@morgen-peschke:
Here’s a quite neat overview of testing styles in ScalaTest: Fixtures in Scala — three simple ways to reduce your test code boilerplate | by Jakub Dzikowski | SoftwareMill Tech Blog
The way you presented has the advantage of easy composability new Fixture with ExtraData1 with ExtraData2 but for now they it suffers from lack of direct support for parametrization (traits do not have parameters yet, but will have in Scala 3) and you don’t get tear down (you would have to wrap the new Fixture with Whatever { <test code> } with some extra function to get e.g. withTeardown(new Fixture with Whatever { <test code > }).
Extension methods are useful when the type interface is deficient. I was presuming that this would be more useful for Java types as Java e.g. for a long time lacked multiple inheritance of behaviour (Java 8 brought that) so making rich APIs was tedious.
OTOH marking something implicit is sometimes required to make something else compile. If I have def myMethod[A: MyContext] ... then when invoking it I need to have implicit MyContext[A] instance in scope. That’s how it worked since the beginning of implicit contexts.
I have already proposed:
function { this =>
<some code using members of new this directly>
}
It’s explicit, comprehensible and use-site configurable.
I don’t see much improvement in:
"say hello" in {
assert(0.greeter.greet(0.name) == s"Hello ${0.name}")
}
over
"say hello" in { f =>
assert(f.greeter.greet(f.name) == s"Hello ${f.name}")
}
Using 0.something instead of f.something wouldn’t pay off when using IDE, as IDE would first suggest methods defined directly on Int and only after that you would see extension methods.
Let’s not mix things together. Rebinding this is a completely different thing than unqualified extension methods / implicit views.
Here’s how you search for binding in current scope:
Bindings of different kinds have a precedence defined on them:
Definitions and declarations that are local, inherited, or made available by a package clause and also defined in the same compilation unit as the reference to them, have highest precedence.
Explicit imports have next highest precedence.
Wildcard imports have next highest precedence.
Definitions made available by a package clause, but not also defined in the same compilation unit as the reference to them, as well as imports which are supplied by the compiler but not explicitly written in source code, have lowest precedence.
Here’s how you search for implicit views:
In a selection e.m with e of type T, if the selector m does not denote an accessible member of T. In this case, a view v is searched which is applicable to e and whose result contains a member named m. The search proceeds as in the case of implicit parameters, where the implicit scope is the one of T. If such a view is found, the selection e.m is converted to v(e).m .
In a selection e.m(args) with e of type T, if the selector m denotes some member(s) of T, but none of these members is applicable to the arguments args. In this case a view v is searched which is applicable to e and whose result contains a method m which is applicable to args. The search proceeds as in the case of implicit parameters, where the implicit scope is the one of T. If such a view is found, the selection e.m is converted to v(e).m(args) .
How do you want to merge searching for implicit views on this to searching for binding in current scope? There has to be some precedence rules. Currently for e.m the precedence rules state that implicit views are tried last. That is good because searching for implicits is costly, so it should be done last. If we carry that rule to searching for binding in scope, then all packages, class members (including class members from outer classes), local variables and methods, functions and methods parameters, etc will have precedence over extension methods / implicit views on this.
Implicits are quite heavy, so we should rather strive to find a way to reduce their compilation performance impact rather than going to enable implicits everywhere. I don’t use scalaz / cats on regular basis but I remembers that when I was adding import scalaz._ ; import scalaz.Scalaz._ to classes a few years ago that slowed down IntelliJ IDE and Scala compiler considerably. Simply being more selective in implicits made performance much better, e.g. import scalaz.std.syntax.options._ (or something like that). Shapeless library is another proof that heavy use of implicits drag compilation speed down. People are inventing extra macros that use few implicits (if any) to implement things that could be done without that extra macros in Shapeless, but with much higher complation performance cost.
As I’ve written before - marking something implicit doesn’t add any named member to any scope.
Your statement is non-sequitur, I’m literaly showing how to add new methods in scope from a programmer’s point of view. The low-level details of how compiler will implement this are completely irrelevant.
Kotlin has a feature that receivers nest - that is, if identifier is not found in inner this, it will be searched in outer this. Seems like your proposal does not have this - new binding of this should make the previous one inaccessible if it follows the rules for variables.
On the contrary, extension methods on 0 do stack, and since in dotty implicits have lexical priority, methods on 0 introduced in inner scopes will override methods in outer scope, but the outer scope conversion will still be available for non-intersecting methods.
From a programmer’s point of view they achieve very similar things - allow introduction of unqualified names within a scope without imports. Latter is much better in presence of the former, of course.
I think it can follow rules for nested classes where there is fallback from inner this to outer this.
I’ve thought a bit on receiver functions in Kotlin in context of type-safe builders and wondered if they are really type safe. It turns out they aren’t until you use special extra annotations: https://kotlinlang.org/docs/reference/type-safe-builders.html
When using DSLs, one might have come across the problem that too many functions can be called in the context. We can call methods of every available implicit receiver inside a lambda and therefore get an inconsistent result, like the tag head inside another head :
html {
head {
head {} // should be forbidden
}
// ...
}
In this example only members of the nearest implicit receiver this@head must be available; head() is a member of the outer receiver this@html , so it must be illegal to call it.
To address this problem, in Kotlin 1.1 a special mechanism to control receiver scope was introduced.
Also somehow I don’t find Kotlin receiver functions particularly succint on definition site (in case of type-safe builders at least). In Dotty 0.19.0-RC1 I can do:
// imagine A, B, C and D are some sort of tags or other nested structures
type X = A|B
type Y = B|D
type Z = X|Y
case class A(items: X|B*)
case class B(items: A|C|D*)
case class C(items: A|Z*)
case class D(items: C|D*)
@main def main = {
val x = A(B(D(C())), A())
// val x = A(B(D(C())), A(), C()) // doesn't typecheck
println(x)
}
How to emulate that with type-safe builders based on receiver functions instead? I guess it will be rather convoluted and verbose.
Actually with Implicit Function Types it can be done more succinct.
“say hello” in greeterFixture{new FixtureContext{
assert(greeter.greet(name) == s"Hello $name")
}}
We write something like that today.
I have understood after this words that I really need to import only implicits. Because overriding “this” has significant disadvantages.
So I can compare template:
aspect{new aspectContext{
}}
with
aspect{implicit context =>
}
for the case where there are single implicit instead of multiple.
Ok, we can live with such boilerplate code and we live with it.
But I do not understand why https://dotty.epfl.ch/docs/reference/contextual/implicit-function-types.html
It is very good for single implicit, and It does not need for multiple.
I need inject multiple implicits more often than a single implicit. Because my main single implicit is declared in the root class.
Conversely, if the expected type of an expression E is an implicit function type (given T_1, ..., T_n) => U and E is not already an implicit function literal, E is converted to an implicit function literal by rewriting to
It doesn’t look like this is limited to a single implicit.
Note: the reason the syntax I mentioned works for tests is that it is generally local to the test, so finding out what’s in scope only involves a quick intra-file check. It wouldn’t work nearly as well if it were used across multiple files.
I’m not quite sure what you’re looking for that the current proposed syntax doesn’t offer. One of the examples in the reference doc does use multiple implicits (both Row and Table are implicitly given to the cell helper).
I was playing around in scastie, and the syntax it supports is pretty much what it sounds like you’re asking for:
You are right, but in practice I will not transfer package\object implicit in the arguments of implicit function. It is the way to emulate:
import implicit it._
But I am afraid such techniques.
I think it is a bad practice. I do not know why it is in documentation.
It is my personal opinion of course. But I never will use such approach for builders.
I would prefer ‘it’ technique that have been introduced here.
It have been discussed already, so I do not know whether it needs to repeat.
I persanaly prefer for orphan tasks and dsl like:
(I do not advartise “basic” but implicits make code more succinct in some case)
new doSomeThingContext{
println("good bye delayInit")
}.execute()
instead of
execute{
import implicit doSomeThingContext._
}
But it seems that it my personal taste. It happens
I do not think so. Actually it help to manage scope. We use objects to implement scope reusing, it is actual for library integration.
I have said that there is bad practice to use implicit import type for builder pattern.
I think so because the grammar of such builder has very low coupling so
it is difficult to navigate
it is difficult to documentate
it has a problem of name clashing
I think the usual people are not smart enough to use such technique in our company.
bad practice to explicitly import new syntax?
I have not said this. It is just less succinct.
There is full analogy with motivation of implicit function type. It just makes code more short.
I like good work of a code assistant.
Scope injection has nothing to do with mutable builders, this is non-sequitur. I want scope injection for ZIO Test’s immutable builder – right now all tests written with it must include boilerplate imports:
import zio.test.Assertion._
import zio.test._
Every time – only because ZIO Test chose to use constructor parameter for test construction instead of inheritance. Having scope injection for top-level members evens the odds and would allow suite, testM and other functions to be used in Spec expression without using inheritance and without constantly repeating boilerplate imports.
Somehow this thread started with discussion on mutable builders for dozens of posts. If scope injection has really nothing to do with mutable builders then maybe this thread should be split into two?
Depends on what could be injected. If author of ZIO doesn’t give you anything to inject then scope injection doesn’t help you at all.
Author of DefaultRunnableSpec could go for e.g. abstract lazy val instead of by-name constructor parameter and that would open the ability to mix in helper classes just as in ScalaTest you’re mixing in lots of them into your test base classes.
The way ZIO DefaultRunnableSpec is made prevents from creating helpful base classes, so even if you get rid of:
This thread is a discussion of scope injection, not scope injection itself, obviously.
The fact that you need “base classes” to extend a scope is a direct consequence of a lack of scope injection – code must be arbitrarily structured ahead of time to allow for scoping – this means inheritance is hugely favored vs. composition – however, Dotty actually adds many tools to boost composition, e.g. export
If author of ZIO doesn’t give you anything to inject then scope injection doesn’t help you at all.
Nope, you don’t need prescience on the part of author with scope injection, you may mix your own arbitrary environment with export:
