Base: Add interaction-environment and environment arg to eval

I'm not 100% sure the `eval` part of this is implemented correctly just
yet. Validation of that will come in subsequent tests when there's more
than just `interaction-environment` available.

Colibri.Core/Macros/CoreMacros.cs

@@ -731,9 +731,21 @@ public static object DelayForce(ColibriRuntime runtime, Scope scope, object?[] a
 
     public static object? Eval(ColibriRuntime runtime, Scope scope, object?[] args)
     {
-        if (args.Length != 1)
+        switch (args.Length)
         {
-            throw new ArgumentException("eval requires one argument");
+            case 0 or > 2:
+                throw new ArgumentException("eval requires one or two arguments");
+            case 2:
+            {
+                if (runtime.Evaluate(scope, args[1]) is not Scope argScope)
+                {
+                    // ReSharper disable once StringLiteralTypo
+                    throw new ArgumentException("eval's second argument must be a scope");
+                }
+
+                scope = argScope;
+                break;
+            }
         }
 
         return runtime.Evaluate(scope, runtime.Evaluate(scope, args[0]));

Colibri.Core/Macros/EnvironmentMacros.cs

@@ -0,0 +1,7 @@
+namespace Colibri.Core.Macros;
+
+public static class EnvironmentMacros
+{
+    public static object InteractionEnvironment(ColibriRuntime runtime, Scope scope, object?[] args) 
+        => runtime.UserScope;
+}

Colibri.Core/StandardLibraries.cs

@@ -24,7 +24,7 @@ static StandardLibraries()
             // TODO: (scheme load) library
             (new LibraryName("scheme", "process-context"), ProcessContext),
             (new LibraryName("scheme", "read"), Read),
-            // TODO: (scheme repl) library
+            (new LibraryName("scheme", "repl"), Repl),
             (new LibraryName("scheme", "time"), Time),
             (new LibraryName("scheme", "write"), Write),
             // TODO: (scheme r5rs) library
@@ -437,8 +437,11 @@ static StandardLibraries()
     {
         ["read"] = (MacroExpression)PortMacros.Read,
     });
-
-    // TODO: (scheme repl) library
+
+    public static readonly Library Repl = new(new Dictionary<string, object?>
+    {
+         ["interaction-environment"] = (MacroExpression)EnvironmentMacros.InteractionEnvironment,
+    });
 
     public static readonly Library Time = new(new Dictionary<string, object?>
     {

Colibri.Tests/EnvironmentTests.cs

@@ -0,0 +1,25 @@
+using Colibri.Core;
+
+namespace Colibri.Tests;
+
+public class EnvironmentTests
+{
+    [Fact]
+    public void InteractionEnvironmentTest()
+    {
+        const string program = @"
+define x 42
+eval 'x (interaction-environment)
+";
+
+        var runtime = new ColibriRuntime();
+
+        var result = runtime.EvaluateProgram(program);
+
+        Assert.Equal(42, result);
+
+        var env = runtime.EvaluateProgram("(interaction-environment)");
+
+        Assert.Same(env, runtime.UserScope);
+    }
+}