gotlin/internal/lang/hir.go

package lang

import "fmt"

type ExprMeaning int

const (
	UnresolvedExpr ExprMeaning = iota
	LiteralExpr
	LocalReferenceExpr
	FunctionReferenceExpr
	ClassReferenceExpr
	EnumReferenceExpr
	ImportReferenceExpr
	GotlinCallExpr
	GoCallExpr
	ClassConstructionExpr
	EnumConstructionExpr
	FieldAccessExpr
	MethodCallExpr
	PropagateResultExpr
	MatchValueExpr
	SQLExpression
	MappingExpression
	CoroutineExpression
)

type ExprMeta struct{ Semantic *HIRExpr }

type HIRExpr struct {
	Type    Type
	Meaning ExprMeaning
	Symbol  *Symbol
	Node    HIRNode
}

type HIRNode interface{ hirNode() }

type HIRLiteral struct{}

func (HIRLiteral) hirNode() {}

type HIRReference struct{ Target *Symbol }

func (HIRReference) hirNode() {}

type HIRGoCall struct {
	Callee *HIRExpr
	Result Type
}

func (HIRGoCall) hirNode() {}

type HIRGotlinCall struct {
	Target *Symbol
	Result Type
}

func (HIRGotlinCall) hirNode() {}

type HIRClassConstruction struct{ Class *ClassSymbol }

func (HIRClassConstruction) hirNode() {}

type HIREnumConstruction struct {
	EnumName, VariantName string
}

func (HIREnumConstruction) hirNode() {}

type HIRPropagateResult struct {
	Value *HIRExpr
	Type  Type
}

func (HIRPropagateResult) hirNode() {}

type HIRMatch struct {
	Value *HIRExpr
	Cases []HIRMatchCase
	Type  Type
}

func (HIRMatch) hirNode() {}

type HIRMatchCase struct {
	EnumName, VariantName string
	Bindings              []string
	Value                 *HIRExpr
}

type HIRCoroutine struct{ Operation string }

func (HIRCoroutine) hirNode() {}

type HIRSQL struct{ Type Type }

func (HIRSQL) hirNode() {}

type HIRMapping struct {
	Source Type
	Target Type
}

func (HIRMapping) hirNode() {}

type HIRFunction struct {
	Symbol *Symbol
	Decl   *FunctionDecl
	Scope  *Scope
}

type HIRProgram struct {
	Functions []*HIRFunction
	Methods   []*HIRFunction
}

func exprMeta(expr Expr) *HIRExpr {
	switch value := expr.(type) {
	case IdentExpr:
		return value.Meta.Semantic
	case IntExpr:
		return value.Meta.Semantic
	case FloatExpr:
		return value.Meta.Semantic
	case StringExpr:
		return value.Meta.Semantic
	case BoolExpr:
		return value.Meta.Semantic
	case NullExpr:
		return value.Meta.Semantic
	case UnaryExpr:
		return value.Meta.Semantic
	case BinaryExpr:
		return value.Meta.Semantic
	case CallExpr:
		return value.Meta.Semantic
	case SelectorExpr:
		return value.Meta.Semantic
	case SafeSelectorExpr:
		return value.Meta.Semantic
	case NonNullExpr:
		return value.Meta.Semantic
	case TryExpr:
		return value.Meta.Semantic
	case IndexExpr:
		return value.Meta.Semantic
	case EnumVariantExpr:
		return value.Meta.Semantic
	case MatchExpr:
		return value.Meta.Semantic
	case LambdaExpr:
		return value.Meta.Semantic
	default:
		return nil
	}
}

func ResolvedType(expr Expr) Type {
	if semantic := exprMeta(expr); semantic != nil {
		return semantic.Type
	}
	return UnknownType{}
}

func ResolvedMeaning(expr Expr) ExprMeaning {
	if semantic := exprMeta(expr); semantic != nil {
		return semantic.Meaning
	}
	return UnresolvedExpr
}

func withExprMeta(expr Expr, semantic *HIRExpr) Expr {
	switch value := expr.(type) {
	case IdentExpr:
		value.Meta.Semantic = semantic
		return value
	case IntExpr:
		value.Meta.Semantic = semantic
		return value
	case FloatExpr:
		value.Meta.Semantic = semantic
		return value
	case StringExpr:
		value.Meta.Semantic = semantic
		return value
	case BoolExpr:
		value.Meta.Semantic = semantic
		return value
	case NullExpr:
		value.Meta.Semantic = semantic
		return value
	case UnaryExpr:
		value.Meta.Semantic = semantic
		return value
	case BinaryExpr:
		value.Meta.Semantic = semantic
		return value
	case CallExpr:
		value.Meta.Semantic = semantic
		return value
	case SelectorExpr:
		value.Meta.Semantic = semantic
		return value
	case SafeSelectorExpr:
		value.Meta.Semantic = semantic
		return value
	case NonNullExpr:
		value.Meta.Semantic = semantic
		return value
	case TryExpr:
		value.Meta.Semantic = semantic
		return value
	case IndexExpr:
		value.Meta.Semantic = semantic
		return value
	case EnumVariantExpr:
		value.Meta.Semantic = semantic
		return value
	case MatchExpr:
		value.Meta.Semantic = semantic
		return value
	case LambdaExpr:
		value.Meta.Semantic = semantic
		return value
	default:
		return expr
	}
}

type semanticResolver struct {
	program *SemanticProgram
	err     error
}

func (resolver *semanticResolver) resolve() error {
	resolver.program.HIR = &HIRProgram{}
	for index := range resolver.program.Syntax.Functions {
		decl := &resolver.program.Syntax.Functions[index]
		symbol, _ := resolver.program.Global.Lookup(decl.Name)
		scope := NewScope(resolver.program.Global)
		for _, param := range decl.Params {
			typ, _ := resolver.program.ResolveType(param.Type)
			_ = scope.Define(&Symbol{Name: param.Name, Kind: VariableSymbol, Type: typ})
		}
		result, _ := resolver.program.ResolveType(decl.ReturnType)
		resolver.resolveStmts(decl.Body, scope, nil, result)
		resolver.program.HIR.Functions = append(resolver.program.HIR.Functions, &HIRFunction{Symbol: symbol, Decl: decl, Scope: scope})
	}
	for classIndex := range resolver.program.Syntax.Classes {
		decl := &resolver.program.Syntax.Classes[classIndex]
		class := resolver.program.ClassInfo[decl.Name]
		for methodIndex := range decl.Methods {
			method := &decl.Methods[methodIndex]
			scope := NewScope(resolver.program.Global)
			_ = scope.Define(&Symbol{Name: "this", Kind: VariableSymbol, Type: ClassType{Class: class}})
			for _, param := range method.Params {
				typ, _ := resolver.program.ResolveType(param.Type)
				_ = scope.Define(&Symbol{Name: param.Name, Kind: VariableSymbol, Type: typ})
			}
			result, _ := resolver.program.ResolveType(method.ReturnType)
			resolver.resolveStmts(method.Body, scope, class, result)
			resolver.program.HIR.Methods = append(resolver.program.HIR.Methods, &HIRFunction{Symbol: class.Methods[method.Name], Decl: method, Scope: scope})
		}
	}
	return resolver.err
}

func (resolver *semanticResolver) resolveStmts(stmts []Stmt, scope *Scope, class *ClassSymbol, returnType Type) {
	for index, stmt := range stmts {
		switch value := stmt.(type) {
		case VarDecl:
			expected := Type(UnknownType{})
			if value.Type != "" {
				expected, _ = resolver.program.ResolveType(value.Type)
			}
			value.Value, _ = resolver.resolveExpr(value.Value, scope, class, expected)
			if isUnknownType(expected) {
				expected = exprMeta(value.Value).Type
			}
			_ = scope.Define(&Symbol{Name: value.Name, Kind: VariableSymbol, Type: expected, Mutable: value.Mutable, Decl: &value})
			stmts[index] = value
		case MultiVarDecl:
			value.Value, _ = resolver.resolveExpr(value.Value, scope, class, UnknownType{})
			valueTypes := make([]Type, len(value.Names))
			for index := range valueTypes {
				valueTypes[index] = UnknownType{}
			}
			if result, ok := ResolvedType(value.Value).(GenericType); ok && result.Base.String() == "Result" && len(result.Args) == 2 && len(valueTypes) == 2 {
				valueTypes[0] = result.Args[0]
				valueTypes[1] = NullableType{Element: result.Args[1]}
			}
			for index, name := range value.Names {
				_ = scope.Define(&Symbol{Name: name, Kind: VariableSymbol, Type: valueTypes[index], Mutable: value.Mutable})
			}
			stmts[index] = value
		case AssignStmt:
			expected := Type(UnknownType{})
			if symbol, ok := scope.Lookup(value.Name); ok {
				expected = symbol.Type
			}
			value.Value, _ = resolver.resolveExpr(value.Value, scope, class, expected)
			stmts[index] = value
		case AddAssignStmt:
			value.Value, _ = resolver.resolveExpr(value.Value, scope, class, UnknownType{})
			stmts[index] = value
		case MultiAssignStmt:
			value.Value, _ = resolver.resolveExpr(value.Value, scope, class, UnknownType{})
			stmts[index] = value
		case ReturnStmt:
			if value.Value != nil {
				value.Value, _ = resolver.resolveExpr(value.Value, scope, class, returnType)
				stmts[index] = value
			}
		case ThrowStmt:
			value.Value, _ = resolver.resolveExpr(value.Value, scope, class, NamedType{Name: "Error"})
			stmts[index] = value
		case DeferStmt:
			value.Value, _ = resolver.resolveExpr(value.Value, scope, class, NamedType{Name: "Unit"})
			stmts[index] = value
		case ExprStmt:
			value.Value, _ = resolver.resolveExpr(value.Value, scope, class, UnknownType{})
			stmts[index] = value
		case IfStmt:
			value.Cond, _ = resolver.resolveExpr(value.Cond, scope, class, NamedType{Name: "Boolean"})
			resolver.resolveStmts(value.Then, NewScope(scope), class, returnType)
			resolver.resolveStmts(value.Else, NewScope(scope), class, returnType)
			stmts[index] = value
		case WhileStmt:
			value.Cond, _ = resolver.resolveExpr(value.Cond, scope, class, NamedType{Name: "Boolean"})
			resolver.resolveStmts(value.Body, NewScope(scope), class, returnType)
			stmts[index] = value
		case ForEachStmt:
			value.Source, _ = resolver.resolveExpr(value.Source, scope, class, UnknownType{})
			bodyScope := NewScope(scope)
			element := collectionElement(exprMeta(value.Source).Type)
			_ = bodyScope.Define(&Symbol{Name: value.Name, Kind: VariableSymbol, Type: element})
			resolver.resolveStmts(value.Body, bodyScope, class, returnType)
			stmts[index] = value
		case MatchStmt:
			value.Value, _ = resolver.resolveExpr(value.Value, scope, class, UnknownType{})
			for caseIndex := range value.Cases {
				matchCase := &value.Cases[caseIndex]
				caseScope := NewScope(scope)
				resolver.defineMatchBindings(caseScope, matchCase.EnumName, matchCase.VariantName, matchCase.Bindings)
				resolver.resolveStmts(matchCase.Body, caseScope, class, returnType)
			}
			resolver.validateStatementMatch(value)
			stmts[index] = value
		case TryCatchStmt:
			resolver.resolveStmts(value.TryBody, NewScope(scope), class, returnType)
			catchScope := NewScope(scope)
			catchType, _ := resolver.program.ResolveType(value.CatchType)
			_ = catchScope.Define(&Symbol{Name: value.CatchName, Kind: VariableSymbol, Type: catchType})
			resolver.resolveStmts(value.CatchBody, catchScope, class, returnType)
		}
	}
}

func (resolver *semanticResolver) resolveExpr(expr Expr, scope *Scope, class *ClassSymbol, expected Type) (Expr, Type) {
	environment := TypeEnvironment{Scope: scope, Class: class}
	switch value := expr.(type) {
	case UnaryExpr:
		value.Value, _ = resolver.resolveExpr(value.Value, scope, class, UnknownType{})
		expr = value
	case BinaryExpr:
		value.Left, _ = resolver.resolveExpr(value.Left, scope, class, UnknownType{})
		value.Right, _ = resolver.resolveExpr(value.Right, scope, class, UnknownType{})
		expr = value
	case CallExpr:
		value.Callee, _ = resolver.resolveExpr(value.Callee, scope, class, UnknownType{})
		for index := range value.Args {
			argumentExpected := Type(UnknownType{})
			if selector, ok := value.Callee.(SelectorExpr); ok {
				if receiver, ok := selector.Receiver.(IdentExpr); ok && receiver.Name == "Result" {
					if selector.Name == "Err" {
						argumentExpected = NamedType{Name: "Error"}
					} else if selector.Name == "Ok" {
						if result, ok := expected.(GenericType); ok && result.Base.String() == "Result" && len(result.Args) == 2 {
							argumentExpected = result.Args[0]
						}
					}
				}
			}
			value.Args[index], _ = resolver.resolveExpr(value.Args[index], scope, class, argumentExpected)
		}
		for index := range value.NamedArgs {
			value.NamedArgs[index].Value, _ = resolver.resolveExpr(value.NamedArgs[index].Value, scope, class, UnknownType{})
		}
		expr = value
	case SelectorExpr:
		value.Receiver, _ = resolver.resolveExpr(value.Receiver, scope, class, UnknownType{})
		expr = value
	case SafeSelectorExpr:
		value.Receiver, _ = resolver.resolveExpr(value.Receiver, scope, class, UnknownType{})
		expr = value
	case NonNullExpr:
		value.Value, _ = resolver.resolveExpr(value.Value, scope, class, UnknownType{})
		expr = value
	case TryExpr:
		value.Value, _ = resolver.resolveExpr(value.Value, scope, class, UnknownType{})
		expr = value
	case IndexExpr:
		value.Receiver, _ = resolver.resolveExpr(value.Receiver, scope, class, UnknownType{})
		value.Index, _ = resolver.resolveExpr(value.Index, scope, class, NamedType{Name: "Int"})
		expr = value
	case EnumVariantExpr:
		for index := range value.Values {
			value.Values[index], _ = resolver.resolveExpr(value.Values[index], scope, class, UnknownType{})
		}
		expr = value
	case MatchExpr:
		value.Value, _ = resolver.resolveExpr(value.Value, scope, class, UnknownType{})
		for index := range value.Cases {
			matchCase := &value.Cases[index]
			caseScope := NewScope(scope)
			resolver.defineMatchBindings(caseScope, matchCase.EnumName, matchCase.VariantName, matchCase.Bindings)
			matchCase.Value, _ = resolver.resolveExpr(matchCase.Value, caseScope, class, expected)
		}
		resolver.validateValueMatch(value)
		expr = value
	case LambdaExpr:
		lambdaScope := NewScope(scope)
		for _, param := range value.Params {
			typ, _ := resolver.program.ResolveType(param.Type)
			_ = lambdaScope.Define(&Symbol{Name: param.Name, Kind: VariableSymbol, Type: typ})
		}
		resolver.resolveStmts(value.Body, lambdaScope, class, functionResult(expected))
		expr = value
	}
	typ := resolver.program.TypeOf(expr, environment)
	if isUnknownType(typ) && !isUnknownType(expected) {
		typ = expected
	}
	if binary, ok := expr.(BinaryExpr); ok {
		switch binary.Op {
		case "==", "!=", "<", "<=", ">", ">=", "&&", "||":
			typ = NamedType{Name: "Boolean"}
		default:
			if meta := exprMeta(binary.Left); meta != nil {
				typ = meta.Type
			}
		}
	}
	meaning, symbol := resolver.meaning(expr, scope)
	resolver.validateEnumExpression(expr, meaning)
	if meaning == GoCallExpr && isResultType(expected) {
		typ = expected
	}
	if nullable, ok := typ.(NullableType); ok && typeEqual(nullable.Element, expected) {
		typ = expected
	}
	semantic := &HIRExpr{Type: typ, Meaning: meaning, Symbol: symbol}
	semantic.Node = resolver.hirNode(expr, semantic)
	return withExprMeta(expr, semantic), typ
}

func (resolver *semanticResolver) hirNode(expr Expr, semantic *HIRExpr) HIRNode {
	switch semantic.Meaning {
	case LiteralExpr:
		return HIRLiteral{}
	case LocalReferenceExpr, FunctionReferenceExpr, ClassReferenceExpr, EnumReferenceExpr, ImportReferenceExpr, FieldAccessExpr:
		return HIRReference{Target: semantic.Symbol}
	case GoCallExpr:
		if call, ok := expr.(CallExpr); ok {
			return HIRGoCall{Callee: exprMeta(call.Callee), Result: semantic.Type}
		}
	case GotlinCallExpr, MethodCallExpr:
		return HIRGotlinCall{Target: semantic.Symbol, Result: semantic.Type}
	case ClassConstructionExpr:
		if class := classTypeOf(semantic.Type); class != nil {
			return HIRClassConstruction{Class: class}
		}
	case EnumConstructionExpr:
		enumName, variantName := enumExpressionName(expr)
		return HIREnumConstruction{EnumName: enumName, VariantName: variantName}
	case PropagateResultExpr:
		if attempt, ok := expr.(TryExpr); ok {
			return HIRPropagateResult{Value: exprMeta(attempt.Value), Type: semantic.Type}
		}
	case MatchValueExpr:
		if match, ok := expr.(MatchExpr); ok {
			cases := make([]HIRMatchCase, len(match.Cases))
			for index, matchCase := range match.Cases {
				cases[index] = HIRMatchCase{EnumName: matchCase.EnumName, VariantName: matchCase.VariantName, Bindings: matchCase.Bindings, Value: exprMeta(matchCase.Value)}
			}
			return HIRMatch{Value: exprMeta(match.Value), Cases: cases, Type: semantic.Type}
		}
	case CoroutineExpression:
		if call, ok := expr.(CallExpr); ok {
			if ident, ok := call.Callee.(IdentExpr); ok {
				return HIRCoroutine{Operation: ident.Name}
			}
		}
	case SQLExpression:
		return HIRSQL{Type: semantic.Type}
	case MappingExpression:
		if call, ok := expr.(CallExpr); ok {
			if selector, ok := call.Callee.(SelectorExpr); ok {
				target := semantic.Type
				return HIRMapping{Source: ResolvedType(selector.Receiver), Target: target}
			}
		}
	}
	return nil
}

func enumExpressionName(expr Expr) (string, string) {
	switch value := expr.(type) {
	case CallExpr:
		if selector, ok := value.Callee.(SelectorExpr); ok {
			if receiver, ok := selector.Receiver.(IdentExpr); ok {
				return receiver.Name, selector.Name
			}
		}
	case SelectorExpr:
		if receiver, ok := value.Receiver.(IdentExpr); ok {
			return receiver.Name, value.Name
		}
	case EnumVariantExpr:
		return value.EnumName, value.VariantName
	}
	return "", ""
}

func (resolver *semanticResolver) meaning(expr Expr, scope *Scope) (ExprMeaning, *Symbol) {
	switch value := expr.(type) {
	case IntExpr, FloatExpr, StringExpr, BoolExpr, NullExpr:
		return LiteralExpr, nil
	case IdentExpr:
		if symbol, ok := scope.Lookup(value.Name); ok {
			switch symbol.Kind {
			case FunctionSymbolKind:
				return FunctionReferenceExpr, symbol
			case ClassSymbolKind:
				return ClassReferenceExpr, symbol
			case EnumSymbolKind:
				return EnumReferenceExpr, symbol
			case ImportSymbolKind:
				return ImportReferenceExpr, symbol
			default:
				return LocalReferenceExpr, symbol
			}
		}
	case CallExpr:
		if _, _, _, ok := splitSQLChain(value); ok {
			return SQLExpression, nil
		}
		if _, ok := value.Callee.(SelectorExpr); ok {
			selector := value.Callee.(SelectorExpr)
			if receiver, ok := selector.Receiver.(IdentExpr); ok && receiver.Name == "json" && selector.Name == "decode" {
				return GotlinCallExpr, nil
			}
			if selector.Name == "mapTo" {
				return MappingExpression, nil
			}
			if receiver, ok := selector.Receiver.(IdentExpr); ok {
				if receiver.Name == "Result" {
					return EnumConstructionExpr, nil
				}
				if _, ok := resolver.program.Enums[receiver.Name]; ok {
					return EnumConstructionExpr, nil
				}
			}
			if root, ok := selectorRootAlias(selector); ok && resolver.program.Imports[root] {
				return GoCallExpr, nil
			}
			receiverType := ResolvedType(selector.Receiver)
			if !isUnknownType(resolver.program.goMethodType(receiverType, selector.Name)) {
				return GoCallExpr, nil
			}
			return MethodCallExpr, nil
		}
		if ident, ok := value.Callee.(IdentExpr); ok {
			if coroutineBuiltins[ident.Name] {
				return CoroutineExpression, nil
			}
			if symbol, ok := resolver.program.Global.Lookup(ident.Name); ok {
				if symbol.Kind == ClassSymbolKind {
					return ClassConstructionExpr, symbol
				}
				if symbol.Kind == FunctionSymbolKind {
					return GotlinCallExpr, symbol
				}
			}
		}
		return GoCallExpr, nil
	case SelectorExpr:
		if receiver, ok := value.Receiver.(IdentExpr); ok {
			if _, ok := resolver.program.Enums[receiver.Name]; ok {
				return EnumReferenceExpr, nil
			}
		}
		return FieldAccessExpr, nil
	case SafeSelectorExpr:
		return FieldAccessExpr, nil
	case EnumVariantExpr:
		return EnumConstructionExpr, nil
	case TryExpr:
		return PropagateResultExpr, nil
	case MatchExpr:
		return MatchValueExpr, nil
	}
	return UnresolvedExpr, nil
}

func (resolver *semanticResolver) defineMatchBindings(scope *Scope, enumName, variantName string, bindings []string) {
	decl, ok := resolver.program.Enums[enumName]
	if !ok {
		return
	}
	variant := enumVariant(decl, variantName)
	if variant == nil {
		return
	}
	for index, binding := range bindings {
		if index >= len(variant.PayloadTypes) {
			break
		}
		typ, _ := resolver.program.ResolveType(variant.PayloadTypes[index])
		_ = scope.Define(&Symbol{Name: binding, Kind: VariableSymbol, Type: typ})
	}
}

func (resolver *semanticResolver) validateStatementMatch(match MatchStmt) {
	patterns := make([]matchPattern, len(match.Cases))
	for index, matchCase := range match.Cases {
		patterns[index] = matchPattern{enumName: matchCase.EnumName, variantName: matchCase.VariantName, bindings: matchCase.Bindings}
	}
	resolver.validateMatchPatterns(patterns)
}

func (resolver *semanticResolver) validateValueMatch(match MatchExpr) {
	patterns := make([]matchPattern, len(match.Cases))
	var result Type = UnknownType{}
	for index, matchCase := range match.Cases {
		patterns[index] = matchPattern{enumName: matchCase.EnumName, variantName: matchCase.VariantName, bindings: matchCase.Bindings}
		armType := ResolvedType(matchCase.Value)
		if isUnknownType(result) {
			result = armType
		} else if !isUnknownType(armType) && !typeEqual(result, armType) {
			resolver.fail(fmt.Errorf("match expression arm %s.%s has type %s, expected %s", matchCase.EnumName, matchCase.VariantName, armType.String(), result.String()))
		}
	}
	if isUnknownType(result) {
		resolver.fail(fmt.Errorf("match expression result type cannot be inferred"))
	}
	resolver.validateMatchPatterns(patterns)
}

type matchPattern struct {
	enumName, variantName string
	bindings              []string
}

func (resolver *semanticResolver) validateMatchPatterns(patterns []matchPattern) {
	if len(patterns) == 0 {
		resolver.fail(fmt.Errorf("match requires at least one case"))
		return
	}
	enumName := patterns[0].enumName
	decl, ok := resolver.program.Enums[enumName]
	if !ok {
		resolver.fail(fmt.Errorf("match value is not a known enum"))
		return
	}
	seen := map[string]bool{}
	for _, pattern := range patterns {
		if pattern.enumName != enumName {
			resolver.fail(fmt.Errorf("match case %s.%s does not match enum %s", pattern.enumName, pattern.variantName, enumName))
			return
		}
		if seen[pattern.variantName] {
			resolver.fail(fmt.Errorf("duplicate match case %s.%s", enumName, pattern.variantName))
			return
		}
		seen[pattern.variantName] = true
		variant := enumVariant(decl, pattern.variantName)
		if variant == nil {
			resolver.fail(fmt.Errorf("unknown variant %s.%s", enumName, pattern.variantName))
			return
		}
		if len(pattern.bindings) != len(variant.PayloadTypes) {
			resolver.fail(fmt.Errorf("match case %s.%s expects %d bindings", enumName, pattern.variantName, len(variant.PayloadTypes)))
			return
		}
	}
	for _, variant := range decl.Variants {
		if !seen[variant.Name] {
			resolver.fail(fmt.Errorf("non-exhaustive match for %s: missing %s", enumName, variant.Name))
			return
		}
	}
}

func (resolver *semanticResolver) fail(err error) {
	if resolver.err == nil {
		resolver.err = err
	}
}

func (resolver *semanticResolver) validateEnumExpression(expr Expr, meaning ExprMeaning) {
	if meaning != EnumConstructionExpr {
		return
	}
	var enumName, variantName string
	valueCount := 0
	switch value := expr.(type) {
	case CallExpr:
		selector, ok := value.Callee.(SelectorExpr)
		if !ok {
			return
		}
		receiver, ok := selector.Receiver.(IdentExpr)
		if !ok {
			return
		}
		enumName, variantName, valueCount = receiver.Name, selector.Name, len(value.Args)
	case SelectorExpr:
		receiver, ok := value.Receiver.(IdentExpr)
		if !ok {
			return
		}
		enumName, variantName = receiver.Name, value.Name
	default:
		return
	}
	if enumName == "Result" {
		if valueCount != 1 {
			resolver.fail(fmt.Errorf("Result.%s expects one value", variantName))
		}
		return
	}
	decl, ok := resolver.program.Enums[enumName]
	if !ok {
		return
	}
	variant := enumVariant(decl, variantName)
	if variant == nil {
		resolver.fail(fmt.Errorf("unknown variant %s.%s", enumName, variantName))
		return
	}
	if len(variant.PayloadTypes) != valueCount {
		resolver.fail(fmt.Errorf("variant %s.%s expects %d values", enumName, variantName, len(variant.PayloadTypes)))
	}
}

func collectionElement(typ Type) Type {
	if generic, ok := typ.(GenericType); ok && len(generic.Args) > 0 {
		return generic.Args[len(generic.Args)-1]
	}
	return UnknownType{}
}

func functionResult(typ Type) Type {
	if function, ok := typ.(FunctionType); ok {
		return function.Result
	}
	return UnknownType{}
}

func isResultType(typ Type) bool {
	result, ok := typ.(GenericType)
	return ok && result.Base.String() == "Result" && len(result.Args) == 2
}