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

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@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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