λ1003: Use this method with argument names

Methods such as Match and Switch should be called with named arguments.

Cause

A method annotated with [UseWithArgumentNames] is called and at least one argument is passed positionally. This applies to Match and Switch on Option<T>, Either<L, R>, Result<T> and EitherOrBoth<L, R>.

Calls inside expression trees (for example an Expression<Func<...>> passed to Entity Framework) are exempt, because named arguments are not supported there.

Reason for rule

The arguments of Match and Switch are all functions of a similar shape. Without names it is easy to confuse which branch handles which case, and swapping two branches often compiles without error. Naming the arguments (none:, some:, left:, right:, ok:, error:) documents the intent at the call site and makes the order of the branches irrelevant.

How to fix violations

Add the parameter name to every argument. A code fix is available.

Examples

Disallowed

static string Example(Option<int> option)
    => option.Match(() => "none", value => value.ToString());

Allowed

static string Example(Option<int> option)
    => option.Match(
        none: () => "none",
        some: value => value.ToString());

Using the attribute on your own methods

The attribute is internal to Funcky, but the analyzer matches it by its full name Funcky.CodeAnalysis.UseWithArgumentNamesAttribute, so a copy in your own assembly works just as well. Let Funcky.Analyzers generate that copy by listing the attribute in your project file:

<PropertyGroup>
  <FunckyCodeAnalysisAttributes>UseWithArgumentNames</FunckyCodeAnalysisAttributes>
</PropertyGroup>

Alternatively, add the attribute by hand:

namespace Funcky.CodeAnalysis;

[AttributeUsage(AttributeTargets.Method)]
internal sealed class UseWithArgumentNamesAttribute : Attribute;

Either way, the rule then applies to every method you annotate:

[UseWithArgumentNames]
public static Money Create(decimal amount, string currency) => ...

Money.Create(amount: 10m, currency: "CHF"); // ok
Money.Create(10m, "CHF");                   // λ1003

See Using the attributes in your own code for the details of the generator.