Move MonadIO to base

[Heinrich Apfelmus]

David Menendez wrote:
> wren ng thornton wrote:
>> Heinrich Apfelmus wrote:
>>
>>> In particular, it's not possible to implement
>>>
>>>    lift :: (Monad m, MonadTrans t) => m a -> t m a
>>
>> Why not?
>> * morph       says m(t m a) is a subset of (t m a)
>> * Monad m     says we can fmap :: (a->b) -> (m a->m b)
>> * Monad (t m) says we can return :: a -> t m a
>>
>>    lift ma = morph (\k -> k (fmap return ma))
> 
> Maybe something like this?
> 
> lift m = morph (\k -> m >>= k . return)
>     -- n.b., return and >>= are from different monads

Nice! And it's consistent with the proposed equation

    morph (\down -> m >>= h down)  =  lift m >>= morph . flip h

which says that effects that happen before applying  down  can be lifted
out of  morph .


Regards,
Heinrich Apfelmus

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