I'd like to create a procedure that exists only within a scope of a different proc in TCL.I.e. that just 1 proc can call it. Is it possible? According to this following link, no. :http://wiki.tcl.tk/463But maybe someone knows another way to do it.Thanks.
You can't limit procedures like that, but you can use a lambda term which is almost as good:
proc outside {a b c} {
# Lambda terms are two- or three-element lists.
set inside {{d e f} {
return [expr {$d + $e * $f}]
}}
set total 0
for {set i $a} {$i < $b} {incr i} {
# Lambdas have to be explicitly applied with [apply]
set total [apply $inside $total $c $i]
}
return $total
}
puts [outside 3 7 18]
First element of lambda: list of formal arguments (as for proc)
Second element of lambda: body (as for proc)
Third OPTIONAL element of lambda: context namespace, defaults to the global namespace (::)
The philosophy of Tcl is enabling rather than restricting. The programmer is trusted to do the right thing.
One can use namespaces, same-interpreter aliases, or OO to soft-restrict procedures. They can't be called by mistake, but they can still be accessed by deliberate action. Module procedures are often restricted this way: look at the code in e.g. struct::matrix to see how it can be done.
One can hard-restrict procedures by running the program in a sandbox, an interpreter that withholds a procedure completely or only permits it to be called under special circumstances, such as if called from a certain procedure.
(One can also write a procedure that simply checks who the caller is, but that's easy to spoof.)
Restricting access to a procedure by making its identifier local is a feature of lexical scoping, which Tcl doesn't use. The nearest corresponding mechanism is namespaces.
Related
Define the function in basic calculator bc as
define void f () { test=42; print "all done\n"; }
I'd have thought the value of test would be limited to the scope of the function f, but no, test equals 42 globally. Is there no way to limit the scope of variables in bc functions? I.e. is there are way to define local variables in bc?
You need to specify an AUTO_LIST in your function definition. From the bc manual,
`define' NAME `(' PARAMETERS `)' `{' NEWLINE
AUTO_LIST STATEMENT_LIST `}'
[...]
The AUTO_LIST is an optional list of variables that are for "local"
use. The syntax of the auto list (if present) is "`auto' NAME, ... ;".
(The semicolon is optional.) Each NAME is the name of an auto
variable. Arrays may be specified by using the same notation as used
in parameters. These variables have their values pushed onto a stack
at the start of the function. The variables are then initialized to
zero and used throughout the execution of the function. At function
exit, these variables are popped so that the original value (at the
time of the function call) of these variables are restored. The
parameters are really auto variables that are initialized to a value
provided in the function call. Auto variables are different than
traditional local variables because if function A calls function B, B
may access function A's auto variables by just using the same name,
unless function B has called them auto variables. Due to the fact that
auto variables and parameters are pushed onto a stack, `bc' supports
recursive functions.
So to keep the test variable "local" in your function, you'd use
define void f () { auto test; test=42; print "all done\n"; }
I am aware that a variable can be dynamically typed with the def keyword in Groovy. But I have also noticed that in some circumstances it can be left out, such as when defining method parameters, eg func(p1, p2) instead of func(def p1, def p2). The latter form is discouraged.
I have noticed that this is extendable to all code - anytime you want to define a variable and set its value, eg var = 2 the def keyword can be safely left out. It only appears to be required if not instantiating the variable on creation, ie. def var1 so that it can be instantiated as a NullObject.
Is this the only time def is useful? Can it be safely left out in all other declarations, for example, of classes and methods?
Short answer: you can't. There are some use cases where skipping the type declaration (or def keyword) works, but it is not a general rule. For instance, Groovy scripts allow you to use variables without specific type declaration, e.g.
x = 10
However, it works because groovy.lang.Script class implements getProperty and setProperty methods that get triggered when you access a missing property. In this case, such a variable is promoted to be a global binding, not a local variable. If you try to do the same on any other class that does not implement those methods, you will end up getting groovy.lang.MissingPropertyException.
Skipping types in a method declaration is supported, both in dynamically compiled and statically compiled Groovy. But is it useful? It depends. In most cases, it's much better to declare the type for a better readability and documentation purpose. I would not recommend doing it in the public API - the user of your API will see Object type, while you may expect some specific type. It shows that this may work if your intention is to receive any object, no matter what is its specific type. (E.g. a method like dump(obj) could work like that.)
And last but not least, there is a way to skip type declaration in any context. You can use a final keyword for that.
class Foo {
final id = 1
void bar(final name) {
final greet = "Hello, "
println greet + name + "!"
}
}
This way you can get a code that compiles with dynamic compilation, as well as with static compilation enabled. Of course, using final keyword prevents you from re-assigning the variable, but for the compiler, this is enough information to infer the proper type.
For more information, you can check a similar question that was asked on SO some time ago: Groovy: "def" keyword vs concrete type
in Groovy it plays an important role in Global and Local variable
if the variable name is same with and without def
def is considered local and without def its global
I have explained here in detail https://stackoverflow.com/a/45994227/2986279
So if someone use with and without it will make a difference and can change things.
The following (rightfully) doesn't work:
package main
import (
"os"
"time"
)
func main() {
os.Args[0] = "custom name"
println("sleeping")
time.Sleep(1000 * time.Second)
println("done")
}
Some languages provide this feature of setting process name as a built-in functionality (in Ruby, for instance, it is only a matter of assigning to $0) or as a third-party library (Python).
I'm looking for a solution that works, at least, on Linux.
There are multiple ways to accomplish this, and many of them only work in certain situations. I don't really recommend doing it, as (for one thing) it can result in your process showing up with different names in different situations. They require using syscall and/or unsafe, and so you're deliberately subverting the safety of the Go language. That said, however, your options seem to be:
Modify argv[0]
func SetProcessName(name string) error {
argv0str := (*reflect.StringHeader)(unsafe.Pointer(&os.Args[0]))
argv0 := (*[1 << 30]byte)(unsafe.Pointer(argv0str.Data))[:argv0str.Len]
n := copy(argv0, name)
if n < len(argv0) {
argv0[n] = 0
}
return nil
}
In Go, you don't have access to the actual argv array itself (without calling internal runtime functions), so you are limited to a new name no longer than the length of the current process name.
This seems to mostly work on both Darwin and Linux.
Call PR_SET_NAME
func SetProcessName(name string) error {
bytes := append([]byte(name), 0)
ptr := unsafe.Pointer(&bytes[0])
if _, _, errno := syscall.RawSyscall6(syscall.SYS_PRCTL, syscall.PR_SET_NAME, uintptr(ptr), 0, 0, 0, 0); errno != 0 {
return syscall.Errno(errno)
}
return nil
}
The new name can be at most 16 bytes.
This doesn't work on Darwin, and doesn't seem to do much on Linux, though it succeeds and PR_GET_NAME reports the correct name afterward. This may be something peculiar about my Linux VM, though.
To change a process name on Linux, you need to use the prctl system call combined with the PR_SET_NAME option.
At the moment, I don't think you can do this in Go code. You can, however, build a small C module to do this and then integrate it into your Go build.
I don't think that "process title" is a well defined term. Anyway, what has Ruby to do with Go? The documentation for os.Args doesn't mention any "process title", nor it says any magic will happen on assigning to a slice item. The later is actually a general property of Go. There's no magic getters/setters for struct fields, variables of array/slice items, so a simple assignment simply assigns and does nothing more and cannot do anything more.
In short, the lack of magic is the expected, correct behavior.
For fiddling with process properties other than the portably accessible ones via the 'os' package, one has to use package 'syscall' in a platform specific way. But then the build constraints (discussed here) can help to correctly handle stuff across platforms.
I'm having a look at the code at this page:
http://golang.org/pkg/net/http/
And there's one thing I don't understand - at some point, a new structure is created and initialized like this:
client := &http.Client{
CheckRedirect: redirectPolicyFunc,
}
Why use & when creating this structure?
I've also read this blog post and structs are initialized like this:
r := Rectangle{}
What is the difference between both and how should I know which one to use?
The difference is in the type of your variable.
client := &http.Client{
makes client of type *http.Client
while
client := http.Client{
builds a http.Client.
The top one is returning a pointer. It is a Go idiom instead of using new. The second one is just a value object. If you need a pointer use the top.
Check the effective go doc for more about this
http://golang.org/doc/effective_go.html#allocation_new
In object-oriented programming, in order for an object to have dynamic lifetime (i.e. not tied to the current function call), it needs to be dynamically allocated in a place other than the current stack frame, thus you manipulate the object through a pointer. This is such a common pattern that in many object-oriented languages, including Java, Python, Ruby, Objective-C, Smalltalk, JavaScript, and others, you can only deal with pointers to objects, never with an "object as a value" itself. (Some languages though, like C++, do allow you to have "objects as values"; it comes with the RAII idiom which adds some complexity.)
Go is not an object-oriented language, but its ability to define custom types and define methods that operates on that custom type, can be made to work very much like classes and methods. Returning a pointer to the type from the "constructor" function allows the "object" to have a dynamic lifetime.
When we use reference, we use a single item throughout the program runtime. Even if we assign that to a new variable or pass through a function. But when we use value, we make new copies of individual items.
( Reference is not right word according to golang convention. "Address of value" would be more appropriate here https://golang.org/ref/spec#Package_initialization )
An example will make it much clear I hope.
type Employee struct {
ID int
Name string
Address string
}
func main() {
andy := &Employee{}
andy.Name = "Andy"
brad := andy
brad.Name = "Brad"
fmt.Println(andy.Name)
}
The result of this code block would be:
Brad
As we made new variable from it but still referring to same data. But if we use value instead of reference and keep the rest of the code same.
// from
andy := &Employee{}
// to
andy := Employee{}
This time the result would be:
Andy
As this time they both are individual items and not referring to same data anymore.
I am making a simple debugger window in ActionScript for myself where I can add and remove variables I want to track. I was to be able to add variables to the list by just doing something like
DebuggerMonitor.trackVar(variable).
My question is, is there any way I can turn "variable" itself (the name, not the value) into a String to be added into a text field?
Depending on how "intelligent" your debugger should be, you could just pass the name along:
DebuggerMonitor.trackVar( variable, "variable" );
since obviously, when used in a context like this, the name should be known at the time you are writing the program.
You can also do some reflection magic to get instance variable names, but it won't work for temp variables (their names are dropped at compilation time):
public function getVariableName( instance:*, match:* ):String {
var typeDescription:XML = describeType( instance );
var variables:XMLList = typeDescription..variable;
var accessors:XMLList = typeDescription..accessor;
for each(var variable:XML in variables)
if(matchesXMLName( instance, variable, match ))
return variable.#name;
for each(var accessor:XML in accessors)
if(matchesXMLName( instance, accessor, match ))
return accessor.#name;
return "No name found.";
}
private function matchesXMLName( instance:*, xml:XML, match:* ):Boolean {
return match == instance[xml.#name.toString()];
}
var varName:String = getVariableName ( myObject, variable );
Using reflections like this will also be quite costly, if used often - you will have to think of a way to cache the type descriptions.
I recommend you check out the as3commons reflections package - there is a lot of useful functionality in there...
Short answer - No :(
You can access the type name but not individual instance names, as these are lost at run-time.
There is a confusion caused by the keyword 'var' because it is used to create several types of bindings.
Lexical bindings (the keyword 'var' was used inside a function).
Dynamic bindings (the keyword 'var' was used to declare a class' field).
Lexical bindings are interpreted by the compiler at compile time as addresses of the registers of the registers space occupied by the function. The names given to lexical bindings perish at this time and it is not possible to restore them at runtime - therefore you can't get the "name" of the variable.
Dynamic bindings are a kind of "public API" of the objects that declare them, they may be accessed from the code that was not compiled together with the code that created them, this is why, for the purpose of reflection the names of these bindings are stored in compiled code. However, ActionScript has no way of referencing LHS values, so you cannot, even if you know the name of the variable and the object declaring it, pass it to another function. But you can look it up in the debugger or by calling describeType on the object declaring the variable. Note that describeType will not show information on private variables even if you are calling it from the scope of the object in question.