A peek past lambda
brian.goetz at oracle.com
Thu Aug 18 11:10:59 PDT 2011
Along the lines that we've been discussing today, here's a peek at where
we're heading. We explored the road of "maybe lambdas should just be
inner class instances, that would be really simple", but eventually came
to the position of "functions are a better direction for the future of
This exploration played out in stages: first internally before the EG
was formed, and then again when the EG discussed the issues. The
following is my position on the issue. Hopefully this fills in some of
the gaps between what the spec currently says and what we say about it.
The issues that have been raised about whether lambdas are objects or
not largely come down to philosophical questions like "what are lambdas
really", "why will Java benefit from lambdas", and ultimately "how best
to evolve the Java language and platform."
Oracle's position is that Java must evolve -- carefully, of course -- in
order to remain competitive. This is, of course, a difficult balancing act.
It is my belief that the best direction for evolving Java is to
encourage a more functional style of programming. The role of Lambda is
primarily to support the development and consumption of more
functional-like libraries; I've offered examples such as
filter-map-reduce to illustrate this direction.
There is plenty of evidence in the ecosystem to support the hypothesis
that, if given the tools to do so easily, object-oriented programmers
are ready to embrace functional techniques (such as immutability) and
work them into an object-oriented view of the world, and will write
better, less error-prone code as a result. Simply put, we believe the
best thing we can do for Java developers is to give them a gentle push
towards a more functional style of programming. We're not going to turn
Java into Haskell, nor even into Scala. But the direction is clear.
Lambda is the down-payment on this evolution, but it is far from the end
of the story. The rest of the story isn't written yet, but preserving
our options are a key aspect of how we evaluate the decisions we make here.
This is why I've been so dogged in my insistence that lambdas are not
objects. I believe the "lambdas are just objects" position, while very
comfortable and tempting, slams the door on a number of potentially
useful directions for language evolution.
As a single example, let's take function types. The lambda strawman
offered at devoxx had function types. I insisted we remove them, and
this made me unpopular. But my objection to function types was not that
I don't like function types -- I love function types -- but that
function types fought badly with an existing aspect of the Java type
system, erasure. Erased function types are the worst of both worlds.
So we removed this from the design.
But I am unwilling to say "Java never will have function types" (though
I recognize that Java may never have function types.) I believe that in
order to get to function types, we have to first deal with erasure. That
may, or may not be possible. But in a world of reified structural
types, function types start to make a lot more sense.
The lambdas-are-objects view of the world conflicts with this possible
future. The lambdas-are-functions view of the world does not, and
preserving this flexibility is one of the points in favor of not
burdening lambdas with even the appearance of object-ness.
You might think that the current design is tightly tied to an object box
for lambdas -- SAM types -- making them effectively objects anyway. But
this has been carefully hidden from the surface area so as to allow us
to consider 'naked' lambdas in the future, or consider other conversion
contexts for lambdas, or integrate lambdas more tightly into control
constructs. We're not doing that now, and we don't even have a concrete
plan for doing so, but the ability to possibly do that in the future is
a critical part of the design.
I am optimistic about Java's future, but to move forward we sometimes
have to let go of some comfortable ideas. Lambdas-are-functions opens
doors. Lambdas-are-objects closes them. We prefer to see those doors
More information about the lambda-dev