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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package template
import (
"fmt"
"io"
"os"
"reflect"
"runtime"
"strings"
"template/parse"
)
// state represents the state of an execution. It's not part of the
// template so that multiple executions of the same template
// can execute in parallel.
type state struct {
tmpl *Template
wr io.Writer
line int // line number for errors
vars []variable // push-down stack of variable values.
}
// variable holds the dynamic value of a variable such as $, $x etc.
type variable struct {
name string
value reflect.Value
}
// push pushes a new variable on the stack.
func (s *state) push(name string, value reflect.Value) {
s.vars = append(s.vars, variable{name, value})
}
// mark returns the length of the variable stack.
func (s *state) mark() int {
return len(s.vars)
}
// pop pops the variable stack up to the mark.
func (s *state) pop(mark int) {
s.vars = s.vars[0:mark]
}
// setVar overwrites the top-nth variable on the stack. Used by range iterations.
func (s *state) setVar(n int, value reflect.Value) {
s.vars[len(s.vars)-n].value = value
}
// varValue returns the value of the named variable.
func (s *state) varValue(name string) reflect.Value {
for i := s.mark() - 1; i >= 0; i-- {
if s.vars[i].name == name {
return s.vars[i].value
}
}
s.errorf("undefined variable: %s", name)
return zero
}
var zero reflect.Value
// errorf formats the error and terminates processing.
func (s *state) errorf(format string, args ...interface{}) {
format = fmt.Sprintf("template: %s:%d: %s", s.tmpl.Name(), s.line, format)
panic(fmt.Errorf(format, args...))
}
// error terminates processing.
func (s *state) error(err os.Error) {
s.errorf("%s", err)
}
// errRecover is the handler that turns panics into returns from the top
// level of Parse.
func errRecover(errp *os.Error) {
e := recover()
if e != nil {
if _, ok := e.(runtime.Error); ok {
panic(e)
}
*errp = e.(os.Error)
}
}
// Execute applies a parsed template to the specified data object,
// writing the output to wr.
func (t *Template) Execute(wr io.Writer, data interface{}) (err os.Error) {
defer errRecover(&err)
value := reflect.ValueOf(data)
state := &state{
tmpl: t,
wr: wr,
line: 1,
vars: []variable{{"$", value}},
}
if t.Root == nil {
state.errorf("must be parsed before execution")
}
state.walk(value, t.Root)
return
}
// Walk functions step through the major pieces of the template structure,
// generating output as they go.
func (s *state) walk(dot reflect.Value, n parse.Node) {
switch n := n.(type) {
case *parse.ActionNode:
s.line = n.Line
// Do not pop variables so they persist until next end.
// Also, if the action declares variables, don't print the result.
val := s.evalPipeline(dot, n.Pipe)
if len(n.Pipe.Decl) == 0 {
s.printValue(n, val)
}
case *parse.IfNode:
s.line = n.Line
s.walkIfOrWith(parse.NodeIf, dot, n.Pipe, n.List, n.ElseList)
case *parse.ListNode:
for _, node := range n.Nodes {
s.walk(dot, node)
}
case *parse.RangeNode:
s.line = n.Line
s.walkRange(dot, n)
case *parse.TemplateNode:
s.line = n.Line
s.walkTemplate(dot, n)
case *parse.TextNode:
if _, err := s.wr.Write(n.Text); err != nil {
s.error(err)
}
case *parse.WithNode:
s.line = n.Line
s.walkIfOrWith(parse.NodeWith, dot, n.Pipe, n.List, n.ElseList)
default:
s.errorf("unknown node: %s", n)
}
}
// walkIfOrWith walks an 'if' or 'with' node. The two control structures
// are identical in behavior except that 'with' sets dot.
func (s *state) walkIfOrWith(typ parse.NodeType, dot reflect.Value, pipe *parse.PipeNode, list, elseList *parse.ListNode) {
defer s.pop(s.mark())
val := s.evalPipeline(dot, pipe)
truth, ok := isTrue(val)
if !ok {
s.errorf("if/with can't use %v", val)
}
if truth {
if typ == parse.NodeWith {
s.walk(val, list)
} else {
s.walk(dot, list)
}
} else if elseList != nil {
s.walk(dot, elseList)
}
}
// isTrue returns whether the value is 'true', in the sense of not the zero of its type,
// and whether the value has a meaningful truth value.
func isTrue(val reflect.Value) (truth, ok bool) {
if !val.IsValid() {
// Something like var x interface{}, never set. It's a form of nil.
return false, true
}
switch val.Kind() {
case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
truth = val.Len() > 0
case reflect.Bool:
truth = val.Bool()
case reflect.Complex64, reflect.Complex128:
truth = val.Complex() != 0
case reflect.Chan, reflect.Func, reflect.Ptr, reflect.Interface:
truth = !val.IsNil()
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
truth = val.Int() != 0
case reflect.Float32, reflect.Float64:
truth = val.Float() != 0
case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
truth = val.Uint() != 0
case reflect.Struct:
truth = true // Struct values are always true.
default:
return
}
return truth, true
}
func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
defer s.pop(s.mark())
val, _ := indirect(s.evalPipeline(dot, r.Pipe))
// mark top of stack before any variables in the body are pushed.
mark := s.mark()
oneIteration := func(index, elem reflect.Value) {
// Set top var (lexically the second if there are two) to the element.
if len(r.Pipe.Decl) > 0 {
s.setVar(1, elem)
}
// Set next var (lexically the first if there are two) to the index.
if len(r.Pipe.Decl) > 1 {
s.setVar(2, index)
}
s.walk(elem, r.List)
s.pop(mark)
}
switch val.Kind() {
case reflect.Array, reflect.Slice:
if val.Len() == 0 {
break
}
for i := 0; i < val.Len(); i++ {
oneIteration(reflect.ValueOf(i), val.Index(i))
}
return
case reflect.Map:
if val.Len() == 0 {
break
}
for _, key := range val.MapKeys() {
oneIteration(key, val.MapIndex(key))
}
return
case reflect.Chan:
if val.IsNil() {
break
}
i := 0
for ; ; i++ {
elem, ok := val.Recv()
if !ok {
break
}
oneIteration(reflect.ValueOf(i), elem)
}
if i == 0 {
break
}
return
case reflect.Invalid:
break // An invalid value is likely a nil map, etc. and acts like an empty map.
default:
s.errorf("range can't iterate over %v", val)
}
if r.ElseList != nil {
s.walk(dot, r.ElseList)
}
}
func (s *state) walkTemplate(dot reflect.Value, t *parse.TemplateNode) {
set := s.tmpl.set
if set == nil {
s.errorf("no set defined in which to invoke template named %q", t.Name)
}
tmpl := set.tmpl[t.Name]
if tmpl == nil {
s.errorf("template %q not in set", t.Name)
}
// Variables declared by the pipeline persist.
dot = s.evalPipeline(dot, t.Pipe)
newState := *s
newState.tmpl = tmpl
// No dynamic scoping: template invocations inherit no variables.
newState.vars = []variable{{"$", dot}}
newState.walk(dot, tmpl.Root)
}
// Eval functions evaluate pipelines, commands, and their elements and extract
// values from the data structure by examining fields, calling methods, and so on.
// The printing of those values happens only through walk functions.
// evalPipeline returns the value acquired by evaluating a pipeline. If the
// pipeline has a variable declaration, the variable will be pushed on the
// stack. Callers should therefore pop the stack after they are finished
// executing commands depending on the pipeline value.
func (s *state) evalPipeline(dot reflect.Value, pipe *parse.PipeNode) (value reflect.Value) {
if pipe == nil {
return
}
for _, cmd := range pipe.Cmds {
value = s.evalCommand(dot, cmd, value) // previous value is this one's final arg.
// If the object has type interface{}, dig down one level to the thing inside.
if value.Kind() == reflect.Interface && value.Type().NumMethod() == 0 {
value = reflect.ValueOf(value.Interface()) // lovely!
}
}
for _, variable := range pipe.Decl {
s.push(variable.Ident[0], value)
}
return value
}
func (s *state) notAFunction(args []parse.Node, final reflect.Value) {
if len(args) > 1 || final.IsValid() {
s.errorf("can't give argument to non-function %s", args[0])
}
}
func (s *state) evalCommand(dot reflect.Value, cmd *parse.CommandNode, final reflect.Value) reflect.Value {
firstWord := cmd.Args[0]
switch n := firstWord.(type) {
case *parse.FieldNode:
return s.evalFieldNode(dot, n, cmd.Args, final)
case *parse.IdentifierNode:
// Must be a function.
return s.evalFunction(dot, n.Ident, cmd.Args, final)
case *parse.VariableNode:
return s.evalVariableNode(dot, n, cmd.Args, final)
}
s.notAFunction(cmd.Args, final)
switch word := firstWord.(type) {
case *parse.BoolNode:
return reflect.ValueOf(word.True)
case *parse.DotNode:
return dot
case *parse.NumberNode:
return s.idealConstant(word)
case *parse.StringNode:
return reflect.ValueOf(word.Text)
}
s.errorf("can't evaluate command %q", firstWord)
panic("not reached")
}
// idealConstant is called to return the value of a number in a context where
// we don't know the type. In that case, the syntax of the number tells us
// its type, and we use Go rules to resolve. Note there is no such thing as
// a uint ideal constant in this situation - the value must be of int type.
func (s *state) idealConstant(constant *parse.NumberNode) reflect.Value {
// These are ideal constants but we don't know the type
// and we have no context. (If it was a method argument,
// we'd know what we need.) The syntax guides us to some extent.
switch {
case constant.IsComplex:
return reflect.ValueOf(constant.Complex128) // incontrovertible.
case constant.IsFloat && strings.IndexAny(constant.Text, ".eE") >= 0:
return reflect.ValueOf(constant.Float64)
case constant.IsInt:
n := int(constant.Int64)
if int64(n) != constant.Int64 {
s.errorf("%s overflows int", constant.Text)
}
return reflect.ValueOf(n)
case constant.IsUint:
s.errorf("%s overflows int", constant.Text)
}
return zero
}
func (s *state) evalFieldNode(dot reflect.Value, field *parse.FieldNode, args []parse.Node, final reflect.Value) reflect.Value {
return s.evalFieldChain(dot, dot, field.Ident, args, final)
}
func (s *state) evalVariableNode(dot reflect.Value, v *parse.VariableNode, args []parse.Node, final reflect.Value) reflect.Value {
// $x.Field has $x as the first ident, Field as the second. Eval the var, then the fields.
value := s.varValue(v.Ident[0])
if len(v.Ident) == 1 {
return value
}
return s.evalFieldChain(dot, value, v.Ident[1:], args, final)
}
// evalFieldChain evaluates .X.Y.Z possibly followed by arguments.
// dot is the environment in which to evaluate arguments, while
// receiver is the value being walked along the chain.
func (s *state) evalFieldChain(dot, receiver reflect.Value, ident []string, args []parse.Node, final reflect.Value) reflect.Value {
n := len(ident)
for i := 0; i < n-1; i++ {
receiver = s.evalField(dot, ident[i], nil, zero, receiver)
}
// Now if it's a method, it gets the arguments.
return s.evalField(dot, ident[n-1], args, final, receiver)
}
func (s *state) evalFunction(dot reflect.Value, name string, args []parse.Node, final reflect.Value) reflect.Value {
function, ok := findFunction(name, s.tmpl, s.tmpl.set)
if !ok {
s.errorf("%q is not a defined function", name)
}
return s.evalCall(dot, function, name, args, final)
}
// evalField evaluates an expression like (.Field) or (.Field arg1 arg2).
// The 'final' argument represents the return value from the preceding
// value of the pipeline, if any.
func (s *state) evalField(dot reflect.Value, fieldName string, args []parse.Node, final, receiver reflect.Value) reflect.Value {
if !receiver.IsValid() {
return zero
}
typ := receiver.Type()
receiver, _ = indirect(receiver)
// Unless it's an interface, need to get to a value of type *T to guarantee
// we see all methods of T and *T.
ptr := receiver
if ptr.Kind() != reflect.Interface && ptr.CanAddr() {
ptr = ptr.Addr()
}
if method, ok := methodByName(ptr, fieldName); ok {
return s.evalCall(dot, method, fieldName, args, final)
}
hasArgs := len(args) > 1 || final.IsValid()
// It's not a method; is it a field of a struct?
receiver, isNil := indirect(receiver)
if receiver.Kind() == reflect.Struct {
tField, ok := receiver.Type().FieldByName(fieldName)
if ok {
field := receiver.FieldByIndex(tField.Index)
if hasArgs {
s.errorf("%s is not a method but has arguments", fieldName)
}
if tField.PkgPath == "" { // field is exported
return field
}
}
}
// If it's a map, attempt to use the field name as a key.
if receiver.Kind() == reflect.Map {
nameVal := reflect.ValueOf(fieldName)
if nameVal.Type().AssignableTo(receiver.Type().Key()) {
if hasArgs {
s.errorf("%s is not a method but has arguments", fieldName)
}
return receiver.MapIndex(nameVal)
}
}
if isNil {
s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
}
s.errorf("can't evaluate field %s in type %s", fieldName, typ)
panic("not reached")
}
// TODO: delete when reflect's own MethodByName is released.
func methodByName(receiver reflect.Value, name string) (reflect.Value, bool) {
typ := receiver.Type()
for i := 0; i < typ.NumMethod(); i++ {
if typ.Method(i).Name == name {
return receiver.Method(i), true // This value includes the receiver.
}
}
return zero, false
}
var (
osErrorType = reflect.TypeOf((*os.Error)(nil)).Elem()
fmtStringerType = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
)
// evalCall executes a function or method call. If it's a method, fun already has the receiver bound, so
// it looks just like a function call. The arg list, if non-nil, includes (in the manner of the shell), arg[0]
// as the function itself.
func (s *state) evalCall(dot, fun reflect.Value, name string, args []parse.Node, final reflect.Value) reflect.Value {
if args != nil {
args = args[1:] // Zeroth arg is function name/node; not passed to function.
}
typ := fun.Type()
numIn := len(args)
if final.IsValid() {
numIn++
}
numFixed := len(args)
if typ.IsVariadic() {
numFixed = typ.NumIn() - 1 // last arg is the variadic one.
if numIn < numFixed {
s.errorf("wrong number of args for %s: want at least %d got %d", name, typ.NumIn()-1, len(args))
}
} else if numIn < typ.NumIn()-1 || !typ.IsVariadic() && numIn != typ.NumIn() {
s.errorf("wrong number of args for %s: want %d got %d", name, typ.NumIn(), len(args))
}
if !goodFunc(typ) {
s.errorf("can't handle multiple results from method/function %q", name)
}
// Build the arg list.
argv := make([]reflect.Value, numIn)
// Args must be evaluated. Fixed args first.
i := 0
for ; i < numFixed; i++ {
argv[i] = s.evalArg(dot, typ.In(i), args[i])
}
// Now the ... args.
if typ.IsVariadic() {
argType := typ.In(typ.NumIn() - 1).Elem() // Argument is a slice.
for ; i < len(args); i++ {
argv[i] = s.evalArg(dot, argType, args[i])
}
}
// Add final value if necessary.
if final.IsValid() {
argv[i] = final
}
result := fun.Call(argv)
// If we have an os.Error that is not nil, stop execution and return that error to the caller.
if len(result) == 2 && !result[1].IsNil() {
s.errorf("error calling %s: %s", name, result[1].Interface().(os.Error))
}
return result[0]
}
// validateType guarantees that the value is valid and assignable to the type.
func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Value {
if !value.IsValid() {
s.errorf("invalid value; expected %s", typ)
}
if !value.Type().AssignableTo(typ) {
// Does one dereference or indirection work? We could do more, as we
// do with method receivers, but that gets messy and method receivers
// are much more constrained, so it makes more sense there than here.
// Besides, one is almost always all you need.
switch {
case value.Kind() == reflect.Ptr && value.Type().Elem().AssignableTo(typ):
value = value.Elem()
case reflect.PtrTo(value.Type()).AssignableTo(typ) && value.CanAddr():
value = value.Addr()
default:
s.errorf("wrong type for value; expected %s; got %s", typ, value.Type())
}
}
return value
}
func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) reflect.Value {
switch arg := n.(type) {
case *parse.DotNode:
return s.validateType(dot, typ)
case *parse.FieldNode:
return s.validateType(s.evalFieldNode(dot, arg, []parse.Node{n}, zero), typ)
case *parse.VariableNode:
return s.validateType(s.evalVariableNode(dot, arg, nil, zero), typ)
}
switch typ.Kind() {
case reflect.Bool:
return s.evalBool(typ, n)
case reflect.Complex64, reflect.Complex128:
return s.evalComplex(typ, n)
case reflect.Float32, reflect.Float64:
return s.evalFloat(typ, n)
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
return s.evalInteger(typ, n)
case reflect.Interface:
if typ.NumMethod() == 0 {
return s.evalEmptyInterface(dot, n)
}
case reflect.String:
return s.evalString(typ, n)
case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
return s.evalUnsignedInteger(typ, n)
}
s.errorf("can't handle %s for arg of type %s", n, typ)
panic("not reached")
}
func (s *state) evalBool(typ reflect.Type, n parse.Node) reflect.Value {
if n, ok := n.(*parse.BoolNode); ok {
value := reflect.New(typ).Elem()
value.SetBool(n.True)
return value
}
s.errorf("expected bool; found %s", n)
panic("not reached")
}
func (s *state) evalString(typ reflect.Type, n parse.Node) reflect.Value {
if n, ok := n.(*parse.StringNode); ok {
value := reflect.New(typ).Elem()
value.SetString(n.Text)
return value
}
s.errorf("expected string; found %s", n)
panic("not reached")
}
func (s *state) evalInteger(typ reflect.Type, n parse.Node) reflect.Value {
if n, ok := n.(*parse.NumberNode); ok && n.IsInt {
value := reflect.New(typ).Elem()
value.SetInt(n.Int64)
return value
}
s.errorf("expected integer; found %s", n)
panic("not reached")
}
func (s *state) evalUnsignedInteger(typ reflect.Type, n parse.Node) reflect.Value {
if n, ok := n.(*parse.NumberNode); ok && n.IsUint {
value := reflect.New(typ).Elem()
value.SetUint(n.Uint64)
return value
}
s.errorf("expected unsigned integer; found %s", n)
panic("not reached")
}
func (s *state) evalFloat(typ reflect.Type, n parse.Node) reflect.Value {
if n, ok := n.(*parse.NumberNode); ok && n.IsFloat {
value := reflect.New(typ).Elem()
value.SetFloat(n.Float64)
return value
}
s.errorf("expected float; found %s", n)
panic("not reached")
}
func (s *state) evalComplex(typ reflect.Type, n parse.Node) reflect.Value {
if n, ok := n.(*parse.NumberNode); ok && n.IsComplex {
value := reflect.New(typ).Elem()
value.SetComplex(n.Complex128)
return value
}
s.errorf("expected complex; found %s", n)
panic("not reached")
}
func (s *state) evalEmptyInterface(dot reflect.Value, n parse.Node) reflect.Value {
switch n := n.(type) {
case *parse.BoolNode:
return reflect.ValueOf(n.True)
case *parse.DotNode:
return dot
case *parse.FieldNode:
return s.evalFieldNode(dot, n, nil, zero)
case *parse.IdentifierNode:
return s.evalFunction(dot, n.Ident, nil, zero)
case *parse.NumberNode:
return s.idealConstant(n)
case *parse.StringNode:
return reflect.ValueOf(n.Text)
case *parse.VariableNode:
return s.evalVariableNode(dot, n, nil, zero)
}
s.errorf("can't handle assignment of %s to empty interface argument", n)
panic("not reached")
}
// indirect returns the item at the end of indirection, and a bool to indicate if it's nil.
// We indirect through pointers and empty interfaces (only) because
// non-empty interfaces have methods we might need.
func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
for ; v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface; v = v.Elem() {
if v.IsNil() {
return v, true
}
if v.Kind() == reflect.Interface && v.NumMethod() > 0 {
break
}
}
return v, false
}
// printValue writes the textual representation of the value to the output of
// the template.
func (s *state) printValue(n parse.Node, v reflect.Value) {
if v.Kind() == reflect.Ptr {
v, _ = indirect(v) // fmt.Fprint handles nil.
}
if !v.IsValid() {
fmt.Fprint(s.wr, "<no value>")
return
}
if !v.Type().Implements(fmtStringerType) {
if v.CanAddr() && reflect.PtrTo(v.Type()).Implements(fmtStringerType) {
v = v.Addr()
} else {
switch v.Kind() {
case reflect.Chan, reflect.Func:
s.errorf("can't print %s of type %s", n, v.Type())
}
}
}
fmt.Fprint(s.wr, v.Interface())
}