feat: digit refinement

Tuxedo.Tests/DigitTests.cs

@@ -0,0 +1,24 @@
+namespace Tuxedo.Tests;
+
+public static class DigitTests
+{
+    [Fact(DisplayName = "Characters can be refined as digits")]
+    public static void Case1()
+    {
+        Refined<char, Digit> digit = '1';
+        digit.Value.Should().Be('1');
+        char raw = digit;
+        raw.Should().Be('1');
+    }
+
+    [Fact(DisplayName = "Characters that are not digits cannot be refined")]
+    public static void Case2()
+    {
+        Action act = () => _ = (Refined<char, Digit>)'a';
+        act.Should()
+            .Throw<RefinementFailureException>()
+            .WithMessage("Value must be a digit, instead found 'a'")
+            .And.StackTrace.Should()
+            .StartWith("   at Tuxedo.Tests.DigitTests");
+    }
+}

Tuxedo/Refinements/Digit.cs

@@ -0,0 +1,21 @@
+using System.Diagnostics.CodeAnalysis;
+
+namespace Tuxedo;
+
+/// <summary>
+/// Enforces that a character is a digit
+/// </summary>
+public readonly struct Digit : IRefinement<Digit, char>
+{
+    /// <inheritdoc />
+    public bool CanBeRefined(char value, [NotNullWhen(false)] out string? failureMessage)
+    {
+        if (char.IsDigit(value))
+        {
+            failureMessage = null;
+            return true;
+        }
+        failureMessage = $"Value must be a digit, instead found '{value}'";
+        return false;
+    }
+}