[Haskell-cafe] Newtype derivation understanding problem

[Konstantine Rybnikov]

Yes, sorry! I also for some reason added "deriving MonadCatchIO" which is
not there :) I needed to see it's not derived, that's why I should do a
search for it.

Thank you.


On Thu, Jul 31, 2014 at 2:00 PM, Mikael Brockman <mbrock at goula.sh> wrote:

> Konstantine Rybnikov <k-bx at k-bx.com> writes:
>
> > Excuse me if the question is silly.
> >
> > Thought this MonadCatchIO will be removed in next version of snap,
> > current version has it and I can't understand this derivation: [0]
> >
> > L.Lensed seems like just a newtype wrapper around a function [1], and
> > there is no instance of MonadCatchIO for a function.
> >
> > How is it derived?
>
> You probably just missed it, but there's a MonadCatchIO instance for
> L.Lensed, and so the derived instance for Handler is trivial.
>
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