unit Myc.Ast.Compiler.TypeChecker;

interface

uses
    System.SysUtils,
    System.Classes,
    System.Generics.Collections,
    Myc.Data.Scalar,
    Myc.Data.Value,
    Myc.Ast.Nodes,
    Myc.Ast.Visitor,
    Myc.Ast.Scope,
    Myc.Ast.Types,
    Myc.Ast.Identities,
    Myc.Ast;

type
    IAstTypeChecker = interface(IAstVisitor)
        function Execute(const RootNode: IAstNode): IAstNode;
    end;

    TTypeChecker = class(TAstTransformer, IAstTypeChecker)
    private
        type
            TTypeContext = class
            private
                FParent: TTypeContext;
                FLayout: IScopeLayout;
                FSlotTypes: TArray<IStaticType>;
                FUpvalueTypes: TArray<IStaticType>;
            public
                constructor Create(
                    AParent: TTypeContext;
                    ALayout: IScopeLayout;
                    const AUpvalueTypes: TArray<IStaticType>;
                    ADescriptor: IScopeDescriptor
                );
                function LookupType(const Address: TResolvedAddress): IStaticType;
                procedure SetType(SlotIndex: Integer; AType: IStaticType);
                property Types: TArray<IStaticType> read FSlotTypes;
            end;

    private
        FCurrentContext: TTypeContext;
        FLog: ICompilerLog;
        FRootScope: IExecutionScope;

        function CreateContextChain(L: IScopeLayout): TTypeContext;

        // Helpers for Null Propagation
        function PrepareBaseType(const BaseNode: IAstNode; out IsOptional: Boolean): IStaticType;
        function ApplyOptionality(const AType: IStaticType; IsOptional: Boolean): IStaticType;

    protected
        function VisitIdentifier(const Node: IIdentifierNode): IAstNode; override;
        function VisitVariableDeclaration(const Node: IVariableDeclarationNode): IAstNode; override;
        function VisitAssignment(const Node: IAssignmentNode): IAstNode; override;
        function VisitLambdaExpression(const Node: ILambdaExpressionNode): IAstNode; override;
        function VisitFunctionCall(const Node: IFunctionCallNode): IAstNode; override;
        function VisitBlockExpression(const Node: IBlockExpressionNode): IAstNode; override;
        function VisitIfExpression(const Node: IIfExpressionNode): IAstNode; override;
        function VisitMemberAccess(const Node: IMemberAccessNode): IAstNode; override;
        function VisitIndexer(const Node: IIndexerNode): IAstNode; override;
        function VisitCreateSeries(const Node: ICreateSeriesNode): IAstNode; override;
        function VisitSeriesLength(const Node: ISeriesLengthNode): IAstNode; override;
        function VisitRecurNode(const Node: IRecurNode): IAstNode; override;
        function VisitNop(const Node: INopNode): IAstNode; override;
        function VisitRecordLiteral(const Node: IRecordLiteralNode): IAstNode; override;

        function VisitConstant(const Node: IConstantNode): IAstNode; override;
        function VisitKeyword(const Node: IKeywordNode): IAstNode; override;

        // Pipe Support
        function VisitPipeInput(const Node: IPipeInputNode): IAstNode; override;
        function VisitPipe(const Node: IPipeNode): IAstNode; override;

    public
        constructor Create(const RootLayout: IScopeLayout; const RootScope: IExecutionScope; const ALog: ICompilerLog);
        destructor Destroy; override;

        function Execute(const RootNode: IAstNode): IAstNode;

        class function CheckTypes(
            const RootNode: IAstNode;
            const Layout: IScopeLayout;
            const RootScope: IExecutionScope;
            const ALog: ICompilerLog
        ): IAstNode; static;
    end;

implementation

uses
    System.Generics.Defaults,
    Myc.Data.Keyword;

{ TTypeChecker.TTypeContext }

constructor TTypeChecker.TTypeContext.Create(
    AParent: TTypeContext;
    ALayout: IScopeLayout;
    const AUpvalueTypes: TArray<IStaticType>;
    ADescriptor: IScopeDescriptor
);
var
    i: Integer;
begin
    inherited Create;
    FParent := AParent;
    FLayout := ALayout;
    FUpvalueTypes := AUpvalueTypes;

    if Assigned(FLayout) then
    begin
        SetLength(FSlotTypes, FLayout.SlotCount);

        if Assigned(ADescriptor) then
        begin
            // Load known types from descriptor (e.g. for Root Scope / RTL)
            for i := 0 to High(FSlotTypes) do
                FSlotTypes[i] := ADescriptor.GetSymbolType(i);
        end
        else
        begin
            // Initialize with Unknown for new scopes
            for i := 0 to High(FSlotTypes) do
                FSlotTypes[i] := TTypes.Unknown;
        end;
    end;
end;

function TTypeChecker.TTypeContext.LookupType(const Address: TResolvedAddress): IStaticType;
var
    ctx: TTypeContext;
    i: Integer;
begin
    case Address.Kind of
        akLocalOrParent:
        begin
            ctx := Self;
            for i := 1 to Address.ScopeDepth do
            begin
                if not Assigned(ctx.FParent) then
                begin
                    Result := TTypes.Unknown;
                    Exit;
                end;
                ctx := ctx.FParent;
            end;

            if (Address.SlotIndex >= 0) and (Address.SlotIndex < Length(ctx.FSlotTypes)) then
                Result := ctx.FSlotTypes[Address.SlotIndex]
            else
                Result := TTypes.Unknown;
        end;
        akUpvalue:
        begin
            if (Address.SlotIndex >= 0) and (Address.SlotIndex < Length(FUpvalueTypes)) then
                Result := FUpvalueTypes[Address.SlotIndex]
            else
                Result := TTypes.Unknown;
        end;
    else
        Result := TTypes.Unknown;
    end;
end;

procedure TTypeChecker.TTypeContext.SetType(SlotIndex: Integer; AType: IStaticType);
begin
    if (SlotIndex >= 0) and (SlotIndex < Length(FSlotTypes)) then
        FSlotTypes[SlotIndex] := AType;
end;

{ TTypeChecker }

function TTypeChecker.CreateContextChain(L: IScopeLayout): TTypeContext;
var
    p: TTypeContext;
    desc: IScopeDescriptor;
begin
    if L = nil then
        exit(nil);

    p := CreateContextChain(L.Parent);

    desc := nil;
    if (L.Parent = nil) and Assigned(FRootScope) then
    begin
        desc := FRootScope.Descriptor;
    end;

    Result := TTypeContext.Create(p, L, [], desc);
end;

constructor TTypeChecker.Create(const RootLayout: IScopeLayout; const RootScope: IExecutionScope; const ALog: ICompilerLog);
begin
    inherited Create;
    FLog := ALog;
    FRootScope := RootScope;

    FCurrentContext := CreateContextChain(RootLayout);
    if FCurrentContext = nil then
        FCurrentContext := TTypeContext.Create(nil, nil, [], nil);
end;

destructor TTypeChecker.Destroy;
begin
    while Assigned(FCurrentContext) do
    begin
        var temp := FCurrentContext;
        FCurrentContext := FCurrentContext.FParent;
        temp.Free;
    end;
    inherited;
end;

class function TTypeChecker.CheckTypes(
    const RootNode: IAstNode;
    const Layout: IScopeLayout;
    const RootScope: IExecutionScope;
    const ALog: ICompilerLog
): IAstNode;
var
    startLayout: IScopeLayout;
begin
    if Assigned(Layout) then
        startLayout := Layout.Parent
    else
        startLayout := nil;

    var checker := TTypeChecker.Create(startLayout, RootScope, ALog) as IAstTypeChecker;
    Result := checker.Execute(RootNode);
end;

function TTypeChecker.Execute(const RootNode: IAstNode): IAstNode;
begin
    Result := Accept(RootNode);
    if not Assigned(Result) then
        Result := TAst.Block([], nil);
end;

// --- Helper ---

function TTypeChecker.PrepareBaseType(const BaseNode: IAstNode; out IsOptional: Boolean): IStaticType;
var
    baseType: IStaticType;
begin
    baseType := BaseNode.AsTypedNode.StaticType;
    IsOptional := baseType.IsOptional;
    Result := TTypes.Unwrap(baseType);
end;

function TTypeChecker.ApplyOptionality(const AType: IStaticType; IsOptional: Boolean): IStaticType;
begin
    if IsOptional and (AType.Kind <> stUnknown) then
        Result := TTypes.MakeOptional(AType)
    else
        Result := AType;
end;

// --- Visits ---

function TTypeChecker.VisitConstant(const Node: IConstantNode): IAstNode;
begin
    Result := Node;
end;

function TTypeChecker.VisitKeyword(const Node: IKeywordNode): IAstNode;
begin
    Result := Node;
end;

function TTypeChecker.VisitIdentifier(const Node: IIdentifierNode): IAstNode;
var
    typ: IStaticType;
    adr: TResolvedAddress;
    identity: INamedIdentity;
begin
    adr := Node.Address;
    identity := Node.Identity.AsNamed;

    if adr.Kind = akUnresolved then
    begin
        Result := TAst.Identifier(identity, adr, TTypes.Unknown);
        Exit;
    end;

    typ := FCurrentContext.LookupType(adr);
    Result := TAst.Identifier(identity, adr, typ);
end;

function TTypeChecker.VisitRecurNode(const Node: IRecurNode): IAstNode;
var
    newArgs: TArray<IAstNode>;
    i: Integer;
begin
    SetLength(newArgs, Node.Arguments.Count);
    for i := 0 to Node.Arguments.Count - 1 do
        newArgs[i] := Accept(Node.Arguments[i]);

    var argList := TArgumentList.Create(newArgs, Node.Arguments.Identity);
    Result := TAst.Recur(Node.Identity, argList, TTypes.Void);
end;

function TTypeChecker.VisitVariableDeclaration(const Node: IVariableDeclarationNode): IAstNode;
var
    initType: IStaticType;
    newInitializer, newIdent: IAstNode;
    adr: TResolvedAddress;
    identNode: IIdentifierNode;
begin
    identNode := Node.Target.AsIdentifier;
    adr := identNode.Address;
    initType := TTypes.Unknown;

    if adr.Kind = akUnresolved then
    begin
        if Assigned(Node.Initializer) then
            Accept(Node.Initializer);
        Result := Node;
        Exit;
    end;

    if Assigned(Node.Initializer) then
        newInitializer := Accept(Node.Initializer)
    else
        newInitializer := nil;

    if Assigned(newInitializer) then
        initType := newInitializer.AsTypedNode.StaticType;

    if initType.Kind <> stUnknown then
        FCurrentContext.SetType(adr.SlotIndex, initType);

    newIdent := TAst.Identifier(identNode.Identity.AsNamed, adr, initType);
    Result := TAst.VarDecl(Node.Identity, newIdent, newInitializer, initType, Node.IsBoxed);
end;

function TTypeChecker.VisitAssignment(const Node: IAssignmentNode): IAstNode;
var
    targetType, sourceType: IStaticType;
    newIdent, newValue: IAstNode;
    adr: TResolvedAddress;
    identNode: IIdentifierNode;
begin
    identNode := Node.Target.AsIdentifier;
    newIdent := Accept(Node.Target);
    targetType := newIdent.AsTypedNode.StaticType;
    adr := identNode.Address;

    if adr.Kind = akUnresolved then
    begin
        Accept(Node.Value);
        Result := Node;
        Exit;
    end;

    newValue := Accept(Node.Value);
    sourceType := newValue.AsTypedNode.StaticType;

    if (targetType.Kind <> stUnknown) and (sourceType.Kind <> stUnknown) then
    begin
        if not TTypeRules.CanAssign(targetType, sourceType) then
        begin
            if Assigned(FLog) then
                FLog.AddError(Format('Cannot assign type %s to %s', [sourceType.ToString, targetType.ToString]), Node);
        end;
    end;

    if (targetType.Kind = stUnknown) and (sourceType.Kind <> stUnknown) then
    begin
        FCurrentContext.SetType(adr.SlotIndex, sourceType);
        newIdent := TAst.Identifier(identNode.Identity.AsNamed, adr, sourceType);
        targetType := sourceType;
    end;

    Result := TAst.Assign(Node.Identity, newIdent, newValue, targetType);
end;

function TTypeChecker.VisitBlockExpression(const Node: IBlockExpressionNode): IAstNode;
var
    blockType: IStaticType;
    newExprs: TArray<IAstNode>;
    i: Integer;
begin
    SetLength(newExprs, Node.Expressions.Count);
    for i := 0 to Node.Expressions.Count - 1 do
        newExprs[i] := Accept(Node.Expressions[i]);

    if Length(newExprs) > 0 then
        blockType := newExprs[High(newExprs)].AsTypedNode.StaticType
    else
        blockType := TTypes.Void;

    var exprList := TExpressionList.Create(newExprs, Node.Expressions.Identity);
    Result := TAst.Block(Node.Identity, exprList, blockType);
end;

function TTypeChecker.VisitLambdaExpression(const Node: ILambdaExpressionNode): IAstNode;
var
    newParams: TArray<IIdentifierNode>;
    newBody: IAstNode;
    bodyType, methodType: IStaticType;
    paramTypes: TArray<IStaticType>;
    upvalueTypes: TArray<IStaticType>;
    i: Integer;
    finalDescriptor: IScopeDescriptor;
    paramIdent: IIdentifierNode;
    injectedType: IStaticType;
begin
    // 1. Resolve Upvalue Types
    var upvalueAddrs := Node.Upvalues;
    SetLength(upvalueTypes, Length(upvalueAddrs));

    for i := 0 to High(upvalueAddrs) do
    begin
        var lookupAddr := upvalueAddrs[i];
        if lookupAddr.Kind = akLocalOrParent then
        begin
            if lookupAddr.ScopeDepth > 0 then
                Dec(lookupAddr.ScopeDepth);
        end;
        upvalueTypes[i] := FCurrentContext.LookupType(lookupAddr);
    end;

    // 2. Enter New Scope
    FCurrentContext := TTypeContext.Create(FCurrentContext, Node.Layout, upvalueTypes, nil);
    try
        SetLength(newParams, Node.Parameters.Count);
        SetLength(paramTypes, Node.Parameters.Count);

        for i := 0 to Node.Parameters.Count - 1 do
        begin
            paramIdent := Node.Parameters[i];

            // Check if there is already a type assigned (e.g. injected by Pipe Visitor)
            injectedType := paramIdent.AsTypedNode.StaticType;

            if injectedType.Kind = stUnknown then
                paramTypes[i] := TTypes.Unknown
            else
                paramTypes[i] := injectedType;

            if paramIdent.Address.Kind <> akUnresolved then
                FCurrentContext.SetType(paramIdent.Address.SlotIndex, paramTypes[i]);

            newParams[i] := TAst.Identifier(paramIdent.Identity.AsNamed, paramIdent.Address, paramTypes[i]);
        end;

        newBody := Accept(Node.Body);

        bodyType := newBody.AsTypedNode.StaticType;
        methodType := TTypes.CreateMethod(paramTypes, bodyType);

        finalDescriptor := TScope.CreateDescriptor(Node.Layout, FCurrentContext.Types);
    finally
        var temp := FCurrentContext;
        FCurrentContext := FCurrentContext.FParent;
        temp.Free;
    end;

    var paramList := TParameterList.Create(newParams, Node.Parameters.Identity);
    Result :=
        TAst.LambdaExpr(
            Node.Identity,
            paramList,
            newBody,
            Node.Layout,
            finalDescriptor,
            Node.Upvalues,
            Node.HasNestedLambdas,
            Node.IsPure,
            methodType
        );
end;

function TTypeChecker.VisitFunctionCall(const Node: IFunctionCallNode): IAstNode;
var
    calleeType, retType: IStaticType;
    i, j: Integer;
    newCallee: IAstNode;
    newArgs: TArray<IAstNode>;
    argTypes: TArray<IStaticType>;
    hasUnknownArgs: Boolean;
    bestSig: IMethodSignature;
    match: Boolean;
begin
    newCallee := Accept(Node.Callee);
    SetLength(newArgs, Node.Arguments.Count);
    SetLength(argTypes, Node.Arguments.Count);
    hasUnknownArgs := False;

    for i := 0 to Node.Arguments.Count - 1 do
    begin
        newArgs[i] := Accept(Node.Arguments[i]);
        argTypes[i] := newArgs[i].AsTypedNode.StaticType;
        if argTypes[i].Kind = stUnknown then
            hasUnknownArgs := True;
    end;

    calleeType := newCallee.AsTypedNode.StaticType;
    retType := TTypes.Unknown;

    if calleeType.Kind = stMethod then
    begin
        if not hasUnknownArgs then
        begin
            bestSig := nil;
            for var sig in calleeType.Signatures do
            begin
                if Length(sig.ParamTypes) <> Length(argTypes) then
                    continue;
                match := True;
                for j := 0 to High(argTypes) do
                    if not TTypeRules.CanAssign(sig.ParamTypes[j], argTypes[j]) then
                    begin
                        match := False;
                        break;
                    end;
                if match then
                begin
                    bestSig := sig;
                    break;
                end;
            end;

            if Assigned(bestSig) then
                retType := bestSig.ReturnType
            else if Assigned(FLog) then
                FLog.AddError(Format('No matching signature found for method call on %s', [calleeType.ToString]), Node);
        end;
    end
    else if calleeType.Kind <> stUnknown then
        if Assigned(FLog) then
            FLog.AddError(Format('Cannot invoke type %s as a function.', [calleeType.ToString]), Node);

    var argList := TArgumentList.Create(newArgs, Node.Arguments.Identity);
    Result := TAst.FunctionCall(Node.Identity, newCallee, argList, retType, Node.IsTailCall, nil, False);
end;

function TTypeChecker.VisitIfExpression(const Node: IIfExpressionNode): IAstNode;
var
    newCond, newThen, newElse: IAstNode;
    resType: IStaticType;
begin
    newCond := Accept(Node.Condition);
    newThen := Accept(Node.ThenBranch);
    newElse := Accept(Node.ElseBranch);

    // Simple promotion logic
    if (newElse <> nil) then
        resType := TTypeRules.Promote(newThen.AsTypedNode.StaticType, newElse.AsTypedNode.StaticType)
    else
        resType := TTypes.MakeOptional(newThen.AsTypedNode.StaticType);

    Result := TAst.IfExpr(Node.Identity, newCond, newThen, newElse, resType);
end;

function TTypeChecker.VisitIndexer(const Node: IIndexerNode): IAstNode;
var
    newBase, newIndex: IAstNode;
    baseType, elemType: IStaticType;
    isOpt: Boolean;
begin
    newBase := Accept(Node.Base);
    newIndex := Accept(Node.Index);
    baseType := PrepareBaseType(newBase, isOpt);

    elemType := TTypes.Unknown;
    if baseType.Kind = stSeries then
        elemType := baseType.ElementType
    else if baseType.Kind = stRecordSeries then
        elemType := TTypes.CreateRecord(baseType.Definition);

    elemType := ApplyOptionality(elemType, isOpt);
    Result := TAst.Indexer(Node.Identity, newBase, newIndex, elemType);
end;

function TTypeChecker.VisitMemberAccess(const Node: IMemberAccessNode): IAstNode;
var
    newBase: IAstNode;
    baseType, resType: IStaticType;
    idx: Integer;
    isOpt: Boolean;
begin
    newBase := Accept(Node.Base);
    baseType := PrepareBaseType(newBase, isOpt);
    resType := TTypes.Unknown;

    if (baseType.Kind = stRecord) or (baseType.Kind = stRecordSeries) then
    begin
        idx := baseType.Definition.IndexOf(Node.Member.Value);
        if idx >= 0 then
        begin
            var fieldType := TTypes.FromScalarKind(baseType.Definition.Items[idx].Value);
            if baseType.Kind = stRecordSeries then
                resType := TTypes.CreateSeries(fieldType)
            else
                resType := fieldType;
        end
        else if Assigned(FLog) then
            FLog.AddError('Member not found', Node);
    end;

    resType := ApplyOptionality(resType, isOpt);
    Result := TAst.MemberAccess(Node.Identity, newBase, Node.Member, resType);
end;

function TTypeChecker.VisitRecordLiteral(const Node: IRecordLiteralNode): IAstNode;
var
    i: Integer;
    newFields: TArray<IRecordFieldNode>;
    fieldTypes: TArray<TPair<IKeyword, IStaticType>>;
    scalarFieldTypes: TArray<TPair<IKeyword, TScalar.TKind>>;
    isScalar: Boolean;
    valType: IStaticType;
    key: IKeyword;
    visitedValue: IAstNode;
    newFieldList: IRecordFieldList;
begin
    SetLength(newFields, Node.Fields.Count);
    SetLength(fieldTypes, Node.Fields.Count);
    SetLength(scalarFieldTypes, Node.Fields.Count);
    isScalar := True;

    for i := 0 to Node.Fields.Count - 1 do
    begin
        var oldField := Node.Fields[i];
        visitedValue := Accept(oldField.Value);

        // Keys are guaranteed to be keywords by parser
        key := oldField.Key.Value;

        // Recreate the field node with the typed value
        newFields[i] := TAst.RecordField(oldField.Identity, oldField.Key, visitedValue);

        // Analyze Type
        valType := visitedValue.AsTypedNode.StaticType;

        // Check if strict scalar (no optionals allowed in packed ScalarRecord)
        if (valType.Kind in [stOrdinal, stFloat, stBoolean, stDateTime, stKeyword]) and (not valType.IsOptional) then
        begin
            var kind: TScalar.TKind;
            // Map StaticType Kind to Scalar Kind
            case valType.Kind of
                stOrdinal: kind := TScalar.TKind.Ordinal;
                stFloat: kind := TScalar.TKind.Float;
                stBoolean: kind := TScalar.TKind.Boolean;
                stDateTime: kind := TScalar.TKind.DateTime;
                stKeyword: kind := TScalar.TKind.Keyword;
            else
                kind := TScalar.TKind.Ordinal; // Should not happen given check above
            end;
            scalarFieldTypes[i] := TPair<IKeyword, TScalar.TKind>.Create(key, kind);
        end
        else
        begin
            isScalar := False;
        end;

        fieldTypes[i] := TPair<IKeyword, IStaticType>.Create(key, valType);
    end;

    // Build Definition
    var scalarDef: IScalarRecordDefinition := nil;
    var genericDef: IGenericRecordDefinition := nil;
    var resultType: IStaticType;

    if isScalar and (Length(newFields) > 0) then
    begin
        scalarDef := TKeywordMappingRegistry<TScalar.TKind>.Intern(scalarFieldTypes);
        resultType := TTypes.CreateRecord(scalarDef);
    end
    else
    begin
        genericDef := TGenericRecordRegistry.Intern(fieldTypes);
        resultType := TTypes.CreateGenericRecord(genericDef);
    end;

    newFieldList := TRecordFieldList.Create(newFields, Node.Fields.Identity);
    Result := TAst.RecordLiteral(Node.Identity, newFieldList, scalarDef, genericDef, resultType);
end;

function TTypeChecker.VisitCreateSeries(const Node: ICreateSeriesNode): IAstNode;
var
    elemType: IStaticType;
    def: string;
begin
    def := Node.Definition;
    // Simple heuristic for type
    if def.StartsWith('[') then
        elemType := TTypes.CreateRecord(nil) // Placeholder, normally parses JSON
    else
        elemType := TTypes.FromScalarKind(TScalar.StringToKind(def));

    Result := TAst.CreateSeries(Node.Identity.AsDefinition, TTypes.CreateSeries(elemType));
end;

function TTypeChecker.VisitSeriesLength(const Node: ISeriesLengthNode): IAstNode;
begin
    Result := TAst.SeriesLength(Node.Identity, Accept(Node.Series).AsIdentifier, TTypes.Ordinal);
end;

function TTypeChecker.VisitNop(const Node: INopNode): IAstNode;
begin
    Result := TAst.Nop(Node.Identity, TTypes.Void);
end;

// =============================================================================
// PIPE IMPLEMENTATION (Type Checking)
// =============================================================================

function TTypeChecker.VisitPipeInput(const Node: IPipeInputNode): IAstNode;
var
    newSource: IIdentifierNode;
    sourceType: IStaticType;
    newSelectors: TArray<IKeywordNode>;
    i: Integer;
begin
    newSource := Accept(Node.StreamSource).AsIdentifier;
    sourceType := newSource.AsTypedNode.StaticType;

    // 1. Verify Source is a Series-compatible type
    if (sourceType.Kind <> stUnknown) then
    begin
        if not ((sourceType.Kind = stSeries) or (sourceType.Kind = stRecordSeries)) then
        begin
            if Assigned(FLog) then
                FLog.AddError(
                    Format('Pipe input "%s" must be a Series or RecordSeries, but got %s.', [newSource.Name, sourceType.ToString]),
                    Node
                );
        end
        else if (sourceType.Kind = stRecordSeries) then
        begin
            // 2. Verify Selectors exist in Record Definition
            var def := sourceType.Definition;
            for var sel in Node.Selectors do
            begin
                if def.IndexOf(sel.Value) < 0 then
                begin
                    if Assigned(FLog) then
                        FLog.AddError(Format('Field ":%s" not found in stream "%s".', [sel.Value.Name, newSource.Name]), sel);
                end;
            end;
        end;
    end;

    // Reconstruct list simply to pass through
    SetLength(newSelectors, Node.Selectors.Count);
    for i := 0 to Node.Selectors.Count - 1 do
        newSelectors[i] := Node.Selectors[i];

    Result := TAst.PipeInput(newSource, TAst.PipeSelectorList(newSelectors, Node.Selectors.Identity.Location), Node.Identity.Location);
end;

function TTypeChecker.VisitPipe(const Node: IPipeNode): IAstNode;
var
    i, k: Integer;
    inputNode: IPipeInputNode;
    newInputs: TArray<IPipeInputNode>;
    streamType: IStaticType;
    paramTypes: TList<IStaticType>;
    lambda: ILambdaExpressionNode;
    newParams: TArray<IIdentifierNode>;
    inferredType: IStaticType;
begin
    SetLength(newInputs, Node.Inputs.Count);
    paramTypes := TList<IStaticType>.Create;
    try
        // 1. Visit Inputs and collect types for Lambda parameters
        for i := 0 to Node.Inputs.Count - 1 do
        begin
            inputNode := Accept(Node.Inputs[i]).AsPipeInput;
            newInputs[i] := inputNode;

            streamType := inputNode.StreamSource.AsTypedNode.StaticType;

            // Flatten logic: One lambda param per selector
            for var sel in inputNode.Selectors do
            begin
                inferredType := TTypes.Unknown;
                if streamType.Kind = stRecordSeries then
                begin
                    // Extract field type from definition
                    var def := streamType.Definition;
                    var idx := def.IndexOf(sel.Value);
                    if idx >= 0 then
                        inferredType := TTypes.FromScalarKind(def.Items[idx].Value);
                end
                else if streamType.Kind = stSeries then
                begin
                    // If simple series, use its element type
                    if Assigned(streamType.ElementType) then
                        inferredType := streamType.ElementType
                    else
                        inferredType := TTypes.Ordinal; // Fallback default
                end;

                paramTypes.Add(inferredType);
            end;
        end;

        // 2. Prepare Lambda with Inferred Types
        lambda := Node.Transformation;

        if lambda.Parameters.Count <> paramTypes.Count then
        begin
            if Assigned(FLog) then
                FLog.AddError(
                    Format(
                        'Pipe lambda expects %d parameters (one per selector), but got %d.',
                        [paramTypes.Count, lambda.Parameters.Count]
                    ),
                    lambda
                );
        end;

        SetLength(newParams, lambda.Parameters.Count);
        for k := 0 to lambda.Parameters.Count - 1 do
        begin
            var oldP := lambda.Parameters[k];
            var typeToInject :=
                if k < paramTypes.Count then paramTypes[k]
                else TTypes.Unknown;

            // Create new identifier with the inferred type!
            newParams[k] := TAst.Identifier(oldP.Identity.AsNamed, oldP.Address, typeToInject);
        end;

        // Recreate Lambda NODE with Typed Parameters
        var preTypedLambda :=
            TAst.LambdaExpr(
                lambda.Identity,
                TParameterList.Create(newParams, lambda.Parameters.Identity),
                lambda.Body,
                lambda.Layout,
                lambda.Descriptor,
                lambda.Upvalues,
                lambda.HasNestedLambdas,
                lambda.IsPure,
                TTypes.Unknown // Will be recalculated in Accept
            );

        // 3. Visit the Lambda (This will now type-check the Body using the types we just injected)
        var typedLambda := Accept(preTypedLambda).AsLambdaExpression;

        // 4. Infer Pipe Return Type & Validate Strictness
        var lambdaRetType := typedLambda.AsTypedNode.StaticType.Signatures[0].ReturnType;
        var pipeType: IStaticType;

        if lambdaRetType.Kind = stRecord then
        begin
            // Valid: Record -> RecordSeries
            pipeType := TTypes.CreateRecordSeries(lambdaRetType.Definition);
        end
        else
        begin
            // STRICT CHECK: Scalars are NOT allowed.
            if (lambdaRetType.Kind <> stUnknown) and (lambdaRetType.Kind <> stVoid) then
            begin
                if Assigned(FLog) then
                    FLog.AddError(
                        Format('Pipe function must return a Record (e.g. {:res ...}). Type "%s" is invalid.', [lambdaRetType.ToString]),
                        lambda
                    );
            end;

            // If Void or Unknown, or Error case:
            pipeType := TTypes.Unknown;
        end;

        Result := TAst.Pipe(Node.Identity, TPipeInputList.Create(newInputs, Node.Inputs.Identity), typedLambda, pipeType);
    finally
        paramTypes.Free;
    end;
end;

end.