missing functions on List - (partition, safeHead) about functionaljava

[ms-tg]

In Scala, is this List#partition and List#headOption?

[amarpotghan]

@ms-tg yup

[knutwalker]

safeHead or headOption is List#toOption

[amarpotghan]

Ah, Sure. Thanks.

On Fri, Feb 27, 2015 at 10:21 PM, Paul Horn [email protected]
wrote:

safeHead or headOption is List#toOption

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#111 (comment)
.

[tonymorris]

+1

However, safeHead is better written as a Prism

https://hackage.haskell.org/package/lens-4.7/docs/Control-Lens-Prism.html

https://hackage.haskell.org/package/lens-4.7/docs/Control-Lens-Cons.html

Note that writing prisms, in general, in Java requires code repetition, due to lack of higher-kinds. The idea here, however, is that _head can perform a "safe head retrieval" as well as "safe update head" operation. Further to this, you can compose prisms, so you might say something like:

safeHead.compose(children).update(upperCase)

which gets the children of an object (e.g. Person => List<Person>), then gets the first child (safeHead) and updates that child by upper-case. The entire expression returns the outer object type.

[julien-truffaut]

@tonymorris safeHead is not a Prism as you cannot recreate the original List. However, if you compose cons: Prism[List[A], (A, List[A])] with _1: Lens[(A, B), A] you get safeHead. The result of the composition is an Optional (in monocle) or a Traversal (in haskell lens if I am not mistaken).

[jbgi]

so, with #130, you would get:

  public static <A> Optional<List<A>, A> _head() {
    return optional(
        l -> l.toOption().toEither(nil()),
        a -> l -> l.list(nil(), h -> tail -> cons(a, tail))
        );
  }

[tonymorris]

@julien-truffaut Yes you are right.

[mperry]

Created #161 to add List#partition.