# Another Lens Optics type difference
**URL:** https://discourse.haskell.org/t/another-lens-optics-type-difference/13335
**Category:** Learn
**Created:** [November 29, 2025, 12:02am UTC](https://discourse.haskell.org/t/another-lens-optics-type-difference/13335 "2025-11-29T00:02:35Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@feature-not-a-bug](https://discourse.haskell.org/u/feature-not-a-bug)
#### Post date: [November 29, 2025, 12:02am UTC](https://discourse.haskell.org/t/another-lens-optics-type-difference/13335/1 "2025-11-29T00:02:35Z")
I’m working through the Servant tutorial using `genericServeTWithContext` to serve an endpoint that’s serving a `JWKSet` as JSON along the lines of this guide [Hoist Server With Context for Custom Monads — Servant documentation](https://docs.servant.dev/en/latest/cookbook/hoist-server-with-context/HoistServerWithContext.html)
```haskell
jwksServe :: ReaderT Env Handler JWKSet
jwksServe = do
t <- asks getJwks
pure $ jwksPub t
jwksPub :: JWKSet -> JWKSet
jwksPub (JWKSet a) = do -- `a` here is [JWK]
let pks = mapM (view asPublicKey) a -- Succeeds with `view` from Lens
case pks of
Just x -> JWKSet x
Nothing -> JWKSet []
```
However if I use Optics with
```haskell
let pks = mapM (view (lensVL asPublicKey)) a -- Fails with `view` from Optics
```
I get the following error leaving me stumped
```haskell
• Could not deduce ‘Data.Functor.Contravariant.Contravariant f’
arising from a use of ‘asPublicKey’
from the context: Functor f
bound by a type expected by the context:
LensVL JWK JWK (Maybe JWK) (Maybe JWK)
at src/Server.hs:189:32-42
Possible fix:
add (Data.Functor.Contravariant.Contravariant f) to the context of
a type expected by the context:
LensVL JWK JWK (Maybe JWK) (Maybe JWK)
• In the first argument of ‘lensVL’, namely ‘asPublicKey’
In the first argument of ‘view’, namely ‘(lensVL asPublicKey)’
In the first argument of ‘mapM’, namely
‘(view (lensVL asPublicKey))’
|
189 | let pks = mapM (view (lensVL asPublicKey)) a -- Fails with Optics
| ^^^^^^^^^^^
```
---
### Author:  [@tomjaguarpaw](https://discourse.haskell.org/u/tomjaguarpaw)
#### Post date: [November 29, 2025, 9:38am UTC](https://discourse.haskell.org/t/another-lens-optics-type-difference/13335/2 "2025-11-29T09:38:16Z")
The reason that this doesn’t work is that `asPublicKey` is a `Getter` not a `Lens`. You really need` getterVL` for this, but it doesn’t exist. I don’t know why not, so I opened [an issue](https://github.com/well-typed/optics/issues/533).
In the meantime maybe you could just paste the definition into your project:
```haskell
-- | Type synonym for a van Laarhoven getter.
type GetterVL s a =
forall f. (Contravariant f, Functor f) => (a -> f a) -> s -> f s
-- | Build a getter from the van Laarhoven representation.
getterVL :: GetterVL s a -> Getter s a
getterVL g = Optic (getting g)
getting :: (Profunctor p, Bicontravariant p) => ((s -> Const s s) -> a -> Const s a) -> p i s c1 -> p i a c2
getting g = lmap (getConst . g Const) . rphantom
```
(for `rphantom` you’ll need to import `Optics.Internal.Bi`)
---
### Author:  [@feature-not-a-bug](https://discourse.haskell.org/u/feature-not-a-bug)
#### Post date: [November 29, 2025, 11:31am UTC](https://discourse.haskell.org/t/another-lens-optics-type-difference/13335/3 "2025-11-29T11:31:31Z")
That worked perfectly, thanks. Tested with ghc 9.12
---
### Author:  [@arybczak](https://discourse.haskell.org/u/arybczak)
#### Post date: [July 22, 2026, 6:21pm UTC](https://discourse.haskell.org/t/another-lens-optics-type-difference/13335/4 "2026-07-22T18:21:00Z")
This will be fixed in optics-core-0.5 (see [Add support for converting to/from VL getters and folds by arybczak · Pull Request #430 · well-typed/optics · GitHub](https://github.com/well-typed/optics/pull/430)).