dinneo

# delete local tag '12345'
git tag -d 12345
# delete remote tag '12345' (eg, GitHub version too)
git push origin :refs/tags/12345
# alternative approach
git push --delete origin tagName
git tag -d tagName

dinneo

Category theory(範疇論)之什麼是Functor, Applicative, Monad, Semigroup, Monoid?

之前曾經寫過一篇 什麼是Monad? ，裡面提到了對於Monad的解釋，而今天進一步解釋這些在Functional Programming中常見到的詞：Functor, Applicative, Monad, Semigroup, Monoid。 透過放在一起比較，讓我們更清楚這些之間的差別與關係。

##Functor

在提到Functor之前，請先了解一下scala中map的作用，map可以讓你傳入一個function，而透過這個function是可以從F[A]轉換成F[B]，這裡的F不管他(就是代表外面的包裝不會變)，而裡面的A型態轉成B型態。

例如：

dinneo

//reduce
//Use reduce to combine all items in a collection to create a single new value.

let numbers = [1, 3, 5, 7, 9]
//sum all values from the array (+)
let result = numbers.reduce(0, +)
//sum all values from the array (+) plus one
let result2 = numbers.reduce(1, +)
let result3 = numbers.reduce(1, *)

dinneo

/// Our custom drop-in replacement `precondition`.
///
/// This will call Swift's `precondition` by default (and terminate the program).
/// But it can be changed at runtime to be tested instead of terminating.
func precondition(@autoclosure condition: () -> Bool, @autoclosure _ message: () -> String = "", file: StaticString = __FILE__, line: UWord = __LINE__) {
    preconditionClosure(condition(), message(), file, line)
}

/// The actual function called by our custom `precondition`.
var preconditionClosure: (Bool, String, StaticString, UWord) -> () = defaultPreconditionClosure

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func isPalindrome(_ word: String) -> Bool {
    let word = word.lowercased().characters.filter{ $0 != " " }
    for (i, character) in word.enumerated() {
        if character != word[word.count-i-1] {
            return false
        }
    }
    return true
}

dinneo

// Numerical matrix examples

let x: Matrix = [[10, 9, 8], [3, 2, 1]]
let y: Matrix = [[1, 2, 3], [4, 5, 6]]
let z: Matrix = [[1, 2], [3, 4], [5, 6]]

x + y              // [[11, 11, 11], [7, 7, 7]]
x * y              // [[10, 18, 24], [12, 10, 6]]
2 * x              // [[20, 18, 16], [6, 4, 2]]
y ** z             // [[22, 28], [49, 64]]

dinneo

//: Rotate a matrix 90 degrees in place
//  "Write a function to rotate an NxN matrix by 90 degrees. You should rotate it in place, meaning you can't use another matrix to perform the rotation, but instead you have to use the same given structure."
//  https://www.shiftedup.com/2015/05/10/programming-challenge-rotating-a-matrix-90-degrees-in-place

//  This implementation of a matrix is taken from Apple's own.
//  https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Subscripts.html
//  The extensions are my own.

//  Xcode 7.0, Swift 2.0

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class ArrayImpl<T> {
    var space: Int
    var count: Int
    var ptr: UnsafeMutablePointer<T>
    
    init(count: Int = 0, ptr: UnsafeMutablePointer<T> = nil) {
        self.count = count
        self.space = count
        

dinneo

indirect enum LinkedList<T> {
    case Empty
    case Node(value: T, next: LinkedList<T>)
    
    init() { self = .Empty }
}

extension LinkedList {
    func cons(x: T) -> LinkedList<T> {
        return .Node(value: x, next: self)

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struct LinkedList<T> {
    var head: Node<T>
    
    init(head: Node<T>) {
        self.head = head
    }
}

indirect enum Node<T> {