Function calls that take a single lambda should not be on a new line

Overview

When there is a only a single argument in a function call it seems unnecessary to place it onto a new line. This misaligns with how other prettier languages work, like in Javascript:

A single argument taking a lambda

When there are multiple arguments, e.g. two lambdas

Steps to reproduce

Prettier-Java 0.8.0

Stream methods (map, filter, etc)

Input:

final List<Integer> values = Stream.of(1, 2)
    .map(n -> {
        // testing method
        return n * 2;
    })
    .collect(Collectors.toList());

Output:

final List<Integer> values = Stream
    .of(1, 2)
    .map(
        n -> {
            // testing method
            return n * 2;
        }
    )
    .collect(Collectors.toList());

Expected behavior:

final List<Integer> values = Stream
    .of(1, 2)
    .map(n -> {
        // testing method
        return n * 2;
     })
    .collect(Collectors.toList());

Completable Futures (single argument)

Input:

CompletableFuture.supplyAsync(() -> {
    // some processing
    return 2;
});

Output:

CompletableFuture.supplyAsync(
    () -> {
        // some processing
        return 2;
    }
);

Expected behavior:

CompletableFuture.supplyAsync(() -> {
    // some processing
    return 2;
});

Completable Future (Multiple Arguments)

Input:

CompletableFuture.supplyAsync(() -> {
       // some processing
       return 2;
}, executor);

Output:

CompletableFuture.supplyAsync(
    () -> {
         // some processing
         return 2;
    },
    executor
);

Expected behavior:

CompletableFuture.supplyAsync(
    () -> {
        // some processing
        return 2;
     },
     executor
);

Function arguments that exceed width

Input:

myFunction((one, two, three, four, five, six) -> {
       // some processing
       return 2;
});

Output:

myFunction(
      (one, two, three, four, five, six) -> {
           // some processing
           return 2;
      }
);

Expected behavior:

myFunction(
      (one, two, three, four, five, six) -> {
           // some processing
           return 2;
      }
);

[clementdessoude]

If I’m not mistaken, I already had a look, and it was not so easy to fix, as the syntax tree is quite complex around lambdas. But it would definitely be better formatted on a single line. I’ll try to have a look soon, I may have more inspiration 😃

[jdubois]

OK, increasing the bounty to $300