 2 => 8 : !! extracts item by index from an array
1 : [] => [1] the colone-func (:) adds a value at the head of a list
[2,4] ++ [8,10] => [2,4,8,10] : ++ concats two arrays
[3,2] > [2,3] => True ;  [2,2] > [2,1] => True : When using <, <=, > and >= to compare lists, they are compared in lexicographical order. First the heads are compared. If they are equal then the second elements are compared, etc.
head, tail, last, init, length, reverse, take, drop => array-funcs (siehe auch gute dokus von ramda zu funktionalitäten)
null [] => True : checks for empty list
elem 3 [1,2,3]=> True : check if first var is in given list (siehe auch infix-funktionen)
further List-funcs
cycle [1,2] => [1,2,1,2,...]; repeat 1 => [1,1,1,...] : produces infinite lists

strings == a list of chars


a char defined via single quote ', a string via double quote "
a string is a list of chars => ['h', 'i'] == "hi"
'h' != "h", da "h" == ['h']
'a' : "sap" => "asap" == ['a', 's', 'a', 'p'] : add the char 'a' to the char-list ['s', 'a', 'p'] == "sap"

range-operator ..


[first,sec..end] => füllt ein array mit der vorschrift: [first,first + step,first + 2 * step,…end] wobei step = sec - first
[1..4] => [1,2,3,4] : here sec is implicitly set to sec = first + 1
Doku allgemein zu 'arithmetic sequence'
can be used if the items implement Enum (read morein the documentation) (in principle strings with a-z and integer; double possible but not necessarily good)

variables


let x = 4 in x * x => 16 : define variables and give them scope (in just separates the expression from the body.)
let villain = (28,"chirs") in fst villain => 28 : returns first value of per variable defined tuple (see variables below for syntax)

extract variables via patterns


let (a,b) = (10,12) in a * 2 => 20 : (10,12) is matched against pattern (a,b) and so a resolves to 10
let (a,_,_) = (10,12,13) in a * 2 => 20 : _ is ignore-operator and can be used multiple in tuple- and list-patterns
let (a:b:c:[]) = "xyz" in a => x : whereby let (a:b:c:[]) == let (a:b:c) (see list for the colone-func)

(a:b:c:d) is short-hand for (a:(b:(c:d))) : so a is the first from b, which is the first from c and c is the rest
let (a:[]) = "xyz" in a => x
let (a:_) = "xyz" in a => x


let ab@(a,b) = (10,20) in (ab,a,b) => ((10,20),10,20) : VAR@... saves the original value of the pattern-matches in VAR

tuples


(28, "JP") => ... : keep a bunch of values together, (can contain different types)
fst (28, "JP") => 28 : returns the first val of a tuple
wenn shape of object is clear : tuple > list, zb, store x-y-coods:

A: [[1,2],[2,3]] vs B: [(1,2),(2,3)] =>
B > A, da

[[1,2],[2,3,4]] => no error for haskell
[(1,2),(2,3,4)] =>  haskell-error


singleton-tuple ((1)) does not exist

functions


let square x = x * x in square 3 => 9

let square x = x * x entspricht quasi (JS) const square = x => x * x
in (siehe #variables) nutzt die vorher definierte funktion square als higher order function


+ is a function (writing + (function) as plus for clarity) : let plus a b = a + b (second + ist die arithmetische operation)
most haskell-functions are curried, so plus (1) (2) == plus (1, 2) where plus 1 == plus (1) returns a function (added braces for clarity)
map (+1) [1..4] => [2,3,4,5] : map is global library-func, so map (+1) [1..4] == map (plus 1) [1...4]
3 `elem` [1,2,3] => True : using elem-func (list) with the infix-operator

where


<expression> where <bindings>
get1 = x where x=1 ; get1 => 1 : define variables after usage
calcSum a b = sum' a b where sum' c d = c + d : where can also define functions
where is no expression, so it cannot be used in expressions (somehow "only in functions") => see let ... in below


where bindings can also be nested. It's a common idiom to make a function and define some helper function in its where clause and then to give those functions helper functions as well, each with its own where clause.

let ... in


let <bindings> in <expression>
let x=1 in x+3 => 4
let ... in is an expression and can be used nearly everywhere

4 * (let a = 9 in a + 1) + 2 => 42
[let square x = x*x in (square 2, square 6)] => [4,36]


let a=2; b=6; in a*b => 12 : semicolons can sperate serval inline variables
other differntations (where/let):

let vs where,
Declaration vs. expression style


[add1 | a <- [1..4], let add1 = a+1] => [2,3,4,5] : The names defined in a let inside a list comprehension are visible to the output function (the part before the |)

List comprehension


abgeleitet von math. Notation: { 2x | x ∈ ℕ, x ≤ 10 } => [x*2 | x <- [1..10]] => [2,4,...,20]
[FUNCTION | DEFINITIONEN, NEBENBEDINGUNGEN, ...]

FUNCTION => wie sollen die einzelnen Elemente sein
DEFINITIONEN => nach Format x <- ...
CONSTRAINTS (Nebenbedingungen) => function (x `mod` 7 == 3, x < 5, ...)


composition: [ [ x | x <- xs, even x ] | xs <- [[1,2],[2,4]]] => [[2],[2,4]]

da FUNCTION pro ele ist kann man es entsprechend composen


use with pattern matching for variables: [a+b | (a,b) <- [(1,2),(6,9)] => [3,15]

function pattern matching

basic

is42 :: (Integral a) => a -> String
is42 42 = "is 42"
is42 x = "not 42"
this is syntactic suggar for a normal case expression:
is42 :: (Integral a) => a -> String
is42 n = case n of
  42 -> "is 42"
  n -> "not 42"
guards

detailedIs42 :: (Integral a) => a -> String
detailedIs42 numb
  | numb < 42 = "too low"
  | numb == 42 = "is 42"
  | otherwise = "too high"
types


:t 'a' => 'a' :: Char : :t returns the type
> 1 params: add x y = x + y, type-def: add :: Int -> Int -> Int

=> no diff between return-type + params (last type is return type) => see currying


read and show are functions, show is like a toString and read a bit like the opposite from show (reads the string and tries to get a type (Int, list, ...) from string) show 3 => "3", read "5" - 1 => 4 (read MUST be directly used afterwards or you have to add eplicit type annotations (see below))

type annotations


type annotations begins with ::
can be inlined read "5" :: Int  and (read "5" :: Float) * 4

type variables / polymorphic functions


:t head => head :: [a] -> a : a here is a type variable and can be any type
functions that have type variables are called polymorphic

Typeclasses / class constraints


a bit like interfaces in OOP
:t (==) => (Eq a) => a -> a -> Bool

Everything before the =>-Operator is called a class constraint (Nebenbedingung)
contraints for a type definition defines how the type variables have to look like
eg Eq a means that every following a MUST have the same type
basic typeclasses:

Eq e.g. :t (==) => (Eq a) => a -> a -> Bool
Ord e.g. (>) :: (Ord a) => a -> a -> Bool (type must have a ordering)
Show e.g. show :: Show a => a -> String (type must presentable as string) ! the show-fn (lowercased!) is like toString; show 3 => "3"
Read e.g. read :: Read a => String -> a (like opposite of Show/show); reads the string and tries to get a type (Int, list, ...) from string
Enum e.g. succ :: Enum a => a -> a : Enum-Nebenbedingung meint dass der Typ Numerierbar (bzw eine interne Ordnung haben muss)
Bounded e.g. minBound :: Bounded a => a : Type must have an upper and lower bound (maxBound- and minBound-fn)
Num e.g. :t 42 => 42 :: (Num t) => t and (*) :: (Num a) => a -> a -> a (product eg in 2*2)
Integral : includes Int and Integer (hat nicht mit einem eigentlichen Integral zu tun)
Floating : Float and Double


working with stack

get started

see documentation

unter windows empfiehlt stack in C:\sr-stack o.ä. zu installieren da osnst die langen Pfade zu Problemen führen können stack#3285 (ebenso auch die project-root für haskell-folder)


... init stack-prject (see docu)
stack build builds your app => stack build --file-watch => watch-mode
stack exec [YOUR_PROJECT_NAME]-exe execute


for VS Code with haskero: you need to install intero (stack build intero)

modules

Lib.hs:
module Lib (someExportFunc, otherExport) where

-- possible type-annotations:
someExportFunc :: IO ()
-- the func:
someExportFunc = putStrLn 'hello world'

-- [...] private implementation-funcs
privateFunc = "this will not be exported"

-- weitere export-funcs
otherExport x = x * x
Main.hs
module Main where

import Lib

main :: IO ()
main = someFunc

module MODULE_NAME (EXPORT_NAME_1, ...) where => defines the moduleName and which functions it will export
module MODULE_NAME where will export every defined function in the file

tools

code prettifier


hindent sieht cooler aus
stylish-haskell hatte bei mir bei der ersten installation nicht funktioniert
brittany another formatter

web-frameworks/libraries

allgemein:

dieser aritkel scheint den anschein zu amche ndas haskell deutlich weniger leisten könnte (aber zum anfang sollte das nicht so wichtig oder problematisch sein)

fjeden ToCheck:

yesod

good starting point for web => scheint älteste und "robusteste" zu sein
scheint aber komplettes web-framework zu sein (MVC-bla)


servant

angeblich speziell für REST-Apis sehr gut
scheint gutes bonus-set zu haben (auto-generate API-Docs in swagger,  JS client, mock server, documentation)


spock

im allgemeinen gilt spock > scotty, scheint aber nicht mehr so aktiv weiterentwickelt zu werden
müsste man sich weiter anschauen


SQL (mySQL, PostgreSQL)

good article which mentions a lot of different libs
List from the best to the worst (after the comments, no further own experiences currently)

project-m36
beam
haskell-opaleye + tisch (postgre-only)
persistent + esqueleto (?)
postgresql-simple + mysql-simple

scheint sehr gut zu sein  weil es am wenigsten tut => aber doku sagt mir spontan nix


examples

recursion

length' :: (Num b) => [a] -> b
length' [] = 0
length' (_:xs) = 1 + length' xs
factorial :: (Integral a) => a -> a
factorial 0 = 1
factorial n = n * factorial (n - 1)
guards

max' :: (Ord a) => a -> a -> a
max' a b
    | a > b     = a
    | otherwise = b
myCompare :: (Ord a) => a -> a -> Ordering
a `myCompare` b
    | a > b     = GT
    | a == b    = EQ
    | otherwise = LT

this example includes the backticks (`myCompare`) for readability

where

initials :: String -> String -> String
initials firstname lastname = [f] ++ ". " ++ [l] ++ "."
    where (f:_) = firstname
          (l:_) = lastname
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