[ceylon-dev] Generic constraints and compiler errors

[Paco Soberón]

I just found that, while this code raises a compiler error, as expected:

value tuple = Tuple<Integer, Boolean>(true, []); // Boolean is not
assignable to upper bound Integer

This other code compiles without problems:

void foo(Tuple<Integer, Boolean> tuple) {}
Tuple<Integer, Boolean> bar() => nothing;

Narrowing my observation to a simpler generic class, this code doesn't
compile:

class Generic<Parent, Child>()
given Child satisfies Parent {}

value generic = Generic<Integer, Boolean>();

But this code does:

void foo(Generic<Integer, Boolean> generic) {}
Generic<Integer, Boolean> bar() => nothing;

Of course, I'm aware that even if it compiles, the code is completely
useless, since you're not going to be able to create a valid object to pass
to foo() or return from bar().

But I am still curious... why the compiler doesn't complain? Doesn't it
check the generic constraints when declaring a type? Does it only do it
when instantiating an object?

Regards.

['Lucas Werkmeister' via ceylon-dev]

Looks like this is https://github.com/ceylon/ceylon/issues/3641
(originally https://github.com/ceylon/ceylon-spec/issues/535)

Post by Paco Soberón
value tuple = Tuple<Integer, Boolean>(true, []); // Boolean is not
assignable to upper bound Integer
void foo(Tuple<Integer, Boolean> tuple) {}
Tuple<Integer, Boolean> bar() => nothing;
Narrowing my observation to a simpler generic class, this code doesn't
class Generic<Parent, Child>()
given Child satisfies Parent {}
value generic = Generic<Integer, Boolean>();
void foo(Generic<Integer, Boolean> generic) {}
Generic<Integer, Boolean> bar() => nothing;
Of course, I'm aware that even if it compiles, the code is completely
useless, since you're not going to be able to create a valid object to
pass to foo() or return from bar().
But I am still curious... why the compiler doesn't complain? Doesn't
it check the generic constraints when declaring a type? Does it only
do it when instantiating an object?
Regards.
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[Paco Soberón]

Interesting stuff. Thanks for answering!

2016-04-01 0:04 GMT+02:00 'Lucas Werkmeister' via ceylon-dev <

Post by 'Lucas Werkmeister' via ceylon-dev
Looks like this is https://github.com/ceylon/ceylon/issues/3641
(originally https://github.com/ceylon/ceylon-spec/issues/535)
value tuple = Tuple<Integer, Boolean>(true, []); // Boolean is not
assignable to upper bound Integer
void foo(Tuple<Integer, Boolean> tuple) {}
Tuple<Integer, Boolean> bar() => nothing;
Narrowing my observation to a simpler generic class, this code doesn't
class Generic<Parent, Child>()
given Child satisfies Parent {}
value generic = Generic<Integer, Boolean>();
void foo(Generic<Integer, Boolean> generic) {}
Generic<Integer, Boolean> bar() => nothing;
Of course, I'm aware that even if it compiles, the code is completely
useless, since you're not going to be able to create a valid object to pass
to foo() or return from bar().
But I am still curious... why the compiler doesn't complain? Doesn't it
check the generic constraints when declaring a type? Does it only do it
when instantiating an object?
Regards.
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