lens | Lens - The way the world runs Kubernetes | Job Orchestrator library
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Lens - The way the world runs Kubernetes
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lens Examples and Code Snippets
public static int findNumberOfLIS(int[] nums) {
int len = nums.length, maxLen = 0;
if (len < 2) return len;
int[] lens = new int[len];
int[] counts = new int[len];
Arrays.fill(lens, 1);
Arrays.fill(c
Community Discussions
Trending Discussions on lens
QUESTION
A type Prism' s a = Prism s s a a
(hackage) can be thought of as a relation between some structure s
and its member a
, such that you can always produce the structure from the member (a -> s
), but can only optionally retrieve the member from the structure (s -> Maybe a
).
This model is helpful in relating a sum type to one of its constructors ... as well as (more relevant here) in route encoding and decoding. If s
is the encoded route URL, and a
is the route type, then we have a -> s
representing the encoding function (always succeeds), and s -> Maybe a
representing the decoding function (can fail).
Now, on to these pairs of functions, I want to add a "context" argument that is to be used in the encoding and decoding process (imagine that the decoding process needs to "look up" some database before successfully producing a relevant route). Basically:
...ANSWER
Answered 2022-Mar-15 at 15:18I'm with @HTNW. You should just be able to define:
QUESTION
I have this extremely messed up dataframe:
...ANSWER
Answered 2022-Mar-09 at 18:02Here is a way using stack
. First remove the _n from the columns names, then set_index
the column id, mask
the cell with empty string that would be remove when stack
ing the data. Then use reset_index
and rename
to fit the expected output.
QUESTION
So I'm trying to code something to tell me the number of elements in any given compound. I'm not even sure where to start: I tried coding something but then realized that it only worked for simple compounds (or did not work at all). Here's an example of what I want:
...ANSWER
Answered 2022-Jan-31 at 12:50Try with this recursive function:
QUESTION
I am trying to access a nested record using lenses and prisms in Haskell:
...ANSWER
Answered 2022-Mar-01 at 21:09Updated: As per comments, I've fixed some errors and added a few asides in [[double square brackets]].
Here's how/why your first mMistake
works...
A prism is an optic that focus on a "part" that may or may not be present in the "whole". [[Technically, it focuses on the sort of part that can be used to reconstruct an entire whole, so it really pertains to a whole that can come in several alternative forms (as in the case of a sum type), with the "part" being one of those alternative forms. However, if you're only using a prism for viewing and not setting, this added functionality isn't too important.]]
In your example, both _StateRun
and _Just
are prisms. The _Just
prism focuses on the a
part of a Maybe a
whole. Such an a
may or may not be present. If the Maybe a
value is Just x
for some x :: a
, the part a
is present and has value x
, and that's what _Just
focuses on. If the Maybe a
value is Nothing
, then the part a
is not present, and _Just
doesn't focus on anything.
It's somewhat similar for your prism _StateRun
. If the whole StateStep
is a StateRun x y
value, then _StateRun
focuses on that "part", represented as a tuple of the fields of the StateRun
constructor, namely (x, y) :: (Int, Maybe Text)
. On the other hand, if the whole StateStep
is a StatePause
, that part isn't present, and the prism doesn't focus on anything.
When you compose prisms, like _StateRun
and _Just
, and lenses, like stStep
and _2
, you create a new optic that combines the composed series of focusing operations.
[[As was pointed out in the comments, this new optic isn't a prism; it's "only" traversal. In fact, it's a specific kind of traversal, called an "affine traversal". A run-of-the-mill traversal can focus on zero or more parts, while an affine traversal focuses on exactly zero (part not present) or one (unique part present). The lens
library doesn't make the distinction between affine traversals and other sorts of traversals, though. The reason the new optic is "only" an affine traversal instead of a prism relates to that earlier technical point. Once you add lenses, you remove your ability to reconstruct the entire "whole" from a single "part". Again, if you're only using the optics for viewing, not setting, it won't really matter.]]
Anyway, consider the optic (affine traversal):
QUESTION
First time using lens
. set
and over
went easy enough and I thought it would be simple with view
: Use the same scheme to reference the inner part, but don't supply a new value or function. But Noooo. tst3 below gives the error below the code
. Anyone know what's going on?
ANSWER
Answered 2022-Feb-20 at 23:35ix 0
doesn't produce a lens, but a traversal.1
Informally, a lens is a "path" that will definitively reach a single value (if you follow it within a hypothetical larger value). A traversal is a path to zero or more values. You can set
or view
the single target of a lens. And you can set
the zero or more targets of a traversal (this simply updates all of them that are present, which is a no-op if there are zero present). But view
ing the targets of a traversal is less straightforward.
If view
simply took a traversal, and an outer structure, and gave you the target value, then it would have a problem. If there are multiple targets, how should it decide which to return? And if there are zero targets, it can't return anything; it would have to be partial. What it needs is a way of condensing zero-or-more values into a single value, so it can return that. And the Monoid
class provides exactly the facilities to do that; mempty
for if there aren't any targets at all, and <>
to condense multiple values to a single one. So view
with a traversal2 actually requires a Monoid
constraint on the returned type, and that's why you're getting the complaint about No instance for (Monoid Int) arising from a use of `ix'
.
And in case it isn't clear, you can often compose (with .
) different types of optics (the general term for "lens-ish things", including lenses, traversals, and several others). But the result has the capabilities of the least capable of the two inputs. So even though your inner
and w
are full lenses, composing them with a traversal produced by ix
results in a traversal, not a lens.
But in this case you know that you're using ix
. The specific kind of traversals ix
makes end up having zero or one target, rather than the zero or more targets that traversals have in general. So you could use preview
instead of view
; for a traversal it will produce a Maybe
containing Just
the first target of the traversal, or Nothing
if there weren't any. A Maybe
is exactly what the type system deems appropriate here, since ix
can't guarantee there will be a target value, but there won't be more than one.
In my experience, when I try to view
something and get this Monoid
instance error, it almost always means I have an optic that can't guarantee a result and I should actually be using preview
to get a Maybe
.
Simple example:
QUESTION
I used the blowup.js
plugin as a base, and I am trying to rotate the image, and the lens to follow the rotation. But it is not working.
When I put the rotate(180deg)
for example, the lens and the image are mismatched, if I remove the rotate(180deg)
they are aligned.
Anybody know how to help me?
...ANSWER
Answered 2022-Feb-16 at 17:42You might rotate the lens in the opposite direction (-180deg) and inverse background position too:
Side note: you don't want to apply background-image on every mousemove, move it to the lens init.
QUESTION
here is a puzzle that I keep on bumping into and that, I believe, no previous SO question has been addressing: How can I best use the lens
library to set or get values within a State
monad managing a nested data structure that involves Map
s when I know for a fact that certain keys are present in the maps involved?
ANSWER
Answered 2022-Feb-09 at 11:43If you are sure that the key is present then you can use fromJust
to turn the Maybe User
into a User
:
QUESTION
I am working on this problem on CSES, Traffic Lights:
There is a street of length 𝑥 whose positions are numbered 0,1,…,𝑥. Initially there are no traffic lights, but 𝑛 sets of traffic lights are added to the street one after another.
Your task is to calculate the length of the longest passage without traffic lights after each addition.
InputThe first input line contains two integers 𝑥 and 𝑛: the length of the street and the number of sets of traffic lights.
Then, the next line contains n integers 𝑝1,𝑝2,…,𝑝𝑛: the position of each set of traffic lights. Each position is distinct.
OutputPrint the length of the longest passage without traffic lights after each addition.
ConstraintsExample
- 1 ≤ 𝑥 ≤ 109
- 1 ≤ 𝑛 ≤ 2⋅105
- 0 < 𝑝𝑖 < 𝑥
Input:
...
ANSWER
Answered 2022-Jan-11 at 10:06The data structure you have for lens
is like a multiset, also available as Counter
. The part of your algorithm that is the bottle neck in terms of time complexity, is this:
QUESTION
While trying to apply a function with multiple layers of functors to a member of a host data structure, as in the following example:
...ANSWER
Answered 2021-Dec-29 at 00:09Not much of a standard approach, perhaps, but anyway: alaf
can handle the Compose
wrapping. For instance:
QUESTION
I am trying to generate lens for data type with a lens field.
...ANSWER
Answered 2021-Nov-26 at 22:00{-# Language ImpredicativeTypes #-}
{-# Language TypeApplications #-}
-- ..
st_lens :: forall st l. Lens' (St st l) (Lens' st l)
st_lens = lens @_ @(Lens' st l) _st_lens \s a -> s { _st_lens = a }
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