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From Jesper Steen Møller <>
Subject Re: About the callable native lambda
Date Thu, 01 Feb 2018 12:00:50 GMT
(Sorry, first version went out too soon, thanks to the silly, silly Touch Bar)

> On 31 Jan 2018, at 23.01, MG <> wrote:
> Hi Jesper,
> seen from a Groovy user perspective your proposal seems to make sense to me.
> (I would at the same time hope you do not dent Daniel Sun's enthusiasm too much, because
as far as I can tell he is currently doing alot of the heavy lifting in this project :-) )
Yes, he is - and the lambda work has come a long way - I'm testing some scenarios, so i hope
to lift a little, too!

> How do you think what you propose fares with regards to "the principle of least surprise"
? Are there any cases where this could lead to hard to track bugs / unexpected behavior ?
From the top of my hat, that would be my biggest concern...

I see static and dynamic as different concerns: For dynamic invocation, there could be some
surprises, like example posted by 'avafanasiev' on GitHub:

class R implements Runnable { void run(){}}

def m(Runnable r1, R r2, r3) {

m(new R(), new R(), new R())

Currently, in the 'native-lambda' branch, r1() succeeds, whereas the latter two fail to run/compile
(depending on dynamic/static compilation), as 'avafanasiev' commented on. I do find that confusing:
Dynamically, my opinion is that the three should work the same.

For static compilation, r1() and r2() should work IMHO, and r3() should be rejected. This
shouldn't surprise anyone, I think.

Also, surprise-wise, consider:

class Q implements Runnable, Supplier<String> { void run(){}; String get() { "Q" } }

def n(Runnable q1, Supplier <String> q2, Q q2, q3) {

n(new Q(), new Q(), new Q(), new Q())

I'm thinking that all four should FAIL dynamically, but statically, q1 and q2 should work,
using the appropriate interface.

> "...only as a fallback if  doesn't exist" seems like the safer choice in this
regard. Default behavior could also be made overridable by a class annotation (then it would
become the programmer's responsibility, to make sure least surprise is not violated).
> Without that the question to me is: Would choosing "fallback if  doesn't exist"
weaken the elegance of the whole concept too much ?

I don't think it would weaken the elegance.
We hardly need annotations - we could simply use the presence of GroovyCallable to find out
which objects would prefer to be call()'ed directly.


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