PowMaybeErr 0.2.0

PowMaybe

Table of content

• Introduction
• Usage
• Example
• License

Introduction

Lightweight Maybe monad library to simplify code that can fail.

It will help you transform code like this:

Before

Person? ParsePage(string url)
{
 var html = client.Query(url);
 if (html == null)
 return null;
 var root = Html.GetRoot(html);
 var node = root.SelectSingleNode("xpath query");
 if (node == null)
 return null;
 var personInfo = Utils.ParseNode(node);
 if (personInfo == null)
 return null;
 return new Person(personInfo);
}

Into this:

After

Maybe<Person> ParsePage(string url) =>
 from html in Utils.Query(url)
 let root = Utils.GetRoot(html)
 from node in root.MaySelectSingleNode("xpath query")
 from personInfo in Utils.MayParseNode(node)
 select new Person(personInfo);

The result is:

• shorter code
• less nested
• no if conditions
• no nullable references

It will not apply for all the code everywhere, but when it does apply it will drastically reduce the potential for bugs.

Usage

Creation

var a = May.Some(47);
var b = May.None<string>();
// nullable reference -> Maybe<>
var mayPerson = person.ToMaybe();
// Maybe<> -> nullable reference
var person = mayPerson.ToNullable();

Combining

Maybe<string> QueryHtml(string url);
Maybe<Person> ParsePerson(string html);

// use any number of from/in statements with a select at the end
Maybe<Person> QueryAndParse(string url) =>
 from html in QueryHtml(url)
 from person in ParsePerson(html)
 where person.Name != "John" // you can also use where statements
 select parson;

Unwrapping

Maybe<Person> mayPerson = ...

if (mayPerson.IsSome(out var person))
{
 // Success, you can access person here
}
else
{
 // Failure
}

Person person = mayPerson.Ensure(); // throws an Exception if mayPerson is None

Person person = mayPerson.FailWith(peter); // returns peter if mayPerson is None

Enumerations

IEnumerable<T> WhereSome<T>(this IEnumerable<Maybe<T>> source);
Maybe<T> FirstOrMaybe<T>(this IEnumerable<T> source, Func<T, bool>? predicate = null)
// and similar LastOrMaybe

// Examples
// ========
new [] { May.Some(4), May.None<int>() May.Some(12) }.WhereSome();
// int[] { 4, 12 }

new [] { 2, 6, 5 }.FirstOrMaybe(e => e % 3 == 0)
// Some(6)

new [] { 2, 6, 5 }.FirstOrMaybe(e => e % 3 == 1)
// None<int>()

Example

Let's say you want to read your configuration from multiple sources:

• environment variables
• command line arguments
• json file

And once a source returns a configuration, you do not want to read the other sources.

You could write it this way:

Maybe<Config> ReadFromEnvVars();
Maybe<Config> ReadFromArgs(string[] args);
Maybe<Config> ReadFromFile(string file);

Maybe<Config> ReadConfig(string[] args, string file) =>
	new[]
	{
		() => ReadFromEnvVars(),
		() => ReadFromArgs(args),
		() => ReadFromFile(file),
	}
	.Select(readFun => readFun())
	.WhereSome()
	.FirstOrMaybe();

License

MIT