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go_funcs.go
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go_funcs.go
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package template
import (
"fmt"
"reflect"
)
var errorType = reflect.TypeOf((*error)(nil)).Elem()
func index(item interface{}, indices ...interface{}) (interface{}, error) {
v := reflect.ValueOf(item)
if !v.IsValid() {
return nil, nil
}
if k := v.Kind(); (k == reflect.Array || k == reflect.Slice || k == reflect.Map) && v.Len() == 0 {
return nil, nil
}
for _, i := range indices {
index := reflect.ValueOf(i)
var isNil bool
if v, isNil = indirect(v); isNil {
return nil, fmt.Errorf("index of nil pointer")
}
switch v.Kind() {
case reflect.Array, reflect.Slice, reflect.String:
var x int64
switch index.Kind() {
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
x = index.Int()
case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
x = int64(index.Uint())
default:
return nil, fmt.Errorf("cannot index slice/array with type %s", index.Type())
}
if x < 0 || x >= int64(v.Len()) {
return nil, fmt.Errorf("index out of range: %d", x)
}
v = v.Index(int(x))
case reflect.Map:
if !index.IsValid() {
index = reflect.Zero(v.Type().Key())
}
if !index.Type().AssignableTo(v.Type().Key()) {
return nil, fmt.Errorf("%s is not index type for %s", index.Type(), v.Type())
}
if x := v.MapIndex(index); x.IsValid() {
v = x
} else {
v = reflect.Zero(v.Type().Elem())
}
default:
return nil, fmt.Errorf("can't index item of type %s", v.Type())
}
}
return v.Interface(), nil
}
func length(item interface{}) (int, error) {
v, isNil := indirect(reflect.ValueOf(item))
if isNil {
return 0, fmt.Errorf("len of nil pointer")
}
switch v.Kind() {
case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String:
return v.Len(), nil
}
return 0, fmt.Errorf("len of type %s", v.Type())
}
func call(fn interface{}, args ...interface{}) (interface{}, error) {
v := reflect.ValueOf(fn)
if !v.IsValid() {
return nil, fmt.Errorf("invalid func %v", v)
}
typ := v.Type()
if typ.Kind() != reflect.Func {
return nil, fmt.Errorf("non-function of type %s", typ)
}
if !goodFunc(typ) {
return nil, fmt.Errorf("function called with %d args; should be 1 or 2", typ.NumOut())
}
numIn := typ.NumIn()
var dddType reflect.Type
if typ.IsVariadic() {
if len(args) < numIn-1 {
return nil, fmt.Errorf("wrong number of args: got %d want at least %d", len(args), numIn-1)
}
dddType = typ.In(numIn - 1).Elem()
} else {
if len(args) != numIn {
return nil, fmt.Errorf("wrong number of args: got %d want %d", len(args), numIn)
}
}
argv := make([]reflect.Value, len(args))
for i, arg := range args {
value := reflect.ValueOf(arg)
// Compute the expected type. Clumsy because of variadics.
var argType reflect.Type
if !typ.IsVariadic() || i < numIn-1 {
argType = typ.In(i)
} else {
argType = dddType
}
if !value.IsValid() && canBeNil(argType) {
value = reflect.Zero(argType)
}
if !value.Type().AssignableTo(argType) {
return nil, fmt.Errorf("arg %d has type %s; should be %s", i, value.Type(), argType)
}
argv[i] = value
}
result := v.Call(argv)
if len(result) == 2 && !result[1].IsNil() {
return result[0].Interface(), result[1].Interface().(error)
}
return result[0].Interface(), nil
}
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
}
func goodFunc(typ reflect.Type) bool {
// We allow functions with 1 result or 2 results where the second is an error.
switch {
case typ.NumOut() == 1:
return true
case typ.NumOut() == 2 && typ.Out(1) == errorType:
return true
}
return false
}
func canBeNil(typ reflect.Type) bool {
switch typ.Kind() {
case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
return true
}
return false
}