Add typelevel boolean if

Author: LiamGoodacre

src/Type/Data/Boolean.purs

@@ -12,8 +12,12 @@ module Type.Data.Boolean
   , or
   , class Not
   , not
+  , class If
+  , if_
   ) where
 
+import Type.Proxy (Proxy(..))
+
 foreign import kind Boolean
 foreign import data True :: Boolean
 foreign import data False :: Boolean
@@ -64,3 +68,15 @@ instance notFalse :: Not False True
 
 not :: forall i o. Not i o => BProxy i -> BProxy o
 not _ = BProxy
+
+-- | If - dispatch based on a boolean
+class If (bool :: Boolean)
+         (onTrue :: Type)
+         (onFalse :: Type)
+         (output :: Type) |
+         bool -> onTrue onFalse output
+instance ifTrue :: If True onTrue onFalse onTrue
+instance ifFalse :: If False onTrue onFalse onFalse
+
+if_ :: forall b t e o. If b t e o => BProxy b -> Proxy t -> Proxy e -> Proxy o
+if_ _ _ _ = Proxy