Keep Go constant-field values and apply instance-field initialisers

Coder.Test/Languages/GoGeneratedSourceCompilesTests.cs

@@ -130,6 +130,59 @@ public void GeneratedSource_CompilesAndIsFormatted()
 		});
 	}
 
+	/// <summary>
+	/// Tests that a constant field keeps its value and that a constructor starts an initialised
+	/// instance field at its value, by running the generated code rather than reading it.
+	/// </summary>
+	/// <remarks>
+	/// Go placed a field by <see cref="FieldDeclaration.IsStatic"/> alone, so a constant became a
+	/// struct member and its value went nowhere, and an instance field's initialiser was dropped from
+	/// the constructor: both printed 0 (issue #111).
+	/// </remarks>
+	[TestMethod]
+	public void ConstantAndInitialisedFields_KeepTheirValuesWhenRun()
+	{
+		if (ToolchainHarness.FindOnPath("version", "go") is null)
+		{
+			Assert.Inconclusive("No Go toolchain on the path, so nothing was compiled.");
+			return;
+		}
+
+		SourceFile file = new("main");
+		ClassDeclaration cfg = new("Cfg") { Kind = TypeDeclarationKind.Struct };
+		cfg.Members.Add(new FieldDeclaration("Ratio", new TypeReference("double")) { IsConstant = true, InitialValue = Literal.DecimalValue(0.5) });
+		cfg.Members.Add(new FieldDeclaration("Hits", new TypeReference("int")) { InitialValue = Literal.Number(5) });
+		cfg.Members.Add(new FunctionDeclaration("Cfg") { Kind = FunctionKind.Constructor });
+		file.Members.Add(cfg);
+
+		const string driver = """
+			package main
+
+			import "fmt"
+
+			func main() {
+				built := NewCfg()
+				fmt.Println(CfgRatio, built.Hits)
+			}
+
+			""";
+
+		ToolchainHarness.InTemporaryDirectory(directory =>
+		{
+			File.WriteAllText(Path.Combine(directory, "go.mod"), Module);
+			File.WriteAllText(Path.Combine(directory, "driver.go"), driver);
+			File.WriteAllText(Path.Combine(directory, "cfg.go"), new GoGenerator().Generate(file));
+
+			(int exitCode, string output) = ToolchainHarness.Run("go", "run .", directory);
+			Assert.AreEqual(0, exitCode, $"go rejected the generated source:{Environment.NewLine}{output}");
+			Assert.AreEqual("0.5 5", output.Trim());
+
+			(int formatted, string differs) = ToolchainHarness.Run("gofmt", "-l cfg.go", directory);
+			Assert.AreEqual(0, formatted, $"gofmt did not run:{Environment.NewLine}{differs}");
+			Assert.AreEqual(string.Empty, differs.Trim(), "gofmt would rewrite the generated source.");
+		});
+	}
+
 	[TestMethod]
 	public void ConditionalExpressions_InferParameterTypesAndLowerNestedReturns()
 	{

Coder.Test/Languages/GoGeneratorTests.cs

@@ -34,6 +34,42 @@
 		Assert.AreEqual("go", Generator.FileExtension);
 	}
 
+	/// <summary>
+	/// Tests that a constructor sets an initialised instance field it does not set itself, leaves the
+	/// one it does set to its own value, and that the struct member says where its value comes from.
+	/// </summary>
+	[TestMethod]
+	public void Constructor_SetsInstanceFieldDefaultsItDoesNotSetItself()
+	{
+		ClassDeclaration cfg = new("Cfg") { Kind = TypeDeclarationKind.Struct };
+		cfg.Members.Add(new FieldDeclaration("Hits", new TypeReference("int")) { InitialValue = Literal.Number(5) });
+		cfg.Members.Add(new FieldDeclaration("Misses", new TypeReference("int")) { InitialValue = Literal.Number(7) });
+		FunctionDeclaration constructor = new("Cfg") { Kind = FunctionKind.Constructor };
+		constructor.Initialisers.Add(new MemberInitialiser("Misses", Literal.Number(1)));
+		cfg.Members.Add(constructor);
+
+		string generated = Generator.Generate(cfg);
+
+		StringAssert.Contains(generated, "return Cfg{Misses: 1, Hits: 5}");
+		StringAssert.Contains(generated, "// defaults to 5: Go has no field initialisers, so only a constructor sets it");
+	}
+
+	/// <summary>
+	/// Tests that a constant field that is not static is still written as package-level storage,
+	/// with its value, rather than as a struct member at its zero.
+	/// </summary>
+	[TestMethod]
+	public void ConstantField_IsPackageLevelEvenWhenNotStatic()
+	{
+		ClassDeclaration cfg = new("Cfg") { Kind = TypeDeclarationKind.Struct };
+		cfg.Members.Add(new FieldDeclaration("Ratio", new TypeReference("double")) { IsConstant = true, InitialValue = Literal.DecimalValue(0.5) });
+
+		string generated = Generator.Generate(cfg);
+
+		StringAssert.Contains(generated, "const CfgRatio float64 = 0.5");
+		Assert.IsFalse(generated.Contains("\tRatio", StringComparison.Ordinal), generated);
+	}
+
 	/// <summary>
 	/// Tests that a function's statements carry no terminator, since Go's lexer supplies the one its
 	/// grammar wants and gofmt deletes any that were written.

Coder/Languages/GoGenerator.cs

@@ -269,7 +269,7 @@
 	/// holds one.
 	/// </remarks>
 	protected override string SpellNonFiniteDouble(double value) =>
 		double.IsNaN(value) ? "math.NaN()" : value > 0 ? "math.Inf(1)" : "math.Inf(-1)";
 
 	/// <inheritdoc/>
 	/// <remarks>
@@ -595,19 +595,47 @@
 		GenerateStruct(classDecl, name, code);
 		WriteInterfaceAssertions(classDecl, name, code);
 
-		foreach (FieldDeclaration field in classDecl.Members.OfType<FieldDeclaration>().Where(field => field.IsStatic))
+		foreach (FieldDeclaration field in classDecl.Members.OfType<FieldDeclaration>().Where(IsPackageLevelField))
 		{
 			code.NewLine();
 			WriteStorage(Join(name, field.Name ?? UnnamedMember), field, code);
 		}
 
-		foreach (FunctionDeclaration function in classDecl.Members.OfType<FunctionDeclaration>())
+		IReadOnlyList<FieldDeclaration> outerDefaults = _instanceFieldDefaults;
+		_instanceFieldDefaults = [.. classDecl.Members.OfType<FieldDeclaration>()
+			.Where(field => !IsPackageLevelField(field) && field.InitialValue is not null)];
+		try
 		{
-			code.NewLine();
-			GenerateFunction(function, code, name);
+			foreach (FunctionDeclaration function in classDecl.Members.OfType<FunctionDeclaration>())
+			{
+				code.NewLine();
+				GenerateFunction(function, code, name);
+			}
+		}
+		finally
+		{
+			_instanceFieldDefaults = outerDefaults;
 		}
 	}
 
+	/// <summary>
+	/// The instance fields of the struct whose functions are being written that say what they
+	/// start at, which its constructors set because a Go struct field cannot say so itself.
+	/// </summary>
+	private IReadOnlyList<FieldDeclaration> _instanceFieldDefaults = [];
+
+	/// <summary>
+	/// Reports whether a field belongs to its type rather than to each value of it, and so is
+	/// written as package-level storage rather than as a struct member.
+	/// </summary>
+	/// <param name="field">The field to place.</param>
+	/// <returns>True for a static or a constant field.</returns>
+	/// <remarks>
+	/// A constant belongs to the type in every other target, and a struct member has nowhere to
+	/// hold its value, so treating one as an instance field would write it at its zero.
+	/// </remarks>
+	private static bool IsPackageLevelField(FieldDeclaration field) => field.IsStatic || field.IsConstant;
+
 	/// <summary>
 	/// Asserts, at compile time, that a type implements what it said it implements.
 	/// </summary>
@@ -746,7 +774,7 @@
 					yield return Field(field.Name, field.Type, field.Visibility);
 					break;
 
-				case FieldDeclaration field when !field.IsStatic:
+				case FieldDeclaration field when !IsPackageLevelField(field):
 					yield return Field(field.Name, field.Type, field.Visibility, field);
 					break;
 
@@ -773,6 +801,11 @@
 			notes.Add((CommentPrefix, note));
 		}
 
+		if (declaration?.InitialValue is Expression initialValue)
+		{
+			notes.Add((CommentPrefix, $"defaults to {GenerateExpression(initialValue)}: Go has no field initialisers, so only a constructor sets it"));
+		}
+
 		return new AlignedLine(notes, name ?? UnnamedMember, SpellType(type ?? new TypeReference(UnknownTypeName)), declaration);
 	}
 
@@ -1153,6 +1186,14 @@
 			value.Arguments.Add(initialiser);
 		}
 
+		// A field the declaration gave a starting value, and this constructor did not set, starts
+		// at that value rather than at its zero, as it would in every other target.
+		foreach (FieldDeclaration field in _instanceFieldDefaults
+			.Where(field => !funcDecl.Initialisers.Any(initialiser => initialiser.Name == field.Name)))
+		{
+			value.Arguments.Add(new MemberInitialiser(field.Name ?? UnnamedMember, (Expression)field.InitialValue!.DeepClone()));
+		}
+
 		code.Write("return ");
 		GenerateConstructionExpression(value, code);
 		EndStatement(code);
@@ -1901,6 +1942,18 @@
 		}
 	}
 
+	/// <summary>
+	/// Writes an expression to a string, for the places a comment has to quote one.
+	/// </summary>
+	/// <param name="expression">The expression to write.</param>
+	/// <returns>Its Go source.</returns>
+	private string GenerateExpression(Expression expression)
+	{
+		using CodeBlocker inline = CodeBlocker.Create(IndentString);
+		GenerateInternal(expression, inline);
+		return inline.ToString().TrimEnd('\r', '\n');
+	}
+
 	/// <summary>
 	/// Spells a type in Go.
 	/// </summary>