Haskell 拓展

16 Apr 2023 | haskell, wip

修订历史

2023.04.16 创建笔记

2024.11.06 移出私密

Reader 和 State 的区别

https://www.reddit.com/r/haskell/comments/7gl2wh/when_to_use_reader_vs_state/

RecordWildCards

https://riptutorial.com/haskell/example/13072/recordwildcards

NamedFieldPuns

todo

OverloadedStrings

Haskell’s numeric literals are polymorphic over Num (in the case of integer literals) or Fractional (in the case of decimal literals)：

a :: Int
a = 1

b :: Double
b = 1

c :: Float
c = 3.5

d :: Rational
d = 3.5

String literals are always of type String, and are not polymorphic at all.

The OverloadedStrings extension simply changes the type of literals to "test" :: Data.String.IsString a => a, making string literals polymorphic over the IsString type class.

a :: String
a = "hello"

b :: Text
b = "hello"

OverloadedStrings also adds IsString to the list of defaultable type classes, so you can use types like String, Text, and Bytestring in a default declaration.

ExtendedDefaultRules

https://stackoverflow.com/questions/26778415/using-overloaded-strings

FlexibleInstances

通常情况下，我们不能给多态类型（polymorphic type）的特化版本（specialized version）写类型类实例

While you can create an instance for [a] you can’t create an instance for specifically [Int].; FlexibleInstances relaxes that:

class C a where

-- works out of the box
instance C [a] where

-- requires FlexibleInstances
instance C [Int] where

BinaryLiterals

Haskell 字面值可以使用十进制、十六进制、八进制。开启 BinaryLiterals 拓展后能够使用二进制字面值

PostfixOperators

感觉没啥用

TupleSections

-- With TupleSections
(a,b) == (,) a b == (a,) b == (,b) a
(,2,) 1 3 == (1,2,3)
map (,"tag") [1,2,3] == [(1,"tag"), (2, "tag"), (3, "tag")]

EmptyCase

允许空 case 语句。不是很实用

MultiWayIf

if | x == 1    -> "a"
   | y <  2    -> "b"
   | otherwise -> "d"

DoAndIfThenElse

Haskell2010 默认开启

RankNTypes

https://riptutorial.com/haskell/example/9606/rankntypes

Memorandum