Introduce Gen.zipWith multi-arity method

[satorg]

Adds a code generator for Gen.zipWith functions.

Example:

case class Foo(pos: Int, ascii: String, hex: String)

Gen.zipWith(Gen.posNum[Int], Gen.asciiStr, Gen.hexStr)(Foo.apply)

Without Gen.zipWith the same was possible to accomplish with Gen.zip, however it would required an additional map over an intermediary tuple instance:

Gen.zip(Gen.posNum[Int], Gen.asciiStr, Gen.hexStr)
  .map { case (pos, ascii, hex) => Foo(pos, ascii, hex)) }

Therefore Gen.zipWith allows to avoid the intermediate conversion which can come in handy in come cases.

Note that Gen.zipWith starts with arity 2 whereas Gen.zip starts with arity 1.
However, I think the latter was an oversight – Gen.zip for arity 1 is a no-op function.

[rossabaker]

Is this intended to behave similarly to mapN, but without the Cats dependency?

[satorg]

Is this intended to behave similarly to mapN, but without the Cats dependency?

Pretty much. To be more accurate, it is supposed to resemble functions map2 through map22 of the Apply typeclass, because mapN requires a tuple to be created first, check this out:

case class Foo(pos: Int, ascii: String, hex: String)

val gen1 = (Gen.posNum[Int], Gen.asciiStr, Gen.hexStr).mapN { Foo.apply }

val gen2 = Apply[Gen].map3(Gen.posNum[Int], Gen.asciiStr, Gen.hexStr) { Foo.apply }

val gen3 = Gen.zipWith(Gen.posNum[Int], Gen.asciiStr, Gen.hexStr) { Foo.apply }

However, there's one catch with the Cats functions: they both use a chain of Functor.product under the hood, which in turn calls to Gen.zip with arity 2. I.e. the Cats functions incur a bunch of intermediary tuples on every evaluation to be created.

[Copilot]

Pull Request Overview

This pull request introduces a multi-arity generator method, Gen.zipWith, which lets users combine multiple generators and apply a mapping function directly without creating an intermediary tuple.

• Adds a new zipWith method implementation in codegen.scala that constructs a chain of flatMap/map calls based on the supplied arity.
• Updates tests in GenSpecification.scala to validate zipWith for various arities, from 2 to 22.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
project/codegen.scala	Introduces the zipWith generator, constructing composed generator expressions via flatMap and map calls.
core/jvm/src/test/scala/org/scalacheck/GenSpecification.scala	Adds property tests for zipWith with multiple arities to ensure correctness.

Comments suppressed due to low confidence (1)

project/codegen.scala:120

• [nitpick] Consider adding an inline comment to clarify the purpose and structure of destructuring the last element of tTtgs; this can help others understand the foldRight initializer logic.

val ((_, ti), gi) = tTtgs.last

[rossabaker]

I lost track of this one in a GitHub bankruptcy. I'm sorry it got so desperately lonely you had to talk to a bot.

[satorg]

@rossabaker , thank you for the review!

I'm sorry it got so desperately lonely you had to talk to a bot.

No worries! Actually, I just wanted to check out that "copilot review" feature – to see how it works, what the output looks like, etc. So I thought – what could be better than some my own PR for that?