naoto-ogawa / haskell-must-read

must-read articles, papers, QAs and so on.

haskell-must-read

must-read articles, papers, QAs and so on.

haskell project structure

• Haskeleton: a Haskell project skeleton
• http://taylor.fausak.me/2014/03/04/haskeleton-a-haskell-project-skeleton/

String

• String types

• https://haskell-lang.org/tutorial/string-types

• Untangling Haskell's Strings

• https://mmhaskell.com/blog/2017/5/15/untangling-haskells-strings

• difference between ByteString ByteString.Char8 and normal string?

• https://www.reddit.com/r/haskell/comments/4zt8xh/difference_between_bytestring_bytestringchar8_and/

• A guided tour through the bytestring library

• http://meiersi.github.io/HaskellerZ/meetups/2012%2001%2019%20-%20The%20bytestring%20library/slides.html#(1)

• Char -> Unicode charactor

• String -> [Char] ie. List of Chars, imutable, arranged in memory as a linked list of characters

• Text -> internally UTF-16

• ByteString -> sequence of bytes, list of Word8

• Char8.ByteString -> treating a ByteString as if ISO-8859-1 encoded.

> S8.pack "abc" :: Data.ByteString.Char8.ByteString
"abc"
> S8.pack "abc" :: Data.ByteString.ByteString
"abc"

• Type Conversion

        -->  T.pack  -->        --> encodeUtf8 -->        
String                    Text                      ByteString
        <-- T.unpack <--        <-- decodeUtf8 <-- 

        -->  C.pack  --> *warning* truncated occured, so .one char -> one byte. 
String                    ByteString
        <-- C.unpack <--

• strict vs lazy (.Lazy packates have conversion functions)

• toStrict lazy -> strict * expensive

• fromStrict strict -> lazy

• Builder

• pack ( https://www.haskell.org/hoogle/?hoogle=pack )

bytestring Data.ByteString       pack :: [Word8] -> ByteString
bytestring Data.ByteString.Lazy  pack :: [Word8] -> ByteString
bytestring Data.ByteString.Char8 pack :: String  -> ByteString
text       Data.Text             pack :: String  -> Text
text       Data.Text.Lazy        pack :: String  -> Text

for all

• http://stackoverflow.com/questions/10753073/whats-the-theoretical-basis-for-existential-types/10753957#10753957 ** Explain why "Existential" of ExistentialQuantification extension means "Unviversal" of for all.
• https://en.wikibooks.org/wiki/Haskell/Existentially_quantified_types
• http://iveselov.info/posts/2012-08-30-existential-types.html

Point Free Style

• (.) . (.)
  • http://stackoverflow.com/questions/17585649/composing-function-composition-how-does-work
  • http://austinrochford.com/posts/2013-10-21-generalized-composition-in-haskell.html

Function Application vs Function Composition

• https://www.haskell.org/tutorial/functions.html

Function application has higher precedence than any infix operator ** Examples

a b . c d
--->
OK --> (a b) . (c d)
NG -->  a (b . c) d

> gcd 5 (5^2)
5
> gcd 5 5^2
25
> (gcd 5 5)^2

let f = undefined :: Char -> Char
let g = undefined :: Char -> Char
let z x = g (f x)    --> OK
let z x = g $ f x    --> OK
let z x = g . f x    --> NG
let z x = (g . f) x  --> OK
let z   = g . f      --> OK

Type Classes

• On Type Class Instance Selection
• Type Class Patterns and Anti-patterns

Type Familly

Injectivity

• http://stackoverflow.com/questions/36268734/type-families-causes-ambiguous-variable-erroro

Monad

Articles

• Application of Monad
• Escaping Hell with Monads

MonadBaseControl

• http://stackoverflow.com/questions/11694299/what-is-monadbasecontrol-for
• http://www.yesodweb.com/book/monad-control

Language extentions

DataKind, PolyKinds

• http://www.konn-san.com/prog/2013-advent-calendar.html

GHC

GHC-mod

• http://stackoverflow.com/questions/30518927/how-to-avoid-main-is-not-defined-in-module-main-when-using-syntastic

Cabal

How to configure for testing

• http://blog.pangyanhan.com/haskell/2013-12-22-cabal-installing-test-dependencies.html

TypeCheck

• Debug Log
  • http://stackoverflow.com/questions/30442003/unreadable-quickcheck-log-file-after-a-test-routine

Language

• Namespace Conflict
  • http://stackoverflow.com/questions/42846194/ambiguous-interface

Servant-must-read

articles

• Servant, Type Families, and Type-level Everything A look at advanced GHC features used in Servant
• https://arow.info/blog/posts/2015-07-10-servant-intro.html
• Implementing a minimal version of haskell-servant
• https://www.well-typed.com/blog/2015/11/implementing-a-minimal-version-of-haskell-servant
• Passing a DB connection to handlers in Servant
• http://oleg.fi/gists/posts/2017-03-03-servant-and-db.html
• Error Handling
• servant-checked-exceptions

Useful QA

• Docs explaining how to actually create a full combinator (ex. one to create/store a DB connection)

migration guide

• to 0.7
• klappvisor/haskell-telegram-api#21

How to do ..

• Access Request Headers
  • http://stackoverflow.com/questions/36307378/how-can-i-get-access-to-http-headers-in-servant

Haskell Quote Unquote

• Type
  • A Type is a kind of label that every expression has.
  • A Type is an invariant condition.
  • :: is pronounced "has type"
• Kind
  • a kind is a type of types
• Boxed And UnBoxed
  • Boxed means that values of that type are represented by a pointer to a heap object.
  • An unboxed type, however, is represented by the value itself, no pointers or heap allocation are involved.
• Covariant and Contravariant
  • Intuitively, a covariant type is produced, and a contravariant type is consumed.
  • If a function type a -> b appears in a covariant position, then a is in a contravariant position and b is in a covariant position.
  • If a function type a -> b appears in a contravariant position, then a is in a covariant position and b is in a contravariant position.
• Ad hoc Parametricity
  • The purpose of " => x" is to constrain the set of possible types that the x type variable can take.
• Type classes
  • Type classes group different types with a common interface.
  • you can think of a type class as a way to group sets of types that support the same operations.
  • Typeclasses are Haskell’s version of interfaces (in languages like Golang or Java).
• data
  • is [d]efined [a] [t]ype [a]nd (values)
    • Note that you can defined a type without values.