Add record lenses (#59)

* Add record lenses

* Import fst, snd

* Use Lens synonym

* Update Record.purs

Author: paf31

bower.json

@@ -18,7 +18,8 @@
     "purescript-profunctor": "^3.0.0",
     "purescript-sets": "^3.0.0",
     "purescript-unsafe-coerce": "^3.0.0",
-    "purescript-transformers": "^3.0.0"
+    "purescript-transformers": "^3.0.0",
+    "purescript-symbols": "^3.0.0"
   },
   "devDependencies": {
     "purescript-console": "^3.0.0"

src/Data/Lens/Record.purs

@@ -0,0 +1,64 @@
+module Data.Lens.Record (prop) where
+
+import Prelude
+import Data.StrMap as S
+import Data.Lens (Lens, lens)
+import Data.Maybe (fromJust)
+import Data.Symbol (class IsSymbol, SProxy, reflectSymbol)
+import Partial.Unsafe (unsafePartial)
+import Unsafe.Coerce (unsafeCoerce)
+
+unsafeGet
+  :: forall r a
+   . String
+  -> Record r
+  -> a
+unsafeGet s = unsafePartial fromJust <<< S.lookup s <<< unsafeCoerce
+
+unsafeSet
+  :: forall r1 r2 a
+   . String
+  -> a
+  -> Record r1
+  -> Record r2
+unsafeSet s a = unsafeCoerce <<< S.insert s a <<< unsafeCoerce
+
+get
+  :: forall r r' l a
+   . IsSymbol l
+  => RowCons l a r' r
+  => SProxy l
+  -> Record r
+  -> a
+get l = unsafeGet (reflectSymbol l)
+
+set
+  :: forall r1 r2 r l a b
+   . IsSymbol l
+  => RowCons l a r r1
+  => RowCons l b r r2
+  => SProxy l
+  -> b
+  -> Record r1
+  -> Record r2
+set l = unsafeSet (reflectSymbol l)
+
+-- | Construct a (type-changing) lens for a record property, by providing a
+-- | proxy for the `Symbol` which corresponds to the property label.
+-- |
+-- | The lens is polymorphic in the rest of the row of property labels.
+-- |
+-- | For example:
+-- |
+-- | ```purescript
+-- | prop (SProxy :: SProxy "foo")
+-- |   :: forall a b r. Lens { foo :: a | r } { foo :: b | r } a b
+-- | ```
+prop
+  :: forall l r1 r2 r a b
+   . IsSymbol l
+  => RowCons l a r r1
+  => RowCons l b r r2
+  => SProxy l
+  -> Lens (Record r1) (Record r2) a b
+prop l = lens (get l) (flip (set l))

test/Main.purs

@@ -6,25 +6,26 @@ import Control.Monad.Eff.Console (CONSOLE, logShow)
 import Control.Monad.State (evalState, get)
 import Data.Distributive (class Distributive)
 import Data.Either (Either(..))
-import Data.Lens (_1, _2, _Just, _Left, collectOf, lens, traversed, view)
+import Data.Lens (view, traversed, _1, _2, _Just, _Left, lens, collectOf)
 import Data.Lens.Fold ((^?))
 import Data.Lens.Fold.Partial ((^?!), (^@?!))
-import Data.Lens.Grate (cloneGrate, grate, zipWithOf)
+import Data.Lens.Grate (Grate, cloneGrate, grate, zipWithOf)
 import Data.Lens.Index (ix)
 import Data.Lens.Lens (ilens, IndexedLens, cloneIndexedLens)
+import Data.Lens.Record (prop)
 import Data.Lens.Setter (iover)
-import Data.Lens.Types (Grate)
 import Data.Lens.Zoom (Traversal, Traversal', Lens, Lens', zoom)
 import Data.Maybe (Maybe(..))
+import Data.Symbol (SProxy(..))
 import Data.Tuple (Tuple(..), fst, snd)
 import Partial.Unsafe (unsafePartial)
 
 -- Traversing an array nested within a record
 foo :: forall a b r. Lens { foo :: a | r } { foo :: b | r } a b
-foo = lens _.foo (_ { foo = _ })
+foo = prop (SProxy :: SProxy "foo")
 
 bar :: forall a b r. Lens { bar :: a | r } { bar :: b | r } a b
-bar = lens _.bar (_ { bar = _ })
+bar = prop (SProxy :: SProxy "bar")
 
 type Foo a = { foo :: Maybe { bar :: Array a } }