Support argument of Regex type by PatternMatchingVisitor

[gao-artur]

Currently, the PatternMatchingVisitor only supports string argument

https://github.com/graphql-dotnet/graphql-dotnet/blob/22052e73d734d3f0b60b52a04897afb12656c55a/src/GraphQL/Utilities/Visitors/Custom/PatternMatchingVisitor.cs#L40-L41

It can be useful also to add support for Regex arguments

Field<string>("Hello")
    .Argument<StringGraphType>("arg", argument => argument.ApplyDirective("pattern", "regex", Patterns.AlphabeticalPattern()))
    .Resolve(ctx => ctx.GetArgument<string>("arg"));

public static partial class Patterns
{
    [GeneratedRegex("[A-Z]+")]
    public static partial Regex AlphabeticalPattern();
}

[Shane32]

Sounds like a great idea!

[Shane32]

You'd probably have to make a custom scalar to allow this however.

[Shane32]

Which also means it's a breaking change unfortunately.

[Shane32]

Maybe a ctor option to the directive to use the custom scalar so it's not breaking by default?

[gao-artur]

I don't think I understand. Why does it require a custom scalar? The ApplyDirective receives an object as an argument, so it can be anything. The only change is required in PatternMatchingVisitor to try cast the applied?.FindArgument("regex")?.Value to Regex in addition to string. Am I missing something?

[Shane32]

Schema validation should fail to coerce the argument to StringGraphType (see https://github.com/graphql-dotnet/graphql-dotnet/blob/master/src/GraphQL/Types/Directives/Custom/PatternMatchingDirective.cs ). Unless we don't have schema validation for applied directives' arguments. In which case the schema printing code would crash when printing the schema within the ScalarGraphType.ToAst method since it won't know how to convert a RegEx class to an AST.

[Shane32]

With a custom scalar, ParseLiteral can support strings, and ParseValue can support both strings and RegEx instances. The ToAST method can be overridden to provide a string representation of the RegEx instance so schema printing still works.

[Shane32]

At least, without trying any code, that's my best guess as to what will happen.

[gao-artur]

You are right. The visitor works correctly, but schema printing fails. But I don't understand your proposal with a custom scalar...