BigInt doesn't support ++ and --

[JayXon]

  var i = BigInt.zero;
  print(++i);

Error: A value of type 'int' can't be assigned to a variable of type 'BigInt'.
 - 'BigInt' is from 'dart:core'.
  print(++i);
        ^

bigint in js supports it

[mraleph]

/cc @lrhn

[lrhn]

That's because BigInt.operator+ does not accept the integer 1 as argument, and both ++v and v++ are defined to perform v = v + 1;.

Nothing much we can do about that without changing the parameter type of BigInt.operator+, which we can't without losing type safety (we'd have to change the parameter type to Object or dynamic, and check that the incoming value is an int or a BigInt).

No current plan to change the behavior.

[JayXon]

I get why this isn't working, but why can't we change ++v to perform v = v + BigInt.one; if v is a BigInt?

[lrhn]

We could special-case BigInt, which is something we have so far refrained from doing.

Doing so will be a language change, which requires the special case to be implemented in all our tools. It's not just a quick library change.

I'd loathe to introduce special casing, and then not get anything from it other than helping BigInt with ++.
I'd at least want to help the fixnum numbers as well, and any other number type that people want to cook up, so that we get some more bang for the special-casing buck.

Maybe:

For e++ (etc.) let T be the static type of e. It's a compile-time error of T does not have an operator+.
Let S be the type of the parameter of T.operator+.
If int is assignable to S, let o be the integer 1.
Otherwise if S is a type whose declaration contains a static constant member named one,
whose type is assignable to S, let o bet the value of S.one.
Otherwise, it's a compile-time error.
... more static type checking, like return type of T.operator+ being assignable to declared type of e ...

Then e++ is evaluated as follows:
Evaluate e to a value v.
Invoke operator+ on v with o as argument, let r be the result.
Assign r to e.

I don't think we'll actually do this (probably still too low reward for the effort, there are other higher-priority things to do to the language.)

[JayXon]

Thanks, that sounds reasonable, if it's a priority thing, can we leave this issue open?

[lrhn]

We can keep it open as a request for allowing user-classes to use ++/-- as well, but then it should be a language issue (in the language repository), so I'll just go file an issue there instead: dart-lang/language#2576

[JayXon]

Thank you!