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https://github.com/mstksg/lens-typelevel
Type-level lenses using singletons because why not
https://github.com/mstksg/lens-typelevel
Last synced: 8 days ago
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Type-level lenses using singletons because why not
- Host: GitHub
- URL: https://github.com/mstksg/lens-typelevel
- Owner: mstksg
- License: bsd-3-clause
- Created: 2018-10-20T22:21:48.000Z (about 6 years ago)
- Default Branch: master
- Last Pushed: 2018-12-19T06:54:16.000Z (almost 6 years ago)
- Last Synced: 2024-05-02T00:12:40.780Z (6 months ago)
- Language: Haskell
- Homepage: https://hackage.haskell.org/package/lens-typelevel
- Size: 1.74 MB
- Stars: 15
- Watchers: 4
- Forks: 1
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- Changelog: CHANGELOG.md
- License: LICENSE
Awesome Lists containing this project
README
# [lens-typelevel][]
[](https://hackage.haskell.org/package/lens-typelevel)
[](https://travis-ci.org/mstksg/lens-typelevel)
([Rendered off-hackage documentation][docs])
[docs]: https://mstksg.github.io/lens-typelevel/
van Laarhoven lenses at the type level using *[singletons][]* defunctionalization.
[lens-typelevel]: http://hackage.haskell.org/package/lens-typelevel
[singletons]: https://hackage.haskell.org/package/singletons
```haskell
ghci> :kind! '("hello", 6 ) & L1_ .~ 'True
'( 'True, 6 )
ghci> :kind! '("hello", 6 ) ^. L2_
6
ghci> :kind! '("hello", 6 ) ^. To_ SndSym0
6
ghci> :kind! '("hello", 'True ) & L2_ %~ NotSym0
'("hello", 'False )
ghci> :kind! '[ 'True, 'False, 'False ] & Traverse_ %~ NotSym0
'[ 'False, 'True, 'True ]
ghci> :kind! '("hello", '(6, 'False ) ) ^. L2_ .@ L1_
6
ghci> type TestList = '[ '("hello", 'True), '("world", 'False), '("curry", 'False)]
ghci> :kind! TestLst ^.. Traverse_ .@ L1_
'["hello", "world", "curry"]
ghci> :kind! '[] ^?! Traverse_
Error "Failed indexing into empty traversal"
ghci> :kind! '["hello", "world", "curry"] & IxList_ ('S 'Z) .~ "haskell"
'["hello", "haskell", "curry"]
```
It's pretty much the exact same representation as the *lens* library; it's
essentially an API-faithful port with the same representation and essentially
the same implementation. We even have `CloneLens_` and `CloneTraversal_`
implemented using type-level versions of `Context` and `Bazaar`:
```haskell
ghci> type CloneExample l = ('( 'True, 'False ) & CloneLens_ l %~ NotSym0)
^. CloneLens_ l
ghci> :kind! CloneExample L1_
'False
ghci> :kind! CloneExample L2_
'True
```
Using prefix function names:
```haskell
ghci> :kind! Set L1_ 'True '("hello", 6 )
'( 'True, 6 )
ghci> :kind! View L2_ '("hello", 6 )
6
ghci> :kind! View (To_ SndSym0) '("hello", 6 )
6
ghci> :kind! Over L2_ NotSym0 '("hello", 'True )
'("hello", 'False )
ghci> :kind! Over Traverse_ NotSym0 '[ 'True, 'False, 'False ]
'[ 'False, 'True, 'True ]
ghci> :kind! View (L2_ .@ L1_) '("hello", '(6, 'False ) )
6
ghci> type TestList = '[ '("hello", 'True), '("world", 'False), '("curry", 'False)]
ghci> :kind! ToListOf (Traverse_ .@ L1_) TestList
'["hello", "world", "curry"]
ghci> :kind! UnsafePreview Traverse_ '[]
Error "Failed indexing into empty traversal"
ghci> :kind! Set (IxList_ ('S 'Z)) "haskell" '["hello", "world", "curry"]
'["hello", "haskell", "curry"]
```
Defining lenses
---------------
There are two main ways to define optics.
First, you can write them by hand using `singletonsOnly`:
```haskell
$(singletonsOnly [d|
l1 :: Functor f => LensLike (a, c) (b, c) a b
l1 f (x, y) = (\x' -> (x', y)) <$> f x
l1Alt :: Functor f => LensLike (a, c) (b, c) a b
l1Alt = mkLens fst (\(_, y) x -> (x', y))
getFirst :: Getting a (a, b) a
getFirst = to fst
|])
```
This creates the *type families* `L1`, `L1Alt`, and `GetFirst`; however, these
aren't lenses, because they aren't partially applied. The actual lenses are
`L1Sym0`, `L1AltSym0`, and `GetFirstSym0`. As a convention, I
recommend aliasing the *actual* lenses with an underscore suffix:
```haskell
-- L1_ :: Functor f => LensLike f (a, c) (b, c) a b
type L1_ = L1Sym0
-- L1Alt_ :: Functor f => LensLike f (a, c) (b, c) a b
type L1Alt_ = L1AltSym0
-- GetFirst_ :: Getting a (a, b) a
type GetFirst_ = GetFirstSym0
```
The number after the `Sym` is determined by how many arguments you need to
apply to your function before you get to the actual lens. For example,
`IxList` requires one argument (the index) to get to the actual traversal, so
the definition in the library is:
```haskell
type IxList_ i = IxListSym1 i
```
Second, you can write them directly at the type level using combinators like
`MkLens_` and `To_`:
```haskell
type GetFirst_ = To_ FstSym0
```
(`FstSym0` is the promotion of `fst` from the *singletons* library)