Move stuff around + add lean and idris experiments

purescript/existentials/src/Main.purs

@@ -0,0 +1,31 @@
+module Main where
+
+import Prelude
+
+import Effect (Effect)
+import Effect.Console (log)
+
+
+main :: Effect Unit
+main = log "hello world"
+
+---------- Existentials
+type ShowConstraint a = Show a => a
+
+type Exists :: forall k. (k -> Type) -> Type
+type Exists c = forall r. (forall a. c a -> r) -> r
+
+type Showable = Exists ShowConstraint
+
+mkShowable :: forall a. Show a => a -> Showable
+mkShowable inner continue = continue inner
+
+demo1 :: Showable
+demo1 = mkShowable 1
+
+demo2:: Showable
+demo2 = mkShowable "a"
+
+demo3:: Showable
+demo3 = mkShowable true
+