Haskell 基础

What's Special With Haskell

Lazy V.S strict

Haskell is a lazy language, which means that no computation takes place unless it is forced to take place when the result of that computation is used.

Destructive update does not exist in Haskell. (e.g. int x = 5; x = x + 1不能运行，但能定义)

Pure Functional

Functions that do not have side effects are called pure(无副作用)

referential transparency : basically states that if two functions f and g produce the same values for the same arguments, then we may replace f with g

Details

• case sensitive: 函数名小写开头，变量类型大写开头

• parentheses: parentheses aren't required around function arguments

• Composition: In mathematics we write $ f\circ g$ to mean "f following g," in Haskell we write f . g also to mean "f following g."

• Infix（中缀）

  • infix to non-infix: parentheses (+)
  • non-infix to infix: backquote ``

内置类型

Tuple

tuples are allowed to be heterogeneous

fst and snd won't work on anything longer than a pair

List

lists must be homogeneous

length

head : returns the first element of a (non-empty) list

tail : returns all but the first element

use function on List: map, filter, foldr

foldr && foldl

folding is like replacing (:) with a particular function and ([]) with an initial element

foldr assumes the function is right-associative：

foldr (-) 1 [4,8,5]
==>  4 - (foldr (-) 1 [8,5])
==>  4 - (8 - foldr (-) 1 [5])
==>  4 - (8 - (5 - foldr (-) 1 []))

foldl uses the opposite bracketing

foldl (-) 1 [4,8,5]
==>  foldl (-) (1 - 4) [8,5]
==>  foldl (-) ((1 - 4) - 8) [5]
==>  foldl (-) (((1 - 4) - 8) - 5) []

String

is simply a list of Chars

concatenated string with： ++

Compiling

• Must have function main in module Main
• ghc --make Test.hs -o test
• ghci Test.hs

Function

structure

if else

• you must have both a **then** and an **else** clause
• The indentation is important. Haskell uses a system called "layout" to structure its code like python

case

let in: local variable

do: procedural