This might be a quick and easy question, but I was still unable to find the answer.
In less, is there any way of using a mixin (say .mixin(#a: 2, #b: 3) { /* css here */ }) and only supplying values for given parameters? I know I can use this as .mixin(5) which would be the same as .mixin(5, 3), but can I do something like .mixin(#b: 5)? Meaning the same as .mixin(2, 5) (however I wouldn't have to know the value of a to use the default).
In less using the dotless compiler you can do
.mixin(#b:3);
This will be in the original less.js in 1.3.1 (the next release)- see https://github.com/cloudhead/less.js/pull/268
Related
Okay, what I really wanted to do is, I have an Array and I want to choose a random element from it. The obvious thing to do is get an integer from a random number generator between 0 and the length minus 1, which I have working already, and then applying Array.get, but that returns a Maybe a. (It appears there's also a package function that does the same thing.) Coming from Haskell, I get the type significance that it's protecting me from the case where my index was out of range, but I have control over the index and don't expect that to happen, so I'd just like to assume I got a Just something and somewhat forcibly convert to a. In Haskell this would be fromJust or, if I was feeling verbose, fromMaybe (error "some message"). How should I do this in Elm?
I found a discussion on the mailing list that seems to be discussing this, but it's been a while and I don't see the function I want in the standard library where the discussion suggests it would be.
Here are some pretty unsatisfying potential solutions I found so far:
Just use withDefault. I do have a default value of a available, but I don't like this as it gives the completely wrong meaning to my code and will probably make debugging harder down the road.
Do some fiddling with ports to interface with Javascript and get an exception thrown there if it's Nothing. I haven't carefully investigated how this works yet, but apparently it's possible. But this just seems to mix up too many dependencies for what would otherwise be simple pure Elm.
(answering my own question)
I found two more-satisfying solutions:
Roll my own partially defined function, which was referenced elsewhere in the linked discussion. But the code kind of feels incomplete this way (I'd hope the compiler would warn me about incomplete pattern matches some day) and the error message is still unclear.
Pattern-match and use Debug.crash if it's a Nothing. This appears similar to Haskell's error and is the solution I'm leaning towards right now.
import Debug
fromJust : Maybe a -> a
fromJust x = case x of
Just y -> y
Nothing -> Debug.crash "error: fromJust Nothing"
(Still, the module name and description also make me hesitate because it doesn't seem like the "right" method intended for my purposes; I want to indicate true programmer error instead of mere debugging.)
Solution
The existence or use of a fromJust or equivalent function is actually code smell and tells you that the API has not been designed correctly. The problem is that you're attempting to make a decision on what to do before you have the information to do it. You can think of this in two cases:
If you know what you're supposed to do with Nothing, then the solution is simple: use withDefault. This will become obvious when you're looking at the right point in your code.
If you don't know what you're supposed to do in the case where you have Nothing, but you still want to make a change, then you need a different way of doing so. Instead of pulling the value out of the Maybe use Maybe.map to change the value while keeping the Maybe. As an example, let's say you're doing the following:
foo : Maybe Int -> Int
foo maybeVal =
let
innerVal = fromJust maybeVal
in
innerVal + 2
Instead, you'll want this:
foo : Maybe Int -> Maybe Int
foo maybeVal =
Maybe.map (\innerVal -> innerVal + 2) maybeVal
Notice that the change you wanted is still done in this case, you've simply not handled the case where you have a Nothing. You can now pass this value up and down the call chain until you've hit a place where it's natural to use withDefault to get rid of the Maybe.
What's happened is that we've separated the concerns of "How do I change this value" and "What do I do when it doesn't exist?". We deal with the former using Maybe.map and the latter with Maybe.withDefault.
Caveat
There are a small number of cases where you simply know that you have a Just value and need to eliminate it using fromJust as you described, but those cases should be few and far between. There's quite a few that actually have a simpler alternative.
Example: Attempting to filter a list and get the value out.
Let's say you have a list of Maybes that you want the values of. A common strategy might be:
foo : List (Maybe a) -> List a
foo hasAnything =
let
onlyHasJustValues = List.filter Maybe.isJust hasAnything
onlyHasRealValues = List.map fromJust onlyHasJustValues
in
onlyHasRealValues
Turns out that even in this case, there are clean ways to avoid fromJust. Most languages with a collection that has a map and a filter have a method to filter using a Maybe built in. Haskell has Maybe.mapMaybe, Scala has flatMap, and Elm has List.filterMap. This transforms your code into:
foo : List (Maybe a) -> List a
foo hasAnything =
let
onlyHasRealValues = List.filterMap (\x -> x) hasAnything
in
onlyHasRealValues
I try to port my pygtk code to gtk3. I get this error:
TypeError: pack_start() takes exactly 5 argument(s) (2 given)
It seams that the default arguments have been removed.
Does gtk3 (accessed from python) not support default arguments?
Since the app is not big, I ask myself if I should port to gtk3 or pyside ...
Removing the default arguments looks like a pointless "job creation programm" for programmers...
I could not find a good porting guide (pygtk to python-gtk3). Only this:
http://wiki.sugarlabs.org/go/Features/GTK3/Porting#HBox.2C_VBox.2C_pack_start_and_pack_end
Code like this is ugly:
box.pack_start(widget, True, True, 0)
I know how to search+replace .... but I don't want to.
There are two options that I can suggest. One is you use the pygtkcompat compability module. This is probably not a good long term solution though.
The other option is to patch just the pack_start method in the same way the compatbility module does. Something like this:
orig_pack_start = Gtk.Box.pack_start
def pack_start(self, child, expand=True, fill=True, padding=0):
orig_pack_start(self, child, expand, fill, padding)
Gtk.Box.pack_start = pack_start
This assumes you only want to patch one or two methods. More than that and it's probably better to stick with the compatibility module.
Is it possible to escape whole declaration?
This complies fine in less.js (I'm using less.app)
margin: e(" 0 10px");
But this throws an error:
e("margin: 0 10px");
I've tried putting 'margin' in a variable but without success.
Sorry that's not possible. A couple things though:
The official escape designation (according to lesscss.org is the tilde-quote, not the e() syntax, like so:
margin: ~"0 10px";
Second, the code you provided is not nearly complex enough to warrant a LESS CSS string literal. Please share the actual code that you are having trouble with. You may want to close this question and ask another one.
Third, remember that you can include a .css file - it will not be compiled, just dumped into the output. The syntax is identical to a standard CSS import:
#import "myfile.css";
After many attempts, I got this to compile and output in my less file.
font-family: e("'object-fit: contain;'");
This is a hack for IE object-fit (I got that idea from object-fit
Consider the case where a certain mocked function is expected to be called several times, each time with a different value in a certain parameter. I would like to validate that the function was indeed called once and only once per value in a certain list of values (e.g. 1,2,5).
On the other hand, I would like to refrain from defining a sequence as that would dictate a certain order, which is an implementation detail I would like to keep free.
Is there some kind of matcher, or other solution for this case?
I'm not sure if this influences the solution in any way but I do intend to use WillOnce(Return(x)) with a different x per value in the list above.
By default gMock expectations can be satisfied in any order (precisely for the reason you mention -- so you don't over specify your tests).
In your case, you just want something like:
EXPECT_CALL(foo, DoThis(1));
EXPECT_CALL(foo, DoThis(2));
EXPECT_CALL(foo, DoThis(5));
And something like:
foo.DoThis(5);
foo.DoThis(1);
foo.DoThis(2);
Would satisfy those expectations.
(Aside: If you did want to constrain the order, you should use InSequence: https://github.com/google/googletest/blob/master/googlemock/docs/cook_book.md#expecting-ordered-calls-orderedcalls)
If you expect a function, DoThing, to be called with many different parameters, you can use the following pattern:
for (auto const param : {1, 2, 3, 7, -1, 2}){
EXPECT_CALL(foo, DoThing(param));
}
This is particularly helpful if your EXPECT_CALL includes many parameters, of which only one is changing, or if your EXPECT_CALL includes many Actions to be repeated.
In algebra if I make the statement x + y = 3, the variables I used will hold the values either 2 and 1 or 1 and 2. I know that assignment in programming is not the same thing, but I got to wondering. If I wanted to represent the value of, say, a quantumly weird particle, I would want my variable to have two values at the same time and to have it resolve into one or the other later. Or maybe I'm just dreaming?
Is it possible to say something like i = 3 or 2;?
This is one of the features planned for Perl 6 (junctions), with syntax that should look like my $a = 1|2|3;
If ever implemented, it would work intuitively, like $a==1 being true at the same time as $a==2. Also, for example, $a+1 would give you a value of 2|3|4.
This feature is actually available in Perl5 as well through Perl6::Junction and Quantum::Superpositions modules, but without the syntax sugar (through 'functions' all and any).
At least for comparison (b < any(1,2,3)) it was also available in Microsoft Cω experimental language, however it was not documented anywhere (I just tried it when I was looking at Cω and it just worked).
You can't do this with native types, but there's nothing stopping you from creating a variable object (presuming you are using an OO language) which has a range of values or even a probability density function rather than an actual value.
You will also need to define all the mathematical operators between your variables and your variables and native scalars. Same goes for the equality and assignment operators.
numpy arrays do something similar for vectors and matrices.
That's also the kind of thing you can do in Prolog. You define rules that constraint your variables and then let Prolog resolve them ...
It takes some time to get used to it, but it is wonderful for certain problems once you know how to use it ...
Damien Conways Quantum::Superpositions might do what you want,
https://metacpan.org/pod/Quantum::Superpositions
You might need your crack-pipe however.
What you're asking seems to be how to implement a Fuzzy Logic system. These have been around for some time and you can undoubtedly pick up a library for the common programming languages quite easily.
You could use a struct and handle the operations manualy. Otherwise, no a variable only has 1 value at a time.
A variable is nothing more than an address into memory. That means a variable describes exactly one place in memory (length depending on the type). So as long as we have no "quantum memory" (and we dont have it, and it doesnt look like we will have it in near future), the answer is a NO.
If you want to program and to modell this behaviour, your way would be to use a an array (with length equal to the number of max. multiple values). With this comes the increased runtime, hence the computations must be done on each of the values (e.g. x+y, must compute with 2 different values x1+y1, x2+y2, x1+y2 and x2+y1).
In Perl , you can .
If you use Scalar::Util , you can have a var take 2 values . One if it's used in string context , and another if it's used in a numerical context .