Add flap function to Apply

[ssanj]

Add the flap function to Apply to facilitate combining a function in an
Apply context with a value which is not.

This is similar to ap:

def ap[A, B](ff: F[A => B])(fa: F[A]): F[B]

with the minor change of the value not being in a context:

def flap[A, B](ff: F[A => B])(a: A): F[B]
                              ^^^^

Thank you for contributing to Cats!

This is a kind reminder to run sbt prePR and commit the changed files before submitting.

[LukaJCB]

Hi @ssanj, thanks for contributing! I don't think Apply is the correct type class here as it could simply be implemented with Functor instead. Futhermore I'm not sure flap is the best name for this, but I'm also not coming up with a better name right now unfortunately, might think about this some more :)

[ssanj]

You're right. This could be moved into Functor. The reason I moved it into Apply was that it had a function in a context (F[A => B]) and was verify similar to ap. Happy to move it.

Yeah, naming this is hard. I hard a look at some of the Haskell equivalents and the variants I saw were:

• flip
• flap
• other options

Another question I had was should this obey any specific Laws?

[ssanj]

When I move the flap function into Functor I get the following compiler error:

[error] /Volumes/Work/projects/code/scala/opensource/cats/core/src/main/scala/cats/Functor.scala:56:21: not found: type A
[error]   def widen[A, B >: A](fa: F[A]): F[B] = fa.asInstanceOf[F[B]]
[error]                     ^
[error] /Volumes/Work/projects/code/scala/opensource/cats/core/src/main/scala/cats/Functor.scala:12:2: type arguments [C,B] do not conform to method widen's type parameter bounds [A,B >: A]
[error] @typeclass trait Functor[F[_]] extends Invariant[F] { self =>
[error]  ^
[error] two errors found
[error] /Volumes/Work/projects/code/scala/opensource/cats/core/src/main/scala/cats/Functor.scala:56:21: not found: type A
[error]   def widen[A, B >: A](fa: F[A]): F[B] = fa.asInstanceOf[F[B]]
[error]                     ^
[error] /Volumes/Work/projects/code/scala/opensource/cats/core/src/main/scala/cats/Functor.scala:12:2: type arguments [C,B] do not conform to method widen's type parameter bounds [A,B >: A]
[error] @typeclass trait Functor[F[_]] extends Invariant[F] { self =>
[error]  ^
[error] two errors found

I've enquired whether this is a Simulcrum issue. I can get this to work if I rewrite widen not to have a upper bound:

def widen[A, B](fa: F[A])(implicit ev: A <:< B): F[B] = fa.asInstanceOf[F[B]]

Has anyone seen this issue before?

[kailuowang]

How about we call this mapApply with a F[_] :Functor bound add it to a Function1 syntax extension trait? We can't add any method to any trait without breaking binary compatibility in Scala 2.11. And we promised to not break it until 2.0

[codecov-io]

Codecov Report

Merging #2588 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2588      +/-   ##
==========================================
+ Coverage   95.16%   95.17%   +0.01%     
==========================================
  Files         361      362       +1     
  Lines        6634     6655      +21     
  Branches      294      302       +8     
==========================================
+ Hits         6313     6334      +21     
  Misses        321      321

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/function1.scala	100% <100%> (ø)
core/src/main/scala/cats/data/Chain.scala	99.58% <0%> (+0.01%)	⬆️
core/src/main/scala/cats/data/NonEmptyChain.scala	99.07% <0%> (+0.07%)	⬆️
...ore/src/main/scala/cats/data/NonEmptyMapImpl.scala	96.15% <0%> (+0.15%)	⬆️

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[kailuowang]

due to binary compatibility restrictions, you can't add it to this trait either. However, you can add it to AllSyntaxBinCompat3 which is a new trait that hasn't been released yet.

[ssanj]

Thanks. 👍

[ssanj]

@kailuowang @LukaJCB does this look ok now?

[kailuowang]

I am going to schedule this to 1.5 so that you don't have to change your BinaryCompat trait.