Flow 0.50.0 throws errors in kefir

[EvHaus]

After upgrading to Flow 0.50.0 I am now getting the following Flow errors coming from kefir:

node_modules/kefir/dist/kefir.js.flow:40
 40:     next?: (value: V) => any,
                        ^ V. invariant position (expected `V` to occur only covariantly)

node_modules/kefir/dist/kefir.js.flow:41
 41:     error?: (error: E) => any,
                         ^ E. invariant position (expected `E` to occur only covariantly)

node_modules/kefir/kefir.js.flow:40
 40:     next?: (value: V) => any,
                        ^ V. invariant position (expected `V` to occur only covariantly)

node_modules/kefir/kefir.js.flow:41
 41:     error?: (error: E) => any,
                         ^ E. invariant position (expected `E` to occur only covariantly)

[mAAdhaTTah]

There are two articles about this on Flowtype that should help here:

https://flow.org/en/docs/lang/variance/
https://flow.org/en/docs/lang/depth-subtyping/

I think the functions may just need to add + to the generics passed into them.

[rpominov]

I have a fix in #246 , but not sure if it's correct. As far as I understand Flow thinks that this code is unsafe because subscribe() may mutate passed object, and I've tricked Flow to think that It's alright using interface.

See also facebook/flow#2354

/cc @hallettj @agentme

[Macil]

Yeahh variance errors are fun. I think that pull request looks right.

[rpominov]

Ok, probably should merge it.