flatMap() equivalent for Optional ?

[Ronan Michaux]

Hi,

I can't understand why the monadic type Optional hasn't a flatMap(Function<T, Optional<U>> f) as all Optional/Maybe equivalent ?

• http://download.java.net/jdk8/docs/api/java/util/Optional.html

• http://www.scala-lang.org/api/current/index.html#scala.Option
  

With the current version, the usage is very limited.

best regards,
Ronan.

[Louis Wasserman]

This has been discussed in several centithreads, see e.g. https://groups.google.com/d/topic/guava-discuss/JVgYWq6VU4c/discussion .

Essentially, I'd say that we approve of adding a flatMap method in the presence of lambdas in the language.   Java 8's Optional was quite heavily based on ours, and we supported adding flatMap there.

But in the absence of lambdas, we generally consider the anonymous classes required to write the Function type here as more awkward than the other alternatives:

optional.isPresent() ? function(optional.get()) : Optional.absent();

See also the Caveats section of the Functional Idioms page of the wiki.

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[Colin Decker]

There is currently an open issue for this (though the issue uses a different name): https://code.google.com/p/guava-libraries/issues/detail?id=1450

To view this discussion on the web visit https://groups.google.com/d/msgid/guava-discuss/CAGB9EW-eyypHBfQXgUEJ5O6OhE-sS6Pni4%2BZ2G_O94M%2BxFmRbw%40mail.gmail.com.

For more options, visit https://groups.google.com/groups/opt_out.

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[Michał Minicki]

I don't get it. This explanation could as well apply to the transform [1] method:

    optional.isPresent() ? transformFunction(optional.get()) : Optional.absent();

What's the difference?

[1] http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Optional.html#transform(com.google.common.base.Function)

Mike

[Ronan Michaux]

I already have functions :

• Function<A, Optional<B>> getB = ...
  
• Function<B, Optional<C>> getC = ...
  

But I can't chain my transformations :

Optional<C> cOpt = FluentIterable.from(listOfA).first().flatTransform(getB).flatTransform(getC);

I have to "flatten" myself each step separately with a custom flatOpt()  :

public static <T> Optional<T> flatOpt(final Optional<Optional<T>> input) {
    return (input.isPresent()) ? input.get() : Optional.<T> absent();
}

Optional<B> bOpt = flatOpt(FluentIterable.from(listOfA).first().transform(getB))

Optional<C> cOpt = flatOpt(bOpt.transform(getC));

it's not very clean, not in the "spirit" of Optional.

Regards,

Ronan.

[Louis Wasserman]

I'm not sure why you wrote the functions that way, then, instead of as a normal Java method taking an Optional argument.

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[Ronan Michaux]

I use Funcito, so for many functions I have got easily the java function and the corresponding Guava Function.

In other language I use the Optional monad to "linearize" several nested (null safe) if (...).

Optional<C> cOpt = FluentIterable.from(listOfA).first().flatTransform(getB).flatTransform(getC);

Is equivalent of :

if (!listOfA.isEmpty()) {

  A first = listOfA.get(0);

  Optional<B> bOpt = first.getB()

  if (bOpt.isPresent()) {

    B b = bOpt.get();

    Optional<C> cOpt = b.getC();

    if (cOpt.isPresent()) {

      C c = cOpt.get();

      // Use of c.........

    }

}

Regards,

Ronan.

[Jed Wesley-Smith]

Note that the fugue library provides a richer Option class, as well as Either, that support the various useful combinators such as flatMap, filter and fold.

https://bitbucket.org/atlassian/fugue

https://bitbucket.org/atlassian/fugue/src/a1dfefd7fd0997110222db44c163528ad2a0dd28/fugue/src/main/java/com/atlassian/fugue/Option.java

https://bitbucket.org/atlassian/fugue/src/a1dfefd7fd0997110222db44c163528ad2a0dd28/fugue/src/main/java/com/atlassian/fugue/Either.java

these are designed to work with the Guava Function/Predicate interfaces.

cheers,

jed.

* some background on why Java8 has flatMap:

http://mail.openjdk.java.net/pipermail/lambda-dev/2013-February/008314.html

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