feat: add algebraic enums (tagged unions) support

- Generate tagged unions in C backend for enums with data
- Add HirEnumVariant type with fields and namedFields
- Support _Tag and _Data field access in sema
- Support enum variant constants (Result_Ok, Result_Err)
- Support _Data union field access (Ok_0, Err_0, etc.)
- Add algebraic_enums.bux example

Example:
  enum Result {
      Ok(int),
      Err(String)
  }

  let r: Result = Result { tag: Result_Ok };
  r.data.Ok_0 = 42;

examples/algebraic_enums.bux

@@ -0,0 +1,29 @@
+// Algebraic Enums - Enums with data (tagged unions)
+extern func Std_Io_PrintLine(s: String);
+extern func Std_Io_PrintInt(n: int);
+
+enum Result {
+    Ok(int),
+    Err(String)
+}
+
+func Main() -> int {
+    let r1: Result = Result { tag: Result_Ok };
+    r1.data.Ok_0 = 42;
+    
+    let r2: Result = Result { tag: Result_Err };
+    r2.data.Err_0 = "error message";
+    
+    if r1.tag == Result_Ok {
+        Std_Io_PrintLine("r1 is Ok:");
+        Std_Io_PrintInt(r1.data.Ok_0);
+        Std_Io_PrintLine("");
+    }
+    
+    if r2.tag == Result_Err {
+        Std_Io_PrintLine("r2 is Err:");
+        Std_Io_PrintLine(r2.data.Err_0);
+    }
+    
+    return 0;
+}

src/c_backend.nim

@@ -355,14 +355,67 @@ proc emitStruct*(be: var CBackend, name: string, fields: seq[tuple[name: string,
   be.emitLine(&"}} {name};")
   be.emitLine("")
 
-proc emitEnum*(be: var CBackend, name: string, variants: seq[string]) =
+proc emitEnum*(be: var CBackend, name: string, variants: seq[HirEnumVariant]) =
+  # Check if this is a simple enum (no data) or algebraic enum (with data)
+  var hasData = false
+  for v in variants:
+    if v.fields.len > 0 or v.namedFields.len > 0:
+      hasData = true
+      break
+  
+  if not hasData:
+    # Simple enum - generate as before
     be.emitLine(&"typedef enum {{")
     inc be.indent
     for i, v in variants:
       if i < variants.len - 1:
-      be.emitLine(&"{name}_{v},")
+        be.emitLine(&"{name}_{v.name},")
       else:
-      be.emitLine(&"{name}_{v}")
+        be.emitLine(&"{name}_{v.name}")
+    dec be.indent
+    be.emitLine(&"}} {name};")
+    be.emitLine("")
+  else:
+    # Algebraic enum - generate tagged union
+    # 1. Generate tag enum
+    be.emitLine(&"typedef enum {{")
+    inc be.indent
+    for i, v in variants:
+      if i < variants.len - 1:
+        be.emitLine(&"{name}_{v.name},")
+      else:
+        be.emitLine(&"{name}_{v.name}")
+    dec be.indent
+    be.emitLine(&"}} {name}_Tag;")
+    be.emitLine("")
+    
+    # 2. Generate union for data
+    be.emitLine(&"typedef union {{")
+    inc be.indent
+    for v in variants:
+      if v.fields.len > 0:
+        # Positional fields
+        for i, f in v.fields:
+          let typ = typeToC(f)
+          be.emitLine(&"{typ} {v.name}_{i};")
+      elif v.namedFields.len > 0:
+        # Named fields - generate as struct
+        be.emitLine(&"struct {{")
+        inc be.indent
+        for nf in v.namedFields:
+          let typ = typeToC(nf.typ)
+          be.emitLine(&"{typ} {nf.name};")
+        dec be.indent
+        be.emitLine(&"}} {v.name};")
+    dec be.indent
+    be.emitLine(&"}} {name}_Data;")
+    be.emitLine("")
+    
+    # 3. Generate main struct with tag + union
+    be.emitLine(&"typedef struct {{")
+    inc be.indent
+    be.emitLine(&"{name}_Tag tag;")
+    be.emitLine(&"{name}_Data data;")
     dec be.indent
     be.emitLine(&"}} {name};")
     be.emitLine("")

src/hir.nim

@@ -124,11 +124,16 @@ type
     body*: HirNode
     isPublic*: bool
 
+  HirEnumVariant* = object
+    name*: string
+    fields*: seq[Type]  # Positional fields for algebraic enums
+    namedFields*: seq[tuple[name: string, typ: Type]]  # Named fields
+
   HirModule* = object
     funcs*: seq[HirFunc]
     externFuncs*: seq[HirFunc]  # Functions declared with extern (no body)
     structs*: seq[tuple[name: string, fields: seq[tuple[name: string, typ: Type]]]]
-    enums*: seq[tuple[name: string, variants: seq[string]]]
+    enums*: seq[tuple[name: string, variants: seq[HirEnumVariant]]]
     consts*: seq[tuple[name: string, typ: Type, value: HirNode]]
 
 # Constructor helpers

src/hir_lower.nim

@@ -433,7 +433,7 @@ proc lowerModule*(module: Module, sema: Sema): HirModule =
   var funcs: seq[HirFunc] = @[]
   var externFuncs: seq[HirFunc] = @[]
   var structs: seq[tuple[name: string, fields: seq[tuple[name: string, typ: Type]]]] = @[]
-  var enums: seq[tuple[name: string, variants: seq[string]]] = @[]
+  var enums: seq[tuple[name: string, variants: seq[HirEnumVariant]]] = @[]
   var consts: seq[tuple[name: string, typ: Type, value: HirNode]] = @[]
 
   for decl in module.items:
@@ -463,9 +463,37 @@ proc lowerModule*(module: Module, sema: Sema): HirModule =
         fields.add((f.name, fType))
       structs.add((decl.declStructName, fields))
     of dkEnum:
-      var variants: seq[string] = @[]
+      var variants: seq[HirEnumVariant] = @[]
       for v in decl.declEnumVariants:
-        variants.add(v.name)
+        var fields: seq[Type] = @[]
+        for f in v.fields:
+          var fType = makeUnknown()
+          if f != nil and f.kind == tekNamed:
+            case f.typeName
+            of "int", "int32": fType = makeInt()
+            of "int64": fType = makeInt64()
+            of "float64": fType = makeFloat64()
+            of "float32": fType = makeFloat32()
+            of "bool": fType = makeBool()
+            of "String", "str": fType = makeStr()
+            else: fType = makeNamed(f.typeName)
+          fields.add(fType)
+        
+        var namedFields: seq[tuple[name: string, typ: Type]] = @[]
+        for nf in v.namedFields:
+          var fType = makeUnknown()
+          if nf.ftype != nil and nf.ftype.kind == tekNamed:
+            case nf.ftype.typeName
+            of "int", "int32": fType = makeInt()
+            of "int64": fType = makeInt64()
+            of "float64": fType = makeFloat64()
+            of "float32": fType = makeFloat32()
+            of "bool": fType = makeBool()
+            of "String", "str": fType = makeStr()
+            else: fType = makeNamed(nf.ftype.typeName)
+          namedFields.add((nf.name, fType))
+        
+        variants.add(HirEnumVariant(name: v.name, fields: fields, namedFields: namedFields))
       enums.add((decl.declEnumName, variants))
     of dkConst:
       let value = ctx.lowerExpr(decl.declConstValue)

src/sema.nim

@@ -132,6 +132,13 @@ proc collectGlobals*(sema: var Sema) =
       if not sema.globalScope.define(sym):
         sema.emitError(decl.loc, &"duplicate symbol '{decl.declEnumName}'")
       sema.typeTable[decl.declEnumName] = t
+      # For algebraic enums, add variant constants with _Tag type
+      for variant in decl.declEnumVariants:
+        let variantName = decl.declEnumName & "_" & variant.name
+        let variantType = makeNamed(decl.declEnumName & "_Tag")
+        let variantSym = Symbol(kind: skConst, name: variantName, typ: variantType,
+                                decl: decl, isPublic: decl.isPublic)
+        discard sema.globalScope.define(variantSym)
     of dkUnion:
       let t = makeNamed(decl.declUnionName)
       let sym = Symbol(kind: skType, name: decl.declUnionName, typ: t,
@@ -398,12 +405,49 @@ proc checkExpr(sema: var Sema, expr: Expr, scope: Scope): Type =
   of ekField:
     let obj = sema.checkExpr(expr.exprFieldObj, scope)
     if obj.kind == tkNamed:
+      # Check if this is a _Data union field access
+      if obj.name.endsWith("_Data"):
+        let enumName = obj.name[0..^6]  # Remove "_Data" suffix
+        let enumSym = sema.globalScope.lookup(enumName)
+        if enumSym != nil and enumSym.decl != nil and enumSym.decl.kind == dkEnum:
+          # Look for the field in enum variants
+          for variant in enumSym.decl.declEnumVariants:
+            # Check positional fields: Ok_0, Ok_1, etc.
+            for i, f in variant.fields:
+              let fieldName = variant.name & "_" & $i
+              if fieldName == expr.exprFieldName:
+                return sema.resolveType(f)
+            # Check named fields
+            for nf in variant.namedFields:
+              if nf.name == expr.exprFieldName:
+                return sema.resolveType(nf.ftype)
+          sema.emitError(expr.loc, &"union '{obj.name}' has no field '{expr.exprFieldName}'")
+        else:
+          sema.emitError(expr.loc, &"cannot access field on type {obj.toString}")
+      else:
         let sym = sema.globalScope.lookup(obj.name)
-      if sym != nil and sym.decl != nil and sym.decl.kind == dkStruct:
+        if sym != nil and sym.decl != nil:
+          if sym.decl.kind == dkStruct:
             for f in sym.decl.declStructFields:
               if f.name == expr.exprFieldName:
                 return sema.resolveType(f.ftype)
             sema.emitError(expr.loc, &"struct '{obj.name}' has no field '{expr.exprFieldName}'")
+          elif sym.decl.kind == dkEnum:
+            # Algebraic enum fields
+            if expr.exprFieldName == "tag":
+              return makeNamed(obj.name & "_Tag")
+            elif expr.exprFieldName == "data":
+              return makeNamed(obj.name & "_Data")
+            else:
+              sema.emitError(expr.loc, &"enum '{obj.name}' has no field '{expr.exprFieldName}'")
+          elif sym.decl.kind == dkUnion:
+            # Union fields
+            for f in sym.decl.declUnionFields:
+              if f.name == expr.exprFieldName:
+                return sema.resolveType(f.ftype)
+            sema.emitError(expr.loc, &"union '{obj.name}' has no field '{expr.exprFieldName}'")
+          else:
+            sema.emitError(expr.loc, &"cannot access field on type {obj.toString}")
         else:
           sema.emitError(expr.loc, &"cannot access field on type {obj.toString}")
     else: