Playing with monads in go. Trying to understand what they are.

monad.go

package main

import (
	"fmt"
	"strings"
)

type stringMonad func(string) (string, error)

func removeString(s string) stringMonad {
	return func(data string) (string, error) {
		i := strings.Index(data, s)
		if i == -1 {
			return data, fmt.Errorf(`string "%s" doesn't contain "%s"`, data, s)
		}

		return data[:i] + data[i+len(s):], nil
	}
}

func appendString(s string) stringMonad {
	return func(data string) (string, error) {
		return data + s, nil
	}
}

type with string

func replaceString(s string, w with) stringMonad {
	return func(data string) (string, error) {
		i := strings.Index(data, s)
		if i == -1 {
			return data, fmt.Errorf(`string "%s" doesn't contain "%s"`, data, s)
		}

		return strings.Replace(data, s, string(w), -1), nil
	}
}

func doStr(s string, ops []stringMonad) string {
	for _, op := range ops {
		s, _ = op(s)
	}
	return s
}

func mustDoStr(s string, ops []stringMonad) (string, error) {
	var err error
	for _, op := range ops {
		s, err = op(s)
		if err != nil {
			return s, err
		}
	}

	return s, err
}

func main() {
	s := doStr("the bcaon had lots of grease", []stringMonad{
		removeString("grease"),
		appendString("flavor"),
		appendString("."),
		replaceString("bcaon", with("bacon")),
		replaceString("pig", with("sheep")),
	})

	fmt.Println(s)

	s, err := mustDoStr("the bcaon had lots of grease", []stringMonad{
		removeString("grease"),
		appendString("flavor"),
		appendString("."),
		replaceString("bcaon", with("bacon")),
		replaceString("pig", with("sheep")),
	})

	if err != nil {
		fmt.Println(err)
	} else {
		fmt.Println(s)
	}
}