expr.go

/*
 Copyright 2021 The GoPlus Authors (goplus.org)

 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
*/

package cl

import (
	"log"
	"math/big"
	"reflect"
	"strconv"
	"strings"

	goast "go/ast"
	gotoken "go/token"
	"go/types"

	"github.com/goplus/gop/ast"
	"github.com/goplus/gop/token"
	"github.com/goplus/gox"
)

/*-----------------------------------------------------------------------------

Name context:
- varVal               (ident)
- varRef = expr        (identLHS)
- pkgRef.member        (selectorExpr)
- pkgRef.member = expr (selectorExprLHS)
- pkgRef.fn(args)      (callExpr)
- fn(args)             (callExpr)
- spx.fn(args)         (callExpr)
- this.member          (classMember)
- this.method(args)    (classMember)

Name lookup:
- local variables
- $recv members (only in class files)
- package globals (variables, constants, types, imported packages etc.)
- $spx package exports (only in class files)
- $universe package exports (including builtins)

// ---------------------------------------------------------------------------*/

const (
	clIdentAutoCall = 1 << iota
	clIdentAllowBuiltin
	clIdentLHS
	clIdentSelectorExpr
	clIdentGoto
)

func compileIdent(ctx *blockCtx, ident *ast.Ident, flags int) *gox.PkgRef {
	fvalue := (flags&clIdentSelectorExpr) != 0 || (flags&clIdentLHS) == 0
	name := ident.Name
	if name == "_" {
		if fvalue {
			panic(ctx.newCodeError(ident.Pos(), "cannot use _ as value"))
		}
		ctx.cb.VarRef(nil)
		return nil
	}

	scope := ctx.pkg.Types.Scope()
	at, o := ctx.cb.Scope().LookupParent(name, token.NoPos)
	if o != nil {
		if at != scope && at != types.Universe { // local object
			goto find
		}
	}

	if ctx.fileType > 0 { // in a Go+ class file
		if fn := ctx.cb.Func(); fn != nil {
			sig := fn.Ancestor().Type().(*types.Signature)
			if recv := sig.Recv(); recv != nil {
				ctx.cb.Val(recv)
				if compileMember(ctx, ident, name, flags) == nil { // class member object
					return nil
				}
				ctx.cb.InternalStack().PopN(1)
			}
		}
	}

	// global object
	if ctx.loadSymbol(name) {
		o, at = scope.Lookup(name), scope
	}
	if o != nil && at != types.Universe {
		goto find
	}

	// pkgRef object
	if (flags & clIdentSelectorExpr) != 0 {
		if pkgRef, ok := ctx.imports[name]; ok {
			return pkgRef
		}
	}

	// object from import . "xxx"
	if compilePkgRef(ctx, nil, ident, flags) {
		return nil
	}

	// universe object
	if (flags&clIdentAllowBuiltin) == 0 && isBuiltin(o) {
		panic(ctx.newCodeErrorf(ident.Pos(), "use of builtin %s not in function call", name))
	}
	if obj := ctx.pkg.Builtin().TryRef(name); obj != nil {
		o = obj
	} else if o == nil {
		if (clIdentGoto & flags) != 0 {
			l := ident.Obj.Data.(*ast.Ident)
			panic(ctx.newCodeErrorf(l.Pos(), "label %v is not defined", l.Name))
		}
		panic(ctx.newCodeErrorf(ident.Pos(), "undefined: %s", name))
	}

find:
	if fvalue {
		ctx.cb.Val(o, ident)
	} else {
		ctx.cb.VarRef(o, ident)
	}
	return nil
}

func isBuiltin(o types.Object) bool {
	if _, ok := o.(*types.Builtin); ok {
		return ok
	}
	return false
}

func compileMember(ctx *blockCtx, v ast.Node, name string, flags int) error {
	cb := ctx.cb
	lhs := (flags & clIdentLHS) != 0
	kind, err := cb.Member(name, lhs, v)
	if kind != 0 {
		return nil
	}
	if c := name[0]; c >= 'a' && c <= 'z' {
		name = string(rune(c)+('A'-'a')) + name[1:]
		switch kind, _ = cb.Member(name, lhs, v); kind {
		case gox.MemberMethod:
			if (flags & clIdentAutoCall) != 0 {
				cb.Call(0)
			}
			return nil
		case gox.MemberField:
			return nil
		}
	}
	return err
}

func compileExprLHS(ctx *blockCtx, expr ast.Expr) {
	switch v := expr.(type) {
	case *ast.Ident:
		compileIdent(ctx, v, clIdentLHS)
	case *ast.IndexExpr:
		compileIndexExprLHS(ctx, v)
	case *ast.SelectorExpr:
		compileSelectorExprLHS(ctx, v)
	case *ast.StarExpr:
		compileStarExprLHS(ctx, v)
	default:
		log.Panicln("compileExpr failed: unknown -", reflect.TypeOf(v))
	}
}

func compileExpr(ctx *blockCtx, expr ast.Expr, twoValue ...bool) {
	switch v := expr.(type) {
	case *ast.Ident:
		compileIdent(ctx, v, clIdentAutoCall)
	case *ast.BasicLit:
		compileBasicLit(ctx, v)
	case *ast.CallExpr:
		compileCallExpr(ctx, v, 0)
	case *ast.SelectorExpr:
		compileSelectorExpr(ctx, v, clIdentAutoCall)
	case *ast.BinaryExpr:
		compileBinaryExpr(ctx, v)
	case *ast.UnaryExpr:
		compileUnaryExpr(ctx, v, twoValue != nil && twoValue[0])
	case *ast.FuncLit:
		compileFuncLit(ctx, v)
	case *ast.CompositeLit:
		compileCompositeLit(ctx, v, nil, false)
	case *ast.SliceLit:
		compileSliceLit(ctx, v)
	case *ast.IndexExpr:
		compileIndexExpr(ctx, v, twoValue != nil && twoValue[0])
	case *ast.SliceExpr:
		compileSliceExpr(ctx, v)
	case *ast.StarExpr:
		compileStarExpr(ctx, v)
	case *ast.ArrayType:
		ctx.cb.Typ(toArrayType(ctx, v), v)
	case *ast.MapType:
		ctx.cb.Typ(toMapType(ctx, v), v)
	case *ast.StructType:
		ctx.cb.Typ(toStructType(ctx, v), v)
	case *ast.ChanType:
		ctx.cb.Typ(toChanType(ctx, v), v)
	case *ast.InterfaceType:
		ctx.cb.Typ(toInterfaceType(ctx, v), v)
	case *ast.ComprehensionExpr:
		compileComprehensionExpr(ctx, v, twoValue != nil && twoValue[0])
	case *ast.TypeAssertExpr:
		compileTypeAssertExpr(ctx, v, twoValue != nil && twoValue[0])
	case *ast.ParenExpr:
		compileExpr(ctx, v.X, twoValue...)
	case *ast.ErrWrapExpr:
		compileErrWrapExpr(ctx, v)
	case *ast.FuncType:
		ctx.cb.Typ(toFuncType(ctx, v, nil), v)
	case *ast.Ellipsis:
		panic("compileEllipsis: ast.Ellipsis unexpected")
	case *ast.KeyValueExpr:
		panic("compileExpr: ast.KeyValueExpr unexpected")
	default:
		log.Panicln("compileExpr failed: unknown -", reflect.TypeOf(v))
	}
}

func compileExprOrNone(ctx *blockCtx, expr ast.Expr) {
	if expr != nil {
		compileExpr(ctx, expr)
	} else {
		ctx.cb.None()
	}
}

func compileUnaryExpr(ctx *blockCtx, v *ast.UnaryExpr, twoValue bool) {
	compileExpr(ctx, v.X)
	ctx.cb.UnaryOp(gotoken.Token(v.Op), twoValue)
}

func compileBinaryExpr(ctx *blockCtx, v *ast.BinaryExpr) {
	compileExpr(ctx, v.X)
	compileExpr(ctx, v.Y)
	ctx.cb.BinaryOp(gotoken.Token(v.Op), v)
}

func compileIndexExprLHS(ctx *blockCtx, v *ast.IndexExpr) {
	compileExpr(ctx, v.X)
	compileExpr(ctx, v.Index)
	ctx.cb.IndexRef(1, v)
}

func compileStarExprLHS(ctx *blockCtx, v *ast.StarExpr) { // *x = ...
	compileExpr(ctx, v.X)
	ctx.cb.ElemRef()
}

func compileStarExpr(ctx *blockCtx, v *ast.StarExpr) { // ... = *x
	compileExpr(ctx, v.X)
	ctx.cb.Star()
}

func compileTypeAssertExpr(ctx *blockCtx, v *ast.TypeAssertExpr, twoValue bool) {
	compileExpr(ctx, v.X)
	if v.Type == nil {
		panic("TODO: x.(type) is only used in type switch")
	}
	typ := toType(ctx, v.Type)
	ctx.cb.TypeAssert(typ, twoValue, v)
}

func compileIndexExpr(ctx *blockCtx, v *ast.IndexExpr, twoValue bool) { // x[i]
	compileExpr(ctx, v.X)
	compileExpr(ctx, v.Index)
	ctx.cb.Index(1, twoValue, v)
}

func compileSliceExpr(ctx *blockCtx, v *ast.SliceExpr) { // x[i:j:k]
	compileExpr(ctx, v.X)
	compileExprOrNone(ctx, v.Low)
	compileExprOrNone(ctx, v.High)
	if v.Slice3 {
		compileExprOrNone(ctx, v.Max)
	}
	ctx.cb.Slice(v.Slice3, v)
}

func compileSelectorExprLHS(ctx *blockCtx, v *ast.SelectorExpr) {
	switch x := v.X.(type) {
	case *ast.Ident:
		if at := compileIdent(ctx, x, clIdentLHS|clIdentSelectorExpr); at != nil {
			ctx.cb.VarRef(at.Ref(v.Sel.Name))
			return
		}
	default:
		compileExpr(ctx, v.X)
	}
	ctx.cb.MemberRef(v.Sel.Name, v)
}

func compileSelectorExpr(ctx *blockCtx, v *ast.SelectorExpr, flags int) {
	switch x := v.X.(type) {
	case *ast.Ident:
		if at := compileIdent(ctx, x, flags|clIdentSelectorExpr); at != nil {
			if compilePkgRef(ctx, at, v.Sel, flags) {
				return
			}
			if token.IsExported(v.Sel.Name) {
				panic(ctx.newCodeErrorf(x.Pos(), "undefined: %s.%s", x.Name, v.Sel.Name))
			}
			panic(ctx.newCodeErrorf(x.Pos(), "cannot refer to unexported name %s.%s", x.Name, v.Sel.Name))
		}
	default:
		compileExpr(ctx, v.X)
	}
	if err := compileMember(ctx, v, v.Sel.Name, flags); err != nil {
		panic(err)
	}
}

func pkgRef(at *gox.PkgRef, name string) (o types.Object, canAutoCall bool) {
	if c := name[0]; c >= 'a' && c <= 'z' {
		name = string(rune(c)+('A'-'a')) + name[1:]
		return at.TryRef(name), true
	}
	return at.TryRef(name), false
}

func lookupPkgRef(ctx *blockCtx, pkg *gox.PkgRef, x *ast.Ident) (o types.Object, canAutoCall bool) {
	if pkg != nil {
		return pkgRef(pkg, x.Name)
	}
	for _, at := range ctx.lookups {
		if o2, canAutoCall2 := pkgRef(at, x.Name); o2 != nil {
			if o != nil {
				panic(ctx.newCodeErrorf(
					x.Pos(), "confliction: %s declared both in \"%s\" and \"%s\"",
					x.Name, at.Types.Path(), pkg.Types.Path()))
			}
			pkg, o, canAutoCall = at, o2, canAutoCall2
		}
	}
	return
}

func compilePkgRef(ctx *blockCtx, at *gox.PkgRef, x *ast.Ident, flags int) bool {
	if v, canAutoCall := lookupPkgRef(ctx, at, x); v != nil {
		cb := ctx.cb
		if (flags & clIdentLHS) != 0 {
			cb.VarRef(v, x)
		} else {
			cb.Val(v, x)
			if canAutoCall && (flags&clIdentAutoCall) != 0 && isFunc(v.Type()) {
				cb.Call(0)
			}
		}
		return true
	}
	return false
}

func isFunc(typ types.Type) bool {
	switch typ.(type) {
	case *types.Signature:
		return true
	}
	return false
}

type fnType struct {
	params   *types.Tuple
	n1       int
	variadic bool
	inited   bool
}

func (p *fnType) arg(i int, ellipsis bool) types.Type {
	if i < p.n1 {
		return p.params.At(i).Type()
	}
	if p.variadic {
		t := p.params.At(p.n1).Type()
		if ellipsis {
			return t
		}
		return t.(*types.Slice).Elem()
	}
	return nil
}

func (p *fnType) init(t *types.Signature) {
	p.params, p.variadic = t.Params(), t.Variadic()
	p.n1 = p.params.Len()
	if p.variadic {
		p.n1--
	}
}

func (p *fnType) initWith(fnt types.Type, idx, nin int) {
	if p.inited {
		return
	}
	p.inited = true
	if t := gox.CheckSignature(fnt, idx, nin); t != nil {
		p.init(t)
	}
}

func compileCallExpr(ctx *blockCtx, v *ast.CallExpr, flags int) {
	switch fn := v.Fun.(type) {
	case *ast.Ident:
		compileIdent(ctx, fn, clIdentAllowBuiltin|flags)
	case *ast.SelectorExpr:
		compileSelectorExpr(ctx, fn, 0)
	default:
		compileExpr(ctx, fn)
	}
	var fn fnType
	fnt := ctx.cb.Get(-1).Type
	ellipsis := (v.Ellipsis != gotoken.NoPos)
	for i, arg := range v.Args {
		if l, ok := arg.(*ast.LambdaExpr); ok {
			fn.initWith(fnt, i, len(l.Lhs))
			if sig, ok := fn.arg(i, true).(*types.Signature); ok {
				compileLambdaExpr(ctx, l, sig.Params())
				continue
			}
		}
		if l, ok := arg.(*ast.LambdaExpr2); ok {
			fn.initWith(fnt, i, len(l.Lhs))
			if sig, ok := fn.arg(i, true).(*types.Signature); ok {
				compileLambdaExpr2(ctx, l, sig.Params())
				continue
			}
		}
		if c, ok := arg.(*ast.CompositeLit); ok && c.Type == nil {
			fn.initWith(fnt, i, -1)
			compileCompositeLit(ctx, c, fn.arg(i, ellipsis), true)
		} else {
			compileExpr(ctx, arg)
		}
	}
	ctx.cb.CallWith(len(v.Args), ellipsis, v)
}

func compileLambdaParams(ctx *blockCtx, pos token.Pos, lhs []*ast.Ident, in *types.Tuple) []*types.Var {
	pkg := ctx.pkg
	n := len(lhs)
	if nin := in.Len(); n != nin {
		fewOrMany := "few"
		if n > nin {
			fewOrMany = "many"
		}
		has := make([]string, n)
		for i, v := range lhs {
			has[i] = v.Name
		}
		panic(ctx.newCodeErrorf(
			pos, "too %s arguments in lambda expression\n\thave (%s)\n\twant %v", fewOrMany, strings.Join(has, ", "), in))
	}
	params := make([]*types.Var, n)
	for i, name := range lhs {
		params[i] = pkg.NewParam(name.Pos(), name.Name, in.At(i).Type())
	}
	return params
}

func compileLambdaExpr(ctx *blockCtx, v *ast.LambdaExpr, in *types.Tuple) {
	pkg := ctx.pkg
	params := compileLambdaParams(ctx, v.Pos(), v.Lhs, in)
	nout := len(v.Rhs)
	results := make([]*types.Var, nout)
	for i := 0; i < nout; i++ {
		results[i] = pkg.NewAutoParam("")
	}
	ctx.cb.NewClosure(types.NewTuple(params...), types.NewTuple(results...), false).BodyStart(pkg)
	for _, v := range v.Rhs {
		compileExpr(ctx, v)
	}
	ctx.cb.Return(nout).End()
}

func compileLambdaExpr2(ctx *blockCtx, v *ast.LambdaExpr2, in *types.Tuple) {
	params := compileLambdaParams(ctx, v.Pos(), v.Lhs, in)
	cb := ctx.cb
	comments := cb.Comments()
	fn := cb.NewClosure(types.NewTuple(params...), nil, false)
	loadFuncBody(ctx, fn, v.Body)
	cb.SetComments(comments, false)
}

func compileFuncLit(ctx *blockCtx, v *ast.FuncLit) {
	cb := ctx.cb
	comments := cb.Comments()
	sig := toFuncType(ctx, v.Type, nil)
	fn := cb.NewClosureWith(sig)
	if body := v.Body; body != nil {
		loadFuncBody(ctx, fn, body)
		cb.SetComments(comments, false)
	}
}

func compileBasicLit(ctx *blockCtx, v *ast.BasicLit) {
	if v.Kind == token.RAT {
		val := v.Value
		bi, _ := new(big.Int).SetString(val[:len(val)-1], 10) // remove r suffix
		ctx.cb.UntypedBigInt(bi, v)
		return
	}
	ctx.cb.Val(&goast.BasicLit{Kind: gotoken.Token(v.Kind), Value: v.Value}, v)
}

const (
	compositeLitVal    = 0
	compositeLitKeyVal = 1
)

func checkCompositeLitElts(ctx *blockCtx, elts []ast.Expr) (kind int) {
	for _, elt := range elts {
		if _, ok := elt.(*ast.KeyValueExpr); ok {
			return compositeLitKeyVal
		}
	}
	return compositeLitVal
}

func compileCompositeLitElts(ctx *blockCtx, elts []ast.Expr, kind int, expected *kvType) {
	for _, elt := range elts {
		if kv, ok := elt.(*ast.KeyValueExpr); ok {
			if key, ok := kv.Key.(*ast.CompositeLit); ok && key.Type == nil {
				compileCompositeLit(ctx, key, expected.Key(), false)
			} else {
				compileExpr(ctx, kv.Key)
			}
			if val, ok := kv.Value.(*ast.CompositeLit); ok && val.Type == nil {
				compileCompositeLit(ctx, val, expected.Elem(), false)
			} else {
				compileExpr(ctx, kv.Value)
			}
		} else {
			if kind == compositeLitKeyVal {
				ctx.cb.None()
			}
			if val, ok := elt.(*ast.CompositeLit); ok && val.Type == nil {
				compileCompositeLit(ctx, val, expected.Elem(), false)
			} else {
				compileExpr(ctx, elt)
			}
		}
	}
}

func compileStructLitInKeyVal(ctx *blockCtx, elts []ast.Expr, t *types.Struct, typ types.Type) {
	for _, elt := range elts {
		kv := elt.(*ast.KeyValueExpr)
		name := kv.Key.(*ast.Ident).Name
		if idx := lookupField(t, name); idx >= 0 {
			ctx.cb.Val(idx)
		} else {
			log.Panicln("TODO: struct member not found -", name)
		}
		compileExpr(ctx, kv.Value)
	}
	ctx.cb.StructLit(typ, len(elts)<<1, true)
}

func lookupField(t *types.Struct, name string) int {
	for i, n := 0, t.NumFields(); i < n; i++ {
		if fld := t.Field(i); fld.Name() == name {
			return i
		}
	}
	return -1
}

type kvType struct {
	underlying types.Type
	key, val   types.Type
	cached     bool
}

func (p *kvType) required() *kvType {
	if !p.cached {
		p.cached = true
		switch t := p.underlying.(type) {
		case *types.Slice:
			p.key, p.val = types.Typ[types.Int], t.Elem()
		case *types.Array:
			p.key, p.val = types.Typ[types.Int], t.Elem()
		case *types.Map:
			p.key, p.val = t.Key(), t.Elem()
		}
	}
	return p
}

func (p *kvType) Key() types.Type {
	return p.required().key
}

func (p *kvType) Elem() types.Type {
	return p.required().val
}

func getUnderlying(ctx *blockCtx, typ types.Type) types.Type {
	u := typ.Underlying()
	if u == nil {
		if t, ok := typ.(*types.Named); ok {
			ctx.loadNamed(ctx.pkg, t)
			u = t.Underlying()
		}
	}
	return u
}

func compileCompositeLit(ctx *blockCtx, v *ast.CompositeLit, expected types.Type, onlyStruct bool) {
	var hasPtr bool
	var typ, underlying types.Type
	var kind = checkCompositeLitElts(ctx, v.Elts)
	if v.Type != nil {
		typ = toType(ctx, v.Type)
		underlying = getUnderlying(ctx, typ)
	} else if expected != nil {
		if t, ok := expected.(*types.Pointer); ok {
			expected, hasPtr = t.Elem(), true
		}
		if onlyStruct {
			if kind == compositeLitKeyVal {
				t := getUnderlying(ctx, expected)
				if _, ok := t.(*types.Struct); ok { // can't omit non-struct type
					typ, underlying = expected, t
				}
			}
		} else {
			typ, underlying = expected, getUnderlying(ctx, expected)
		}
	}
	if t, ok := underlying.(*types.Struct); ok && kind == compositeLitKeyVal {
		compileStructLitInKeyVal(ctx, v.Elts, t, typ)
		if hasPtr {
			ctx.cb.UnaryOp(gotoken.AND)
		}
		return
	}
	compileCompositeLitElts(ctx, v.Elts, kind, &kvType{underlying: underlying})
	n := len(v.Elts)
	if typ == nil {
		if kind == compositeLitVal && n > 0 {
			panic("TODO: mapLit should be in {key: val, ...} form")
		}
		ctx.cb.MapLit(nil, n<<1)
		return
	}
	switch underlying.(type) {
	case *types.Slice:
		ctx.cb.SliceLit(typ, n<<kind, kind == compositeLitKeyVal)
	case *types.Array:
		ctx.cb.ArrayLit(typ, n<<kind, kind == compositeLitKeyVal)
	case *types.Map:
		ctx.cb.MapLit(typ, n<<1)
	case *types.Struct:
		ctx.cb.StructLit(typ, n, false)
	default:
		log.Panicln("compileCompositeLit: unknown type -", reflect.TypeOf(underlying))
	}
	if hasPtr {
		ctx.cb.UnaryOp(gotoken.AND)
	}
}

func compileSliceLit(ctx *blockCtx, v *ast.SliceLit) {
	n := len(v.Elts)
	for _, elt := range v.Elts {
		compileExpr(ctx, elt)
	}
	ctx.cb.SliceLit(nil, n)
}

const (
	comprehensionInvalid = iota
	comprehensionList
	comprehensionMap
	comprehensionSelect
)

func comprehensionKind(v *ast.ComprehensionExpr) int {
	switch v.Tok {
	case token.LBRACK: // [
		return comprehensionList
	case token.LBRACE: // {
		if _, ok := v.Elt.(*ast.KeyValueExpr); ok {
			return comprehensionMap
		}
		return comprehensionSelect
	}
	panic("TODO: invalid comprehensionExpr")
}

// [expr for k, v <- container, cond]
// {for k, v <- container, cond}
// {expr for k, v <- container, cond}
// {kexpr: vexpr for k, v <- container, cond}
func compileComprehensionExpr(ctx *blockCtx, v *ast.ComprehensionExpr, twoValue bool) {
	kind := comprehensionKind(v)
	pkg, cb := ctx.pkg, ctx.cb
	var results *types.Tuple
	var ret *gox.Param
	if v.Elt == nil {
		boolean := pkg.NewParam(token.NoPos, "_gop_ok", types.Typ[types.Bool])
		results = types.NewTuple(boolean)
	} else {
		ret = pkg.NewAutoParam("_gop_ret")
		if kind == comprehensionSelect && twoValue {
			boolean := pkg.NewParam(token.NoPos, "_gop_ok", types.Typ[types.Bool])
			results = types.NewTuple(ret, boolean)
		} else {
			results = types.NewTuple(ret)
		}
	}
	cb.NewClosure(nil, results, false).BodyStart(pkg)
	if kind == comprehensionMap {
		cb.VarRef(ret).ZeroLit(ret.Type()).Assign(1)
	}
	end := 0
	for i := len(v.Fors) - 1; i >= 0; i-- {
		names := make([]string, 0, 2)
		forStmt := v.Fors[i]
		if forStmt.Key != nil {
			names = append(names, forStmt.Key.Name)
		} else {
			names = append(names, "_")
		}
		names = append(names, forStmt.Value.Name)
		cb.ForRange(names...)
		compileExpr(ctx, forStmt.X)
		cb.RangeAssignThen(forStmt.TokPos)
		if forStmt.Cond != nil {
			cb.If()
			if forStmt.Init != nil {
				compileStmt(ctx, forStmt.Init)
			}
			compileExpr(ctx, forStmt.Cond)
			cb.Then()
			end++
		}
		end++
	}
	switch kind {
	case comprehensionList:
		// _gop_ret = append(_gop_ret, elt)
		cb.VarRef(ret)
		cb.Val(pkg.Builtin().Ref("append"))
		cb.Val(ret)
		compileExpr(ctx, v.Elt)
		cb.Call(2).Assign(1)
	case comprehensionMap:
		// _gop_ret[key] = val
		cb.Val(ret)
		kv := v.Elt.(*ast.KeyValueExpr)
		compileExpr(ctx, kv.Key)
		cb.IndexRef(1)
		compileExpr(ctx, kv.Value)
		cb.Assign(1)
	default:
		if v.Elt == nil {
			// return true
			cb.Val(true)
			cb.Return(1)
		} else {
			// return elt, true
			compileExpr(ctx, v.Elt)
			n := 1
			if twoValue {
				cb.Val(true)
				n++
			}
			cb.Return(n)
		}
	}
	for i := 0; i < end; i++ {
		cb.End()
	}
	cb.Return(0).End().Call(0)
}

var (
	tyError = types.Universe.Lookup("error").Type()
)

func compileErrWrapExpr(ctx *blockCtx, v *ast.ErrWrapExpr) {
	pkg, cb := ctx.pkg, ctx.cb
	useClosure := (v.Tok == token.NOT || v.Default != nil)
	if !useClosure && (cb.Scope().Parent() == types.Universe) {
		panic("TODO: can't use expr? in global")
	}

	compileExpr(ctx, v.X)
	x := cb.InternalStack().Pop()
	n := 0
	results, ok := x.Type.(*types.Tuple)
	if ok {
		n = results.Len() - 1
	}

	var ret []*types.Var
	if n > 0 {
		i, retName := 0, "_gop_ret"
		ret = make([]*gox.Param, n)
		for {
			ret[i] = pkg.NewAutoParam(retName)
			i++
			if i >= n {
				break
			}
			retName = "_gop_ret" + strconv.Itoa(i+1)
		}
	}
	sig := types.NewSignature(nil, nil, types.NewTuple(ret...), false)
	if useClosure {
		cb.NewClosureWith(sig).BodyStart(pkg)
	} else {
		cb.CallInlineClosureStart(sig, 0, false)
	}

	cb.NewVar(tyError, "_gop_err")
	err := cb.Scope().Lookup("_gop_err")

	for _, retVar := range ret {
		cb.VarRef(retVar)
	}
	cb.VarRef(err)
	cb.InternalStack().Push(x)
	cb.Assign(n+1, 1)

	cb.If().Val(err).CompareNil(gotoken.NEQ).Then()
	if v.Tok == token.NOT { // expr!
		cb.Val(pkg.Builtin().Ref("panic")).Val(err).Call(1).EndStmt() // TODO: wrap err
	} else if v.Default == nil { // expr?
		cb.Val(err).ReturnErr(true) // TODO: wrap err & return err
	} else { // expr?:val
		compileExpr(ctx, v.Default)
		cb.Return(1)
	}
	cb.End().Return(0).End()
	if useClosure {
		cb.Call(0)
	}
}

// -----------------------------------------------------------------------------