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Why EitherT is not covariant for the input types as Either from scala #2538

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REASY opened this issue Sep 27, 2018 · 3 comments
Closed

Why EitherT is not covariant for the input types as Either from scala #2538

REASY opened this issue Sep 27, 2018 · 3 comments

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@REASY
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REASY commented Sep 27, 2018

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@ceedubs
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ceedubs commented Sep 29, 2018

This has been discussed in #2310 and #556. I'm going to leave this issue open, because the fact that it keeps coming up is a sign that we really need to document this. I'll try to put something together.

ceedubs added a commit to ceedubs/typelevel.github.com that referenced this issue Sep 30, 2018
@ceedubs
Add a blog post aboutn variance and monad transformers
e68556b 
Every once in a while [people](typelevel/cats#556) [ask](typelevel/cats#2310) [about](typelevel/cats#2538) this.

I thought that it would be nice to provide a more detailed answer in a
single place so we can link to it from the Cats FAQ.

Note: I had to add the `-Ypartial-unification` flag for one of the code
samples to compile. It's quite possible that I should be doing this a
different way than I did.
@ceedubs ceedubs mentioned this issue Sep 30, 2018
Merged
ceedubs added a commit to ceedubs/typelevel.github.com that referenced this issue Sep 30, 2018
@ceedubs
Add a blog post about variance and monad transformers
f256419 
Every once in a while [people](typelevel/cats#556) [ask](typelevel/cats#2310) [about](typelevel/cats#2538) this.

I thought that it would be nice to provide a more detailed answer in a
single place so we can link to it from the Cats FAQ.

Note: I had to add the `-Ypartial-unification` flag for one of the code
samples to compile. It's quite possible that I should be doing this a
different way than I did.
@ceedubs
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ceedubs commented Sep 30, 2018

I've submitted typelevel/typelevel.github.com#233 and if it gets merged we can point to the blog post in the Cats FAQ.

ceedubs added a commit to ceedubs/cats that referenced this issue Oct 1, 2018
@ceedubs
Add FAQ for monad transformer variance
d100dce 
Resolves typelevel#2538.
@ceedubs ceedubs mentioned this issue Oct 1, 2018
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@REASY
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REASY commented Oct 2, 2018

Thanks, @ceedubs !

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@ceedubs @REASY