`OptionalOrNull()`

[nblumhardt]

To cleanly support defaulting optional reference type parsers to null I believe we need something like:

        public static TokenListParser<TKind, T?> OptionalOrNull<TKind, T>(
            this TokenListParser<TKind, T> parser)
        where T: notnull
        {
            return parser != null ? parser.Select(r => (T?)r).Or(Parse.Return<TKind, T?>(default)) : throw new ArgumentNullException(nameof (parser));
        }
    }

[samblackburn]

We've hit the same problem (in our case we wanted TextParser rather than TokenListParser).

Unfortunately your suggestion doesn't seem to return a nullable when working with value types. This example compiles even though it assigns the result to a non-nullable variable:

using System;
using Superpower;
using Superpower.Parsers;

#nullable enable

public class Program
{
    public static void Main()
    {
        TextParser<char> a = Character.EqualTo('a');
        TextParser<char> shouldBeNullable = a.OptionalOrNull();
    }
}

public static class Extensions
{
    public static TextParser<T?> OptionalOrNull<T>(
        this TextParser<T> parser)
    {
        return parser != null
            ? parser.Select(r => (T?) r).Or(Parse.Return<T?>(default))
            : throw new ArgumentNullException(nameof(parser));
    }
}

The best solution I've found is to have 2 overloads of the extension method.

In the following sample, the return type is nullable as it should be:

using System;
using Superpower;
using Superpower.Parsers;

#nullable enable

public class Program
{
    public static void Main()
    {
        TextParser<char> a = Character.EqualTo('a');
        TextParser<char?> correctlyNullable = a.OptionalOrNull();
    }
}

internal static class TextParserClassExtensions
{
    public static TextParser<T?> OptionalOrNull<T>(this TextParser<T> parser)
        where T : class
    {
        return parser != null
            ? parser.Select(r => (T?) r).Or(Parse.Return<T?>(null))
            : throw new ArgumentNullException(nameof(parser));
    }
}

internal static class TextParserParserStructExtensions
{
    public static TextParser<T?> OptionalOrNull<T>(this TextParser<T> parser)
        where T : struct
    {
        return parser != null
            ? parser.Select(r => (T?) r).Or(Parse.Return<T?>(null))
            : throw new ArgumentNullException(nameof(parser));
    }
}

Hope this helps!
Sam Blackburn, Redgate Software

[nblumhardt]

Thank you, Sam! Hoping I'll have a chance to loop back around to this soon.