I have not followed the process and evolution of the Java 7 lambda
proposal, I am not even sure of what the latest proposal wording is.
Consider this as a rant/opinion rather than statements of truth. Also,
I have not used Java for ages, so the syntax might be rusty and
incorrect at places.
First, what are lambdas to the Java language? Syntactic sugar. While
in general lambdas enable code to create small function objects in
place, that support was already preset --to some extent-- in the Java
language through the use of inner classes.
So how much better is the syntax of lambdas? Where does it outperform
previous language constructs? Where could it be better?
For starters, I dislike the fact that there are two available syntax
for lambda functions (but this goes in the line of C#, so I guess my
opinion is not widespread. I guess if we want to sugar coat, then
#(int x)(x*x) is sweeter than #(int x){ return x*x; } even if the
double syntax does not add anything else. I would have preferred the
second syntax, more generic at the extra cost of writting return and
; in the short versions.
To be really useful, lambdas can take variables from the scope in
where they are defined and from a closure. Being consistent with
Inner classes, lambdas are restricted to capturing 'effectively
final' variables. Consistency with the previous features of the
language is a nice feature, but for sweetness, it would be nice to be
able to capture variables that can be reassigned. For that purpose,
they are considering that variables present in the context and
annotated with @Shared will be captured by-reference, allowing
assignments. To me this seems weird as how a lambda can use a variable
is determined at the place of declaration of the variable rather than
where the lambda is defined. A single variable could be used in more
than one lambda and this forces the same behavior in all of them.
Lambdas try to simulate actual function objects, but the proposal does
not get completely there: to keep the parser simple, since up to now
an identifier denotes either an object or a method that has been kept
consistent and calling a lambda requires using a ! after the lambda
name: #(int x)(x*x)!(5) will return 25. This brings a new syntax
to use for lambdas that differ from the rest of the language, where
! stands somehow as a synonim for .execute on a virtual generic
interface Lambda<Result,Args...> but, why not make it complete?
A new generic (virtual) interface Lambda could be created. It would
have to be virtual as the interface is not a real interface, but a
family of such: Lambda<Return>, Lambda<Return,Arg1>,
Lambda<Return,Arg1,Arg2>... They could define a single execution
method, which I would like to be like C++ operator(), but if that is
a burden then any other name would be fine, embracing the ! as a
shortcut for the method execution:
interface Lambda<R> {
R exec();
}
interface Lambda<R,A> {
R exec( A a );
}
Then the compiler need only translate identifier!(args) to
identifier.exec( args ), which is simple. The translation of the
lambda syntax would require the compiler to identify the proper
interface being implemented and could be matched as:
#( int x )(x *x)
// translated to
new Lambda<int,int>{ int exec( int x ) { return x*x; } }
This would also allow users to define Inner classes that can be used
as lambdas, in more complex situations. For example, if lambda
function needed to capture a variable annotated as @Shared in a
read-only manner, or maintain the state of the captured object at the
place of capture, manual implementation of the Lambda would be
available:
new Lambda<int,int>{ int value = context_value;
int exec( int x ) { return x * context_value; }
};
In a manner similar to what the current Inner classes definition is,
and thus being natural to current Java users. This could be used,
for example, in a loop to generate multiplier lambdas:
Lambda<int,int> array[10] = new Lambda<int,int>[10]();
for (int i = 0; i < 10; ++i ) {
array[i] = new Lambda<int,int>{ final int multiplier = i;
int exec( int x ) { return x * multiplier; }
};
}
// note this is disallowed in the current proposal, as `i` is
// not effectively final and as such cannot be 'captured'. Also
// if `i` was marked @Shared, then all the lambdas would share
// the same `i` as the loop and thus would produce the same
// result: multiply by 10 --probably quite unexpectedly.
//
// I am aware that this can be rewritten as:
// for (int ii = 0; ii < 10; ++ii ) { final int i = ii; ...
//
// but that is not simplifying the system, just pushing the
// complexity outside of the lambda.
This would allow usage of lambdas and methods that accept lambdas both
with the new simple syntax: #(int x){ return x*x; } or with the more
complex manual approach for specific cases where the sugar coating
interferes with the intended semantics.
Overall, I believe that the lambda proposal can be improved in
different directions, that the way it adds syntactic sugar is a
leaking abstraction (you have deal externally with issues that are
particular to the lambda) and that by not providing a lower level
interface it makes user code less readable in use cases that do not
perfectly fit the simple use case.
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