Surprise | Python scikit for building and analyzing recommender systems | Recommender System library

I like the idea of the lazy ranges you can make with std::views::iota but was surprised to see that iota is currently the only thing like it in the standard; it is the only "range factory" besides views::single and views::empty. There is not currently, for example, the equivalent of std::generate as a range factory.

I note however it is trivial to implement the semantics of generate by using a transform view on iota and just ignoring the value iota passes to transform i.e.

I am reading this book by Fedor Pikus and he has some very very interesting examples which for me were a surprise.
Particularly this benchmark caught me, where the only difference is that in one of them we use || in if and in another we use |.

[NOTE: I asked this question based on an older version of Rakudo. As explained in the accepted answer, the confusing output was the result of Rakudo bugs, which have now been resolved. I've left the original version of the Q below for historical reference.]

Raku sometimes prohibits re-binding; both of the following lines

I was looking for the canonical implementation of MergeSort on Haskell to port to HOVM, and I found this StackOverflow answer. When porting the algorithm, I realized something looked silly: the algorithm has a "halve" function that does nothing but split a list in two, using half of the length, before recursing and merging. So I thought: why not make a better use of this pass, and use a pivot, to make each half respectively smaller and bigger than that pivot? That would increase the odds that recursive merge calls are applied to already-sorted lists, which might speed up the algorithm!

I've done this change, resulting in the following code:

The following difference between Vector{Missing} and Vector{Int} surprised me (in a positive way):

I understand that React Testing Library has an example of testing with react router, but I couldn't get it to work (I think because I am using react router V6).

Basically, I need router testing because I have details component that uses useParams() to get part of the url. I can't render the component without it.

This was my attempt to make it work (yes the page also needs apollo, although it doesn't really need redux).

I am trying to get to grips with the specifics of the (C++20) standards requirements for container classes with a view to writing some container classes that are compatible with the standard library. To begin looking into this matter I have looked up the references for named requirements, specifically around container requirements, and have only found one general container requirement called Container given by the standard. Reading this requirement has given my two queries that I am unsure about and would like some clarification on:

1. The requirement for the expression a == b for two container type C has as precondition on the element type T that it is equality comparable. However, noted later on the same page under the header 'other requirements' is the explicitly requirement that T be always equality comparable. Thus, on my reading the precondition for the aforementioned requirement is redundant and need not be given. Am I correct in this thinking, or is there something else at play here that I should take into account?

2. I was surprised to see explicit requirements on T at all: notably the equality comparable requirement above and the named requirement destructible. Does this mean it is undefined behaviour to ever construct standard containers of types failing these requirements, or only to perform certain standard library function calls on them?

Apologies if these two questions sound asinine, I am currently trying to transition my C++ knowledge from a place of having a basic understanding of how to use features to a robust understanding so that I may write good generic code. Whilst I am trying to use (a draft of) the standard to look up behaviour where possible, its verbiage is oft too verbose for me to completely understand what is actually being said.

In an attempt to seek the answer I cooked up a a quick test .cpp file to try an compile, given below. All uncommented code compiles with MSVC compiler set to C++20. All commented code will not compile, and visa versa all uncommented code will. It seems that what one naively thinks should work does In particular:

• We cannot construct any object without a destructor, though the objects type is valid and can be used for other things (for example as a template parameter!)
• We cannot create an object of vector, where T has no destructor, even if we don't attempt to create any objects T. Presumably because creating the destructor for vector tries to access a destructor for T.
• We can create an object of type vector, T where T has no operator ==, so long as we do not try to use operator ==, which would require T to have operator ==.

However, just because my compiler lets me make an object of vector where T is not equality-comparable does not mean I have achieved standards compliant behaviour/ all of our behaviour is not undefined - which is what I want I concerned about, especially as at least some of the usual requirements on the container object have been violated.

Code:

I have simulation program written in Julia that does something equivalent to this as a part of its main loop:

This is maybe a dumb question, but I'm pretty surprised to see that using the private inner class as a generic type in the outer class isn't allowed.

If I make the inner class protected, it compiles fine.

Additionally, I have to precise Outer.Inner instead of just Inner, otherwise the inner class isn't found. This also looks a little weird.

Why can't Inner be private ? And why it is allowed to be protected ?