I. Functional Programming

Functional Programming VS Function Pointer

• Functions are first-class citizens: parameters, variables, and return values can all be functions
• Higher order function
• Function - > Closure

Orthodox Functional Programming

• Invariability: There can be no states, only constants and functions
• A function can only have one parameter
• When programming in go language, the above strict rules are not necessary

Closure implementation accumulator

package main

import "fmt"

// closure
func adder() func(int) int {
	sum := 0
	return func(v int) int {
		sum += v
		return sum
	}
}

// Orthodox Functional Programming Closure
type iAdder func(int) (int, iAdder)  // recursion

func adder2(base int) iAdder {
	return func(v int) (int, iAdder) {
		return base + v, adder2(base + v)
	}
}

func main() {
	a1 := adder() 
	for i := 0; i < 10; i++ {
		fmt.Printf("0 + 1 + ... + %d = %d\n", i, a1(i))
	}

	a := adder2(0)
	for i := 0; i < 10; i++ {
		var s int
		s, a = a(i)
		fmt.Printf("0 + 1 + ... + %d = %d\n",
			i, s)
	}
}

2. Examples of Functional Programming

Fibonacci sequence

package main

import "fmt"

// 1, 1, 2, 3, 5, 8, 13, ...
func Fibonacci() func() int {
	a, b := 0, 1
	return func() int {
		a, b = b, a+b
		return a
	}
}

func main() {
	f := Fibonacci()

	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
}

Generate interfaces for functions

• Make Fibonacci function, realize reader interface. So you can print it like you read a file.

package main

import (
	"bufio"
	"fmt"
	"io"
	"strings"

	"learngo/lang/functional/fib"
)

type intGen func() int // Fibonacci function interface

func (g intGen) Read(
	p []byte) (n int, err error) {
	next := g()
	if next > 10000 {  // More than 10,000 lets the function end, that is, the file reads to the end.
		return 0, io.EOF
	}
	s := fmt.Sprintf("%d\n", next)

	// TODO: incorrect if p is too small!
	return strings.NewReader(s).Read(p)
}

func printFileContents(reader io.Reader) {
	scanner := bufio.NewScanner(reader)

	for scanner.Scan() {
		fmt.Println(scanner.Text())
	}
}

func main() {
	var f intGen = fib.Fibonacci()
	printFileContents(f)
}