# [lens-typelevel][]

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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> :kind! '("hello", 6) ^. CloneLens_ L1_
"hello"
```

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"]
```