Are there any other languages like Common Lisp or Emacs Lisp that have separate namespaces for variables and functions?
I'm generally interested and the Google results are flooded with non relevant info.
Thanks!
Well, in a sense php can be said to have that. If I understand your question correctly.
For example:
$hello="a variable";
function hello() {
return "a function";
}
echo $hello."\n";
echo hello()."\n";
Would print:
a variable
a function
So both the variable and the function is called hello.
Related
I'm not great at programming, but I was trying to fiddle around with a conky_rc file I liked that I found that seemed pretty straight-forward.
As the title states, I have now learned that the previous command of pre_exec has been long removed and superseded by Lua.
Unfortunately, I cannot seem to find anything directly related to this other than https://github.com/brndnmtthws/conky/issues/62. The thread https://github.com/brndnmtthws/conky/issues/146 references it, and its "solution" states: Basically, there is no replacement and you should use Lua or use a very large interval and execi.
I have found a few more threads that all include the question as to why this function was discontinued, but with no actual answers. So, to reiterate mine, I have absolutely no knowledge of Lua (I've heard of it before, and I've now added a few websites to look at tomorrow since I have spent most of the evening trying to figure out this Conky thing), and I'll probably just give up and do the execi option (my computer can handle it but, I just think it's so horribly inefficient).
Is there an appropriate Lua option? If so, would someone please direct me to either the manual or wiki for it, or explain it? Or is the "proper" Lua solution this?
#Vincent-C It's not working for your script is because the function
ain't getting call. from the quick few tests I did, it seem
lua_startup_hook need the function to be in another file that is
loaded using lua_load, not really sure how the hook function thingy
all works cause I rather just directly use the config as lua since it
is lua.
Basically just call the io.popen stuff and concat it into conky.text
conky.text = [[ a lot of stuff... ${color green} ]];
o = io.popen('fortune -s | cowsay', 'r') conky.text = conky.text ..
o:read('*a')
The comment by asl97 on the first page you cited appears to provide an answer, but a bit of explanation would probably help.
asl97 provides the following general purpose Lua function to use as a substitute for $pre_exec, preceded by a require statement to make io available for use by the function:
require 'io'
function pre_exec(cmd)
local handle = io.popen(cmd)
local output = handle:read("*a")
handle:close()
return output
end
Adding this block of code to your conky configuration file will make the function available for use therein. For testing, I added it above the conky.config = { ... } section.
Calling the Lua pre_exec function will return a string containing the output of the command passed to it. The conky.text section from [[ to ]] is also a string, so it can then be conactenated to the string returned by pre_exec using the .. operator, as shown in the usage section provided by asl97.
In my test, I did the following silly bit, which worked as expected, to display "Hello World!" and the output of the date function with spacing above and below each at the top of my conky display:
conky.text = pre_exec("echo; echo Hello World!; echo; date; echo")..[[
-- lots of boring conky stuff --
]]
More serious commands can, of course, be used with pre_exec, as shown by asl97.
One thing that asl97 didn't explain was how to provide how to concatenate so that the pre_exec output is in the middle of the conky display rather than just the beginning. I tested and found that you can do it like the following:
conky.text = [[
-- some conky stuff --
]]..pre_exec("your_important_command")..[[
-- more conky stuff --
]]
In Advanced Programming in the UNIX Environment, I find function like this
void
err_sys(const char *fmt, ...)
{
}
what does the formal parameter using ... mean? I think this format should be reported as an error.
The phenomenon goes by the name of 'variadic function'.
You may want to try your favorite search engine on that term. Wikipedia has a nice article on that subject too, so you might want to take look there as well.
I'm currently trying to learn Nim (it's going slowly - can't devote much time to it). On the other hand, in the interests of getting some working code, I'd like to prototype out sections of a Nim app I'm working on in ruby.
Since mruby allows embedding a ruby subset in a C app, and since nim allows compiling arbitrary C code into functions, it feels like this should be relatively straightforward. Has anybody done this?
I'm particularly looking for ways of using Nim's funky macro features to break out into inline ruby code. I'm going to try myself, but I figure someone is bound to have tried it and /or come up with more elegant solutions than I can in my current state of learning :)
https://github.com/micklat/NimBorg
This is a project with a somewhat similar goal. It targets python and lua at the moment, but using the same techniques to interface with Ruby shouldn't be too hard.
There are several features in Nim that help in interfacing with a foreign language in a fluent way:
1) Calling Ruby from Nim using Nim's dot operators
These are a bit like method_missing in Ruby.
You can define a type like RubyValue in Nim, which will have dot operators that will translate any expression like foo.bar or foo.bar(baz) to the appropriate Ruby method call. The arguments can be passed to a generic function like toRubyValue that can be overloaded for various Nim and C types to automatically convert them to the right Ruby type.
2) Calling Nim from Ruby
In most scripting languages, there is a way to register a foreign type, often described in a particular data structure that has to be populated once per exported type. You can use a bit of generic programming and Nim's .global. vars to automatically create and cache the required data structure for each type that was passed to Ruby through the dot operators. There will be a generic proc like getRubyTypeDesc(T: typedesc) that may rely on typeinfo, typetraits or some overloaded procs supplied by user, defining what has to be exported for the type.
Now, if you really want to rely on mruby (because you have experience with it for example), you can look into using the .emit. pragma to directly output pieces of mruby code. You can then ask the Nim compiler to generate only source code, which you will compile in a second step or you can just change the compiler executable, which Nim will call when compiling the project (this is explained in the same section linked above).
Here's what I've discovered so far.
Fetching the return value from an mruby execution is not as easy as I thought. That said, after much trial and error, this is the simplest way I've found to get some mruby code to execute:
const mrb_cc_flags = "-v -I/mruby_1.2.0_path/include/ -L/mruby_1.2.0_path/build/host/lib/"
const mrb_linker_flags = "-v"
const mrb_obj = "/mruby_1.2.0_path/build/host/lib/libmruby.a"
{. passC: mrb_cc_flags, passL: mrb_linker_flags, link: mrb_obj .}
{.emit: """
#include <mruby.h>
#include <mruby/string.h>
""".}
proc ruby_raw(str:cstring):cstring =
{.emit: """
mrb_state *mrb = mrb_open();
if (!mrb) { printf("ERROR: couldn't init mruby\n"); exit(0); }
mrb_load_string(mrb, `str`);
`result` = mrb_str_to_cstr(mrb, mrb_funcall(mrb, mrb_top_self(mrb), "test_func", 0));
mrb_close(mrb);
""".}
proc ruby*(str:string):string =
echo ruby_raw("def test_func\n" & str & "\nend")
"done"
let resp = ruby """
puts 'this was a puts from within ruby'
"this is the response"
"""
echo(resp)
I'm pretty sure that you should be able to omit some of the compiler flags at the start of the file in a well configured environment, e.g. by setting LD_LIBRARY_PATH correctly (not least because that would make the code more portable)
Some of the issues I've encountered so far:
I'm forced to use mrb_funcall because, for some reason, clang seems to think that the mrb_load_string function returns an int, despite all the c code I can find and the documentation and several people online saying otherwise:
error: initializing 'mrb_value' (aka 'struct mrb_value') with an expression of incompatible type 'int'
mrb_value mrb_out = mrb_load_string(mrb, str);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~
The mruby/string.h header is needed for mrb_str_to_cstr, otherwise you get a segfault. RSTRING_PTR seems to work fine also (which at least gives a sensible error without string.h), but if you write it as a one-liner as above, it will execute the function twice.
I'm going to keep going, write some slightly more idiomatic nim, but this has done what I needed for now.
To implement a domain specific language, within lua,
I want to add barewords to the language.
So that
print("foo") could be written as print(foo)
The way I have done this is by changing the metatable of the enviroment table _G.
mt = {__index = function(tbl,key) return key end}
setmetatable(_G, mt)
And that works, because retrieving the value of variable foo is done by _G.foo which is equivalent to _G["foo"]
Is this a good approach?
Are there hidden downsides?
Is there a better way?
Can I do it so that barewords only work inside a certain file?
(Perhaps by executing that file, from another luascript, using loadstring)
As soon as someone declares a local with the same name of your "keywords" it will shadow your "keyword/global var" and your mechanism will fail:
print(foo) -- does what you think
local foo = 2
print(foo) -- argh!
and note that you cannot prevent the definition of local variables in a script.
Edit: (answering to a comment).
Are you using a customized Lua engine? You cannot prevent entering local scope, because you are always in local scope. It is exactly the other way around: technically there is no global scope in Lua (with the same meaning as in C, for example). There is a global namespace (implemented as a table), instead. The mechanisms for accessing globals differs between Lua 5.1 (function environments) and Lua 5.2 (implicit _ENV prefixing), but the concept is almost the same.
In particular, when a Lua script is loaded, whether by the interpreter, or by load, loadstring, dofile, etc., it is interpreted as the body of an anonymous function (a closure), usually referred to as the "main chunk". Thus there is always a local scope. In standard Lua you cannot prevent the definition of a local variable when the parser encounters a local statement in the script being loaded.
I have seen a lot of C/C++ based solutions to this problem where we have to write a program that upon execution prints its own source.
some solutions --
http://www.cprogramming.com/challenges/solutions/self_print.html
Quine Page solution in many languages
There are many more solutions on the net, each different from the other. I wonder how do we approach to such a problem, what goes inside the mind of the one who solves it. Lend me some insights into this problem... While solutions in interpreted languages like perl, php, ruby, etc might be easy... i would like to know how does one go about designing it in compiled languages...
Aside from cheating¹ there is no difference between compiled and interpreted languages.
The generic approach to quines is quite easy. First, whatever the program looks like, at some point it has to print something:
print ...
However, what should it print? Itself. So it needs to print the "print" command:
print "print ..."
What should it print next? Well, in the mean time the program grew, so it needs to print the string starting with "print", too:
print "print \"print ...\""
Now the program grew again, so there's again more to print:
print "print \"print \\\"...\\\"\""
And so on.
With every added code there's more code to print.
This approach is getting nowhere,
but it reveals an interesting pattern:
The string "print \"" is repeated over and over again.
It would be nice to put the repeating part
into a variable:
a = "print \""
print a
However, the program just changed,
so we need to adjust a:
a = "a = ...\nprint a"
print a
When we now try to fill in the "...",
we run into the same problems as before.
Ultimately, we want to write something like this:
a = "a = " + (quoted contents of a) + "\nprint a"
print a
But that is not possible,
because even if we had such a function quoted() for quoting,
there's still the problem that we define a in terms of itself:
a = "a = " + quoted(a) + "\nprint a"
print a
So the only thing we can do is putting a place holder into a:
a = "a = #\nprint a"
print a
And that's the whole trick!
Anything else is now clear.
Simply replace the place holder
with the quoted contents of a:
a = "a = #\nprint a"
print a.replace("#", quoted(a))
Since we have changed the code,
we need to adjust the string:
a = "a = #\nprint a.replace(\"#\", quoted(a))"
print a.replace("#", quoted(a))
And that's it!
All quines in all languages work that way
(except the cheating ones).
Well, you should ensure that you replace only
the first occurence of the place holder.
And if you use a second place holder,
you can avoid needing to quote the string.
But those are minor issues
and easy to solve.
If fact, the realization of quoted() and replace()
are the only details in which the various quines really differ.
¹ by making the program read its source file
There are a couple of different strategies to writing quines. The obvious one is to just write code that opens the code and prints it out. But the more interesting ones involve language features that allow for self-embedding, like the %s-style printf feature in many languages. You have to figure out how to embed something so that it ends up resolving to the request to be embedded. I suspect, like palindromes, a lot of trial and error is involved.
The usual approach (when you can't cheat*) is to write something that encodes its source in a string constant, then prints out that constant twice: Once as a string literal, and once as code. That gets around the "every time I write a line of code, I have to write another to print it out!" problem.
'Cheating' includes:
- Using an interpreted language and simply loading the source and printing it
- 0-byte long files, which are valid in some languages, such as C.
For fun, I came up with one in Scheme, which I was pretty proud of for about 5 minutes until I discovered has been discovered before. Anyways, there's a slight modification to the "rules" of the game to better count for the duality of data and code in Lisp: instead of printing out the source of the program, it's an S-expression that returns itself:
((lambda (x) (list x `',x)) '(lambda (x) (list x `',x)))
The one on Wikipedia has the same concept, but with a slightly different (more verbose) mechanism for quoting. I like mine better though.
One idea to think about encoding and how to give something a double meaning so that it can be used to output something in a couple of forms. There is also the cavaet that this type of problem comes with restrictions to make it harder as without any rules other than the program output itself, the empty program is a solution.
How about actually reading and printing your source code? Its not difficult at all!! Heres one in php:
<?php
{
header("Content-Type: text/plain");
$f=fopen("5.php","r");
while(!feof($f))
{
echo fgetc($f);
}
fclose($f);
}
?>
In python, you can write:
s='c=chr(39);print"s="+c+s+c+";"+s';c=chr(39);print"s="+c+s+c+";"+s
inspired from this self printing pseudo-code:
Print the following line twice, the second time with quotes.
"Print the following line twice, the second time with quotes."
I've done a AS3 example for those interested in this
var program = "var program = #; function main(){trace(program.replace('#',
String.fromCharCode(34) + program + String.fromCharCode(34)))} main()";
function main(){
trace(program.replace('#', String.fromCharCode(34) + program + String.fromCharCode(34)))
}
main()
In bash it is really easy
touch test; chmod oug+x test; ./test
Empty file, Empty output
In ruby:
puts File.read(_ _ FILE _ _)