[Haskell-cafe] Polymorphic case for this lens transformer?

[Felix Kunzmann]

Hi Hans,

I think this code should work:

{-# LANGUAGE RankNTypes #-}{-# LANGUAGE TupleSections #-}
import Control.Applicativeimport Control.Lens
dependingOn :: Lens s t (x,a) (x,b) -> (x -> Lens a b c d) -> Lens s t
c ddependingOn l depending f = l (\ (x,a) -> (x,) <$> depending x f a)

The trick here is that Lens s t a b is just an type synonym for Functor f
=> (a -> f b) -> s -> f t.

I think it should also be a speed improvement on more complex lenses, only
going over the structure once whereas the use of lens would examine the
structure twice (first to get and then to set the value).

I hope this helps.

Felix
​

2014-10-28 15:40 GMT+01:00 Hans Höglund <hans at hanshoglund.se>:

> Hi all, I have been using this lens transformer http://lpaste.net/113159.
>
> However, I can't seem to find a way to  implement the polymorphic case
> (i.e. Lens instead of Lens'). What am I missing?
>
> Hans
>
> -
>
> Hans Höglund
> *Composer, conductor and developer*
>
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