In vanilla Lua 5.2, I have a module that contains:
Two local functions, A and B: B will always call A, A will sometimes call B and sometimes will call the functions stored in C;
C: a table (local). It contains tables which contains tables which can contain tables... which will at the end contain functions. These functions may call either A or B;
Then there's the return function, D, which will be returned when my module is loaded using require. It will call A.
At the end, it looks quite like this:
--don't pay attention to what the functions do:
--I am only writing them down to explain how they interact with each other
local A, B, C
C = {
...
{
function(a)
B(a)
end
}
...
}
A = function(a)
...
if (...) then
B(a)
end
...
if (...) then
C[...]...[...](a)
end
...
end
B = function(a)
A(a)
end
return function(s) -- we called this one D
A(s)
end
Now, my problem is this: the declaration of C uses its own local variables, metatables and all that stuff, to the point I enclosed its declaration in a do ... end block.
It also is - with all those tables inside tables and newlines for each curly brace and indentation and so on - quite long. So I wanted to put it in its own module, but then it couldn't access B.
So, my question is: is there a way to pass B and maybe even A to the file where C is declared while loading it? I mean something like this, if it were possible:
--in the original module
local A, B, C
C = require("c", A, B)
...
And then, in c.lua:
local A, B = select(1, ...), select(2, ...)
C = {
...
{
function(a)
B(a)
end
}
...
}
I really have no idea on how to do it.
Is there a way to pass variables from the requiring file to the required file which doesn't involve the variables being inserted in the global namespace?
main module:
local A, B, C
A = function(a)
...
if (...) then
B(a)
end
...
if (...) then
C[...]...[...](a)
end
...
end
B = function(a)
A(a)
end
C = require("c")(A, B)
return function(s) -- we called this one D
A(s)
end
c.lua:
local function C_constructor(A, B)
local C =
{
...
{
function(a)
B(a)
end
}
...
}
return C
end
return C_constructor
Is there a way to pass variables from the requiring file to the required file which doesn't involve the variables being inserted in the global namespace?
Not with the default require function, but that shouldn't stop you from writing your own require function that does this. Obviously this will make the solution specific to your application, so those required files will not function correctly when a standard Lua interpreter (with its require function) is used.
Related
I'm trying to understand why can't a variable that is assigned from a class field change in a function. Consider this example:
class A {
private var a: String? = "a"
fun hello() {
val b = a
if (b != null) {
// The compiler assumes that 'b' is always not null inside this branch
b.length
} else {
// is null
}
}
}
Variable b is going to always be the same throughout the function (later is implicit non null in the if branch). So even if through concurrency I change the field a, the variable b does not change?
How is this possible? does Kotlin make a copy of the variable and assigns it to b? I thought assignments worked with references and such.
This can happen, as far as I'm concerned, with any type (String, Int, custom class, etc)
Strings are immutable, there's no way you can change them after you instantiated them. Thus, if another thread modifies the value of a after b != null was evaluated, it will not affect b's value.
Actually it's not even the real answer because here we're just talking about nullity. b will point to whatever object was pointed by a at the moment of the assignment, but then nothing can change what b is pointing to because b is a val.
I was just mentioning immutability because the content of b could still change through mutation if the type was mutable, but the reference itself will never change.
For more clarity, let's make a scenario:
[thread 1] a = "hello" <state: a => h: String("hello")>
[thread 1] b = a <state: a => h, b => h>
[thread 2] a = "world" <state: a => w: String("world"), b => h>
In short, b points to the same object as a at the time of assignment, but it doesn't point to a itself, so when a changes, b doesn't care.
How can I implement functions that take an optional flag in Perl6? For example, say that I want to invoke my function like this:
format 'a b c';
or like this:
format :pretty 'a b c';
How can I do this? Thanks
It's just a named argument, in case of flags a boolean one. This all works out because :pretty is syntactic sugar for the pair :pretty(True) aka pretty => True.
You can either use the boolean value
sub format($arg, Bool :$pretty = False) {
if $pretty { ... }
else { ... }
}
or use its presence for multi-dispatch
multi format($arg) { ... }
multi format($arg, Bool :$pretty!) { ... }
In the first example, we provided a default value (which isn't really necessary as the undefined value boolifies to False, but it's arguably the 'right thing to do' semantically), in the second one we made it a required parameter by adding the !.
Also note that named arguments still have to be separated by commas, ie you'd use it as
format :pretty, 'a b c';
If you really want that odd syntax, you can use an operator and some subsignature magic. The Bool method is optional and the class Pretty can be empty. It's just there to provide something for the MMD-dispatcher to hold onto.
class Pretty { method Bool {True} };
sub prefix:<:pretty>(|c){ Pretty.new, c };
multi sub format((Pretty:D $pretty, |a)){ dd $pretty.Bool, a };
multi sub format(|c){ dd c };
format :pretty 'a b c'; format 'a b c';
# OUTPUT«Bool::True\(\("a b c"))\("a b c")»
I would like to overload only one type of subsref calls (the '()' type) for a particular class and leave any other calls to Matlab's built in subsref -- specifically, I want Matlab to handle property/method access via the '.' type. But, it seems like Matlab's 'builtin' function doesn't work when subsref is overloaded in a class.
Consider this class:
classdef TestBuiltIn
properties
testprop = 'This is the built in method';
end
methods
function v = subsref(this, s)
disp('This is the overloaded method');
end
end
end
To use the overloaded subsref method, I do this:
t = TestBuiltIn;
t.testprop
>> This is the overloaded method
That's as expected. But now I want to call Matlab's built in subsref method. To make sure I'm doing things right, first I try out a similar call on a struct:
x.testprop = 'Accessed correctly';
s.type = '.';
s.subs = 'testprop';
builtin('subsref', x, s)
>> Accessed correctly
That's as expected as well. But, when I try the same method on TestBuiltIn:
builtin('subsref', t, s)
>> This is the overloaded method
...Matlab calls the overloaded method rather than the built in method. Why does Matlab call the overloaded method when I requested that it call the builtin method?
UPDATE:
In response to #Andrew Janke's answer, that solution almost works but doesn't quite. Consider this class:
classdef TestIndexing
properties
prop1
child
end
methods
function this = TestIndexing(n)
if nargin==0
n = 1;
end
this.prop1 = n;
if n<2
this.child = TestIndexing(n+1);
else
this.child = ['child on instance ' num2str(n)];
end
end
function v = subsref(this, s)
if strcmp(s(1).type, '()')
v = 'overloaded method';
else
v = builtin('subsref', this, s);
end
end
end
end
All of this works:
t = TestIndexing;
t(1)
>> overloaded method
t.prop1
>> 1
t.child
>> [TestIndexing instance]
t.child.prop1
>> 2
But this doesn't work; it uses the built in subsref for the child rather than the overloaded subsref:
t.child(1)
>> [TestIndexing instance]
Note that the above behavior is inconsistent with both of these behaviors (which are as expected):
tc = t.child;
tc(1)
>> overloaded method
x.child = t.child;
x.child(1)
>> overloaded method
It's possible, IIRC. To change () but not {} and '.', write your subsref method to pass those other cases along to the builtin subsref from within your overloaded subsref, instead of trying to explicitly call the builtin from outside.
function B = subsref(A, S)
% Handle the first indexing on your obj itself
switch S(1).type
case '()'
B = % ... do your custom "()" behavior ...
otherwise
% Enable normal "." and "{}" behavior
B = builtin('subsref', A, S(1))
end
end
% Handle "chaining" (not sure this part is fully correct; it is tricky)
orig_B = B; % hold on to a copy for debugging purposes
if numel(S) > 1
B = subsref(B, S(2:end)); % regular call, not "builtin", to support overrides
end
end
(And if that builtin call doesn't work, you can just put in cases that use . and {} directly, because the subsref overload is ignored inside the class definition.)
To make it fully functional, you may need to change B to a varargout, and add chaining behavior in to the "()" case.
To expand on the explanation given on the Mathworks board, builtin only works from within an overloaded method to access the pre-existing (built in) method.
Overloading a method in Matlab effectively shadows the built in implementation from everything except the method doing the shadowing, and the method doing the shadowing must use builtin to access the built in implementation instead of recursing into itself.
In general. You should use builtin(m,s) inside the function that is been overloaded. This is specified clearly in MATLAB documentation.
http://www.mathworks.com/help/matlab/ref/builtin.html
builtin(function,x1,...,xn) executes the built-in function with the
input arguments x1 through xn. Use builtin to execute the original
built-in from within a method that overloads the function. To work
properly, you must never overload builtin.
Consider this code:
classdef TestBuiltIn
properties
testprop = 'This is the built in method';
testprop2 = 'This is the derived subsref ';
end
methods
function v = subsref(m, s)
disp('enter subsref no matter how!');
v = builtin('subsref',m, s);
end
end
end
and test command
clear;
t = TestBuiltIn;
builtin('subsref', t, s)
s.type = '.';
s.subs = 'testprop';
s2 = s;
s2.subs = 'testprop2';
>> builtin('subsref', t, s1)
enter subsref no matter how!
ans =
This is the derived subsref
>> builtin('subsref', t, s)
enter subsref no matter how!
ans =
This is the built in method
In your updated version of this problem, when you call t.child(1), the subsref function will receive argument s with s(1).type='.', s(1).subs='child' and s(2).type='()', s(2).subs='1'. The evaluation of this expression is not in a step-by-step manner, as Andrew Janke mentioned in his answer. As a result, when overriding subsref, you should handle this operation chain, by first processing the '.' operator. Here is a incomplete example for your case,
function v = subsref(this, s)
switch s(1).type
case '.'
member = s(1).subs;
if ismethod(this, member)
% invoke builtin function to access method member
% there is issue about the number of output arguments
v = builtin('subsref',this,s);
elseif isprop(this, member) % property
if length(s) == 1
% invoke builtin function to access method member
v = builtin('subsref', this, s);
elseif length(s) == 2 && strcmp(s(2).type,'()')
% this is where you evaluate 'tc.child(1)'
else
% add other cases when you need, otherwise calling builtin
end
else
% handling error.
end
case '()'
% this is where you evaluate 't(1)'
% you may need to handle something like 't(1).prop1', like the '.' case
otherwise
% by default, calling the builtin.
end
end
You can also find a detailed code sample and instruction at Code Patterns for subsref and subsasgn Methods.
One more thing you may need to know, is that method members in this class will also be invoked through subsref with '.' operation. Look at this subject subsref on classes: how to dispatch methods?, you will find that the builtin function has no return value (since the invoked method has no return value). However, the return value of builtin is assigned to v(even though v is replaced by varargout), which is an obvious error. The author also gives a temporary solution by using try ... catch to resolve this error.
Sometimes, I have a control structure (if, for, ...), and depending on a condition I either want to use the control structure, or only execute the body. As a simple example, I can do the following in C, but it's pretty ugly:
#ifdef APPLY_FILTER
if (filter()) {
#endif
// do something
#ifdef APPLY_FILTER
}
#endif
Also it doesn't work if I only know apply_filter at runtime. Of course, in this case I can just change the code to:
if (apply_filter && filter())
but that doesn't work in the general case of arbitrary control structures. (I don't have a nice example at hand, but recently I had some code that would have benefited a lot from a feature like this.)
Is there any langugage where I can apply conditions to control structures, i.e. have higher-order conditionals? In pseudocode, the above example would be:
<if apply_filter>
if (filter()) {
// ...
}
Or a more complicated example, if a varable is set wrap code in a function and start it as a thread:
<if (run_on_thread)>
void thread() {
<endif>
for (int i = 0; i < 10; i++) {
printf("%d\n", i);
sleep(1);
}
<if (run_on_thread)>
}
start_thread(&thread);
<endif>
(Actually, in this example I could imagine it would even be useful to give the meta condition a name, to ensure that the top and bottom s are in sync.)
I could imagine something like this is a feature in LISP, right?
Any language with first-class functions can pull this off. In fact, your use of "higher-order" is telling; the necessary abstraction will indeed be a higher-order function. The idea is to write a function applyIf which takes a boolean (enabled/disabled), a control-flow operator (really, just a function), and a block of code (any value in the domain of the function); then, if the boolean is true, the operator/function is applied to the block/value, and otherwise the block/value is just run/returned. This will be a lot clearer in code.
In Haskell, for instance, this pattern would be, without an explicit applyIf, written as:
example1 = (if applyFilter then when someFilter else id) body
example2 = (if runOnThread then (void . forkIO) else id) . forM_ [1..10] $ \i ->
print i >> threadDelay 1000000 -- threadDelay takes microseconds
Here, id is just the identity function \x -> x; it always returns its argument. Thus, (if cond then f else id) x is the same as f x if cond == True, and is the same as id x otherwise; and of course, id x is the same as x.
Then you could factor this pattern out into our applyIf combinator:
applyIf :: Bool -> (a -> a) -> a -> a
applyIf True f x = f x
applyIf False _ x = x
-- Or, how I'd probably actually write it:
-- applyIf True = id
-- applyIf False = flip const
-- Note that `flip f a b = f b a` and `const a _ = a`, so
-- `flip const = \_ a -> a` returns its second argument.
example1' = applyIf applyFilter (when someFilter) body
example2' = applyIf runOnThread (void . forkIO) . forM_ [1..10] $ \i ->
print i >> threadDelay 1000000
And then, of course, if some particular use of applyIf was a common pattern in your application, you could abstract over it:
-- Runs its argument on a separate thread if the application is configured to
-- run on more than one thread.
possiblyThreaded action = do
multithreaded <- (> 1) . numberOfThreads <$> getConfig
applyIf multithreaded (void . forkIO) action
example2'' = possiblyThreaded . forM_ [1..10] $ \i ->
print i >> threadDelay 1000000
As mentioned above, Haskell is certainly not alone in being able to express this idea. For instance, here's a translation into Ruby, with the caveat that my Ruby is very rusty, so this is likely to be unidiomatic. (I welcome suggestions on how to improve it.)
def apply_if(use_function, f, &block)
use_function ? f.call(&block) : yield
end
def example1a
do_when = lambda { |&block| if some_filter then block.call() end }
apply_if(apply_filter, do_when) { puts "Hello, world!" }
end
def example2a
apply_if(run_on_thread, Thread.method(:new)) do
(1..10).each { |i| puts i; sleep 1 }
end
end
def possibly_threaded(&block)
apply_if(app_config.number_of_threads > 1, Thread.method(:new), &block)
end
def example2b
possibly_threaded do
(1..10).each { |i| puts i; sleep 1 }
end
end
The point is the same—we wrap up the maybe-do-this-thing logic in its own function, and then apply that to the relevant block of code.
Note that this function is actually more general than just working on code blocks (as the Haskell type signature expresses); you can also, for instance, write abs n = applyIf (n < 0) negate n to implement the absolute value function. The key is to realize that code blocks themselves can be abstracted over, so things like if statements and for loops can just be functions. And we already know how to compose functions!
Also, all of the code above compiles and/or runs, but you'll need some imports and definitions. For the Haskell examples, you'll need the impots
import Control.Applicative -- for (<$>)
import Control.Monad -- for when, void, and forM_
import Control.Concurrent -- for forkIO and threadDelay
along with some bogus definitions of applyFilter, someFilter, body, runOnThread, numberOfThreads, and getConfig:
applyFilter = False
someFilter = False
body = putStrLn "Hello, world!"
runOnThread = True
getConfig = return 4 :: IO Int
numberOfThreads = id
For the Ruby examples, you'll need no imports and the following analogous bogus definitions:
def apply_filter; false; end
def some_filter; false; end
def run_on_thread; true; end
class AppConfig
attr_accessor :number_of_threads
def initialize(n)
#number_of_threads = n
end
end
def app_config; AppConfig.new(4); end
Common Lisp does not let you redefine if. You can, however, invent your own control structure as a macro in Lisp and use that instead.
Is it possible to define a MATLAB class such that the objects from this class can be called like any other function?
IOW, I'm asking whether one can write in MATLAB the equivalent of something like the following Python class:
# define the class FxnClass
class FxnClass(object):
def __init__(self, template):
self.template = template
def __call__(self, x, y, z):
print self.template % locals()
# create an instance of FxnClass
f = FxnClass('x is %(x)r; y is %(y)r; z is %(z)r')
# call the instance of FxnClass
f(3, 'two', False)
...
[OUTPUT]
x is 3; y is 'two'; z is False
Thanks!
I do not know, whether MATLAB directly supports what you want, but MATLAB does support first-class functions; closures might therefore provide a useable substitute, for instance:
function f = count_call(msg)
calls = 0;
function current_count()
disp(strcat(msg, num2str(calls)));
calls = calls + 1;
end
f = #current_count;
end
In this case, current_count closes over calls (and msg). That way you can express functions that depend on some internal state. You would use it this way:
g = count_call('number of calls: ') % returns a new function ("__init__")
g() % "__call__"
I will be interested to see if this is possible without simply creating a java method in Matlab. I know you can do the following
classdef ExampleObject
properties
test;
end
methods
function exampleObject = ExampleObject(inputTest)
exampleObject.test=inputTest;
end
function f(exampleObject,funcInput)
disp(funcInput+exampleObject.test);
end
end
end
>> e=ExampleObject(5);
>> f(e,10)
15
But as far as my knowledge goes, if you tried to override the call function you'd run into a conflict with Matlab's parenthetical subscript reference subsref. You can find a reference here showing how to overwrite that, and you might be able to get it to do what you want...but it doesn't seem like good form to do so. Not sure how Matlab would handle a call to an object (as opposed to a function) without it getting confused with this.
One way is to override the feval function for your class:
classdef FxnClass < handle
properties
template
end
methods
function obj = FxnClass(t)
obj.template = t;
end
function out = feval(obj, varargin)
out = sprintf(obj.template, varargin{:});
end
end
end
This would be used as:
>> f = FxnClass('x = %f, y = %s, z = %d');
>> feval(f, 3,'two',false)
ans =
x = 3.000000, y = two, z = 0
Now if you want to provide additional syntactic sugar, you could redefine the subsref function for your class as #Salain suggested. Add the following to the previous class definition:
classdef FxnClass < handle
...
methods
function out = subsref(obj, S)
switch S(1).type
case '.'
% call builtin subsref, so we dont break the dot notation
out = builtin('subsref', obj, S);
case '()'
out = feval(obj, S.subs{:});
case '{}'
error('Not a supported subscripted reference');
end
end
end
end
Now you could simply write:
>> f = FxnClass('x = %f, y = %s, z = %d');
>> f(3,'two',false)
ans =
x = 3.000000, y = two, z = 0
Personally I don't particularly like overriding the subsref or subsasgn functions. They are used for too many cases, and its sometimes hard to get them write. For example all the following will eventually call the subsref method with different input:
f(..)
f.template
f.template(..)
f(..).template
f(..).template(..)
There is also the case of the end keyword which could appear in indexing, so you might have to also override it as well in some cases. Not to mention that objects can also be concatenated into arrays, which makes things even more complicated:
>> ff = [f,f];
>> ff(1) % not what you expect!
That said, I think #Frank's suggestion to use nested functions with closures is more elegant in this case:
function f = FxnClass(t)
f = #call;
function out = call(varargin)
out = sprintf(t, varargin{:});
end
end
which is called as before:
>> f = FxnClass('x = %f, y = %s, z = %d');
>> f(3, 'two', false)
If you mean that you want a class to hold a method which you use like a normal function (eg. defined in an m-file), then yes, Matlab does support static methods.
A static method runs independently of any instances of that class, in fact, you don't even need to instantiate a class to use its static methods.
Matlab does not support static fields, however, so you would have to instantiate such a class first, and then set its fields before using the functions (which presumably make use of these fields, since you are asking this question).
Given the limitation with static members, you might be better off with closures, as described by Frank.