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Add Reducible for Eval and Id #1475
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should this be combined with the monad/comonad above?
I don't think we can ever implement
Traverse[Eval]
(I'd love to be wrong about that), so it didn't have the same ambiguous implicit concern thatId
has (sinceTraverse[Id]
also extendsFoldable[Id]
, we don't wantReducible[Id]
we have to either use priority, or unify them in the same instance).There was a problem hiding this comment.
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Couldn't a fairly trivial
Traverse[Eval]
be implemented wheretraverse
looks something like this?It's a bit wonky, but the
Comonad
,Reducible
, etc instances are already based on eager calls to.value
, so I don't know if it's any different.There was a problem hiding this comment.
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It occurs to me that such a
Traverse
instance can be defined for any comonad, so it it's probably a known thing with some known properties to people who know more than I do :)There was a problem hiding this comment.
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It also occurs to me that a big difference between this
Traverse
instance and the instances that you've added is that yours shouldn't ever lead to stack overflows while this one is probably pretty likely to bite people with them.There was a problem hiding this comment.
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none of our instances call
.value
unless the return type requires it. It seems to me we should keep that since otherwise you lose the stack safety if Eval, so that is what I really meant. If you are willing to call.value
,Eval
is isomorphic toId
I think, but the best practice ofEval
(though perhaps not required, but the real rule might somewhat complex and difficult to verify) is to only call.value
once at the end.There was a problem hiding this comment.
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I think that limiting ourselves to instances that (if they need to call
.value
at all) only call.value
once at the end is probably a good general rule. 👍There was a problem hiding this comment.
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Hmm sorry for all of the thinking out loud here, but I just realized that the
traverse
implementation above only calls.value
once, just as theReducible
instances do. Is there a reason that we would consider this to be different? Maybe because theEval
in the return type suggests that we aren't calling.value
?