Add an If-Then-Else operation to the Apply. #2609
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The `FlatMap` type-class includes a function `ifM`, which allows evaluating a Boolean computation and, depending on the result, choosing between one effectful computation or other. We add to the `Apply` type-class a very similar function. Although the function is similar to `ifM`, there is an important difference between them, as to how they handle the effect: - The `ifM` will run the effects of the condition `F[Boolean]`, followed by the effects of _only one_ of the branches. - The `ifA` will always run the effect of the condition, that of the first branch, and that of the second branch, in that order. Thus, the if-then-else part in `ifA` only applies to the inner values, not to the effects.
| * scala> val asInt3: Option[Int] = b3.ifA(Some(1), None) | ||
| * asInt2: Option[Int] = None | ||
| * | ||
| * }}} |
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Unfortunately we can't add any methods to type classses without breaking binary compatibility. So for now, we have to settle adding to a new syntax trait. Sorry I am on my phone and couldn't point you to an example.
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@kailuowang If I understand you correctly, we can not add the method to the trait Apply[F[_]] (the type-class) but can only leave it as part of a syntactic extension implicit class?
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sorry for the delayed response. Yes, that's the idea. You already updated the PR, we also need to create a new ApplySyntaxBinCompat trait to include an implicit conversion from F[Boolean] to IfAOps, then there are a couple of more steps,
- here we need to add a new trait
AllSyntaxBinCompat4to extends the newApplySyntaxBinCompat,
let theAllSyntaxBinCompatat the top extend thisAllSyntaxBinCompat4 - [here] https://github.com/typelevel/cats/blob/master/core/src/main/scala/cats/implicits.scala we also need to let the
implicitsobject extends this newAllSyntaxBinCompat4 - then we can probably add a doctest here that
import cats.implicits._to validate that it's all hooked up.
Sorry about the gymnastics you have to go through to overcome the Scala 2.11 binary compatibility restrictions.
To preserve binary compatibility, we can not add methods to an existing type-class, and can only use the syntax extension.
| @@ -77,3 +78,6 @@ trait AllSyntaxBinCompat2 | |||
| with ValidatedSyntaxBincompat0 | |||
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| trait AllSyntaxBinCompat3 extends UnorderedFoldableSyntax with Function1Syntax | |||
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| trait AllSyntaxBinCompat4 extends IfApplyOps | |||
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Here we need to extends ApplySyntaxBincompat0 instead
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after you are done with the changes, please also run |
Codecov Report
@@ Coverage Diff @@
## master #2609 +/- ##
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+ Coverage 95.15% 95.16% +<.01%
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Files 365 365
Lines 6774 6777 +3
Branches 297 297
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+ Hits 6446 6449 +3
Misses 328 328
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Thanks @diesalbla! |
The
FlatMaptype-class includes a functionifM, which allows evaluating a Boolean computation and, depending on the result, choosing between one effectful computation or other.We add to the
Applytype-class a similar function,ifA, which 1) takes as inputs anF[Boolean]and twoF[A], and 2) depending on the value of the boolean result, returns either the first or the secondF[A]on the result.Although the profile is similar to
ifM, they need to be separate functions due to the different way they handle the effects:ifMperforms the effects of the conditionF[Boolean], and the effects of one of the branches. Which branch, depends on the boolean result.ifAwill always perform the effect of theF[Boolean], those of the firstF[A], and those of the secondF[A], in that order. It will apply theif-then-elseselection to the values.In other words,
ifAonly maps the values, but does not allow skipping effects.