[Haskell-cafe] lens and Maybe value

[Oleg Grenrus]

over can be only strict for traversals. Consider mapped [1], you cannot 
turn ordinary `fmap` into strict one.

[1]: 
https://hackage.haskell.org/package/lens-5.2/docs/Control-Lens-Combinators.html#v:mapped

Secondly, don't ask for ALens, if you don't need it. LensLike Solo is 
enough (and works with traversals "for free").

- Oleg

On 20.12.2022 19.43, David Feuer wrote:
> That's nice and clean, but it leaves a thunk inside (since the field 
> is lazy). Is there a strict version of `over` that avoids this? We 
> could write one thus:
>
> over' :: ALens s t a b -> (a -> b) -> s -> t
> over' l f = getSolo . cloneLens l (\old -> Solo $! f old)
>   where
>     getSolo (Solo x) = x
>
> On Tue, Dec 20, 2022, 11:33 AM Tom Ellis 
> <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
>
>     On Tue, Dec 20, 2022 at 05:25:35PM +0100, PICCA Frederic-Emmanuel
>     wrote:
>     > > Please clarify your question. Your code doesn't compile, and I
>     can't tell
>     > > what it's supposed to do.
>     >
>     > replace a (Maybe a) value in a data using lens only if the new
>     (Maybe a) is a Just
>
>     How about
>
>         over a (mb <|>)
>
>     ?
>
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