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QUESTION
Suppose I have three functions foo
, bar
, baz
, all of which return nullable types.
ANSWER
Answered 2022-Feb-21 at 16:50val result = listOf(foo(), bar(), baz())
.reduce { acc, i ->
when {
acc == null || i == null -> null
else -> acc + i
}
}
QUESTION
Given the following text:
...ANSWER
Answered 2022-Jan-18 at 21:53You can use
QUESTION
How can I use use something like variant
(which is in C++) in Julia?
In C++, I can do something like this
...ANSWER
Answered 2022-Jan-05 at 02:02You can use union types to do the same.
QUESTION
Ownership Tree
Hi,
I was trying to understand ownership concepts in Rust and came across this image (attached in this post) in "Programming Rust" book.
In particular am concerned about the "Borrowing a shared reference" part. In the book, the author says
Values borrowed by shared references are read-only. Across the lifetime of a shared reference, neither its referent, nor anything reachable from that referent, can be changed by anything. There exist no live mutable references to anything in that structure, its owner is held read-only, and so on. It’s really frozen
In the image, he goes on to highlight the path along the ownership tree that becomes immutable once a shared reference is taken to a particular section of the ownership tree. But what confused me is that the author also mentions that certain other parts of the ownership tree are not read only.
So I tried to test out with this code:
...ANSWER
Answered 2021-Dec-31 at 02:08Someone should probably check my answer, as I am fairly new to Rust myself. But...
I think this is because a Vec doesn't uphold the same invariance as, say, a tuple or nested structs.
Here's a tuple version of the example you gave (Although tuples don't support pushing, so I'm just incrementing an integer):
QUESTION
Shared aggregation seems to express nothing more than a simple association. It is apparently a "modeling placebo" with no objective reason to use it (except in specific contexts where modelers have agreed between them on specific additional semantic).
Several sources claim however that shared aggregation would imply absence of cycles in the aggregation graph of instances. This would make aggregation a powerful tool in the domain of graph modeling.
But I couldn't find any evidence of those claims in the specifications. Did I miss some indirect evidence (e.g. interplay of several constraints)?
More details The quoteIn their book The Unified Modeling Language Reference Manual, 2nd Edition, Booch, Jacobson and Rumbaugh claim about (shared) aggregation:
The distinction between aggregation and association is often a matter of taste rather than a difference in semantics. Keep in mind that aggregation is association. Aggregation conveys the thought that the aggregate is inherently the sum of its parts. In fact, the only real semantics that it adds to association is the constraint that chains of aggregate links may not form cycles, which is often important to know, however. (...) In spite of the few semantics attached to aggregation, everybody thinks it is necessary (for different reasons). Think of it as a modeling placebo.
Other rather well informed sources(e.g. uml-diagrams.org and several academic papers) keep the directed acyclic graph for granted as well.
Verification against the UML specificationsThe books is from 2004, based on UML 2.0. But most of this claim is still verified in the UML 2.5.1 specification. Shared aggregation is defined therein in very vague terms and without any objective advantage over a simple association (p.112):
Sometimes a Property is used to model circumstances in which one instance is used to group together a set of instances; this is called aggregation. To represent such circumstances, a Property has an aggregation property, of type AggregationKind; (...)
...
shared : Indicates that the Property has shared aggregation semantics. Precise semantics of shared aggregation varies by application area and modeler.
Another clause about aggregation prevents some special cycling situations (p.200):
An end Property of an Association may only be marked as a shared or composite aggregation if the Association is binary and the other end is not marked as a shared or composite aggregation.
I have however not found any evidence that "chains of aggregate links may not form cycles" in the general case. I also did not find other properties such as transitivity and anti-symetry which could allow to infer the absence of cycles.
Is the claim by the UML inventors now obsolete? Or did I miss something in the current specs that would allow to derive this statement ?
...ANSWER
Answered 2021-Dec-12 at 11:32I think my favorite page 110 of UML 2.5 is quite clear about this:
Sometimes a Property is used to model circumstances in which one instance is used to group together a set of instances; this is called aggregation. To represent such circumstances, a Property has an aggregation property, of type AggregationKind; the instance representing the whole group is classified by the owner of the Property, and the instances representing the grouped individuals are classified by the type of the Property. AggregationKind is an enumeration with the following literal values:
none Indicates that the Property has no aggregation semantics. shared Indicates that the Property has shared aggregation semantics. Precise semantics of shared aggregation varies by application area and modeler. composite Indicates that the Property is aggregated compositely, i.e., the composite object has responsibility for the existence and storage of the composed objects (see the definition of parts in 11.2.3).Composite aggregation is a strong form of aggregation that requires a part object be included in at most one composite object at a time. If a composite object is deleted, all of its part instances that are objects are deleted with it.
Emphasis by me. Shared aggregation is out of the run in any case since its semantic is undefined by definition. So the implication is only for composite aggregation.
What is the standard?
UML has quite some history. And there are lots of citations out there. Only the fewest get updates along with UML evolving. Although Booch et al. invented UML, they are no longer defining the standard. That's done by OMG and they publish the ISO standard (for which you can pay extra money if you like). Quite some terms go round which have ancient origins and are outdated the one or other way. Still, they are being used - and in the now wrong context.
Is the standard perfect?
Definitely not. It's evolving and still has quite some flaws or misconceptions. In my opinion the introduction of shared aggregation in UML 2.0 was not a so good idea. To define something that has no definition per definition seems odd. And looking at the confusion about this concept proves me right.
QUESTION
I've been bringing myself up to speed on HTTP Basic Authentication.
I understand that this is fundamentally an insecure access mechanism (even when used over HTTPS, which it always should be) but I recognise that HTTP Basic Auth is not entirely without utility and I would like to be familiar with it, even if situations seldom arise in which I might deploy it.
My understanding so far:After some reading, I understand that:
- a server may request authorisation for a resource by returning
401 (unauthorised)
- a
WWW-Authenticate
response header determines that the authentication to be used to access this resource will beBasic
HTTP Basic Authentication
requires either that:- a) the user submit username and password via a browser-generated console; or that
- b) once successfully submitted manually, the same username and password combination will be automatically submitted via an
Authorization
request header prepended to each HTTP request
So far, so good.
Issues to be aware of:I also understand that there are some issues with HTTP Basic Auth which have evolved over time, like:
- some browsers no longer accept URL syntax such as
https://mylogin:mypassword@example.com/my-resource.html
- where PHP is being run through CGI or FastCGI then the submitted Authorization Credentials will not be passed to
$_SERVER['HTTP_AUTHORIZATION']
unless a hack is deployed - the most common recommendation being a URL rewrite via.htaccess mod_rewrite
and other issues which have persisted from the very beginning, like:
- it's a non-trivial problem to "log out" of HTTP Basic Auth since it was never intended or designed to have a log-out mechanism
However, I'm still confused, because even where the user (or the Authorization
request header) has submitted valid authentication credentials... how does the server know they are valid?
In every document I have come across discussing the mechanics of HTTP Basic Authentication the discussion stops short of the point at which the credentials are actually authenticated.
Question:How are the submitted credentials actually authenticated?
Where is the server comparing the submitted credentials to... anything?
Bonus Question:
N.B. This is related to my main question immediately above because my use of .htaccess
and queryString
parameters to convey credentials (see below) renders deployment of HTTP Basic Auth entirely redundant - if I go down this route, I can convey credentials using .htaccess
and queryString
parameters alone and I don't need to deploy HTTP Basic Auth at all.
As a way to circumvent the CGI/FastCGI issue, I often see variations of these .htaccess
lines cited:
ANSWER
Answered 2021-Oct-11 at 23:55The explanation stops short of the actual authentication because how you do this is entirely up to you.
The username and password are sent as clear text* to you. So, same as when someone would submit a login form with a username
and password
field, you can then decide what to do with it.
*: Yes, they are base64-encoded. But my point was that they aren't hashed or encrypted or anything like that.
So, you may...
- Compare the username & password to some other clear-text value, for example from an environment variable
- Hash the password and compare the hash with something, for example a hashed password in a database
- Forward the credentials to some external authentication service
- ...?
Example of how that could look on the server (this example assumes the use of node.js, Koa, koa-router, Mongoose and bcrypt, and for simplification purposes it assumes neither username nor password are allowed to contain a colon):
QUESTION
I want the f2
to end not itself, but rather completely the parent function f1
while being executed, with a command. I know that return
is used to end a function, but it doesn't work here at a sub-level.
So my question is what are these commands (or sets of lines) and how can I implement them in my code? Example snippet here:
...ANSWER
Answered 2021-Oct-26 at 03:16One way can be to raise an exception from f2
, then catch that exception in f1
and then return early:
QUESTION
I am trying to graph a Bayesian Network (BN) with instantiated nodes using the libraries bnlearn
and Rgraphviz
. My workflow is as follow:
After creating a data frame with random data (the data I am actually using is obviously not random) I then discretise the data, structure learn the directed acyclic graph (DAG), fit the data to the DAG and then plot the DAG. I also plot a DAG which shows the posterior probabilities of each of the nodes.
...ANSWER
Answered 2021-Sep-14 at 21:41How about using cpdist
to draw samples from the posterior given the evidence. You can then estimate the updated parameters using bn.fit
using the cpdist
samples. Then plot as before.
An example:
QUESTION
I’m adding a thematic break inside a unordered list and I’m wondering what is the correct way to do that that will keep the list accessible and semantically correct.
What I would like to do would look something like this:
...ANSWER
Answered 2021-Sep-01 at 23:55Personally I would break it into 2 lists, perhaps with a surrounding one to keep them semantical related.
QUESTION
I have a graph of tasks, where each task may have on_success
and on_error
follow-up tasks. Success path is guaranteed to be acyclic (e.g. if all tasks run fine then no on_success
handlers link to tasks that have already ran) and guaranteed to have some final task (e.g. without on_success
follow-up handler). But on_error
tasks might link to any tasks, creating new flows (which themselves are positive-acyclic and finite). Example graph:
ANSWER
Answered 2021-Aug-11 at 17:48I worked out a solution to only queue single task once the currently running task finishes (either with success or failure), which looks similar to this:
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