1 files changed, 51 insertions, 0 deletions
diff --git a/src/Expr.hs b/src/Expr.hs
new file mode 100644
index 0000000..fa17158
--- /dev/null
+++ b/src/Expr.hs
@@ -0,0 +1,51 @@
+module Expr (
+    Expr(..),
+    ExprContext(..),
+    getContext,
+
+    addDependency,
+    collectDependencies,
+
+    exprIO,
+) where
+
+import Data.Kind
+
+
+data Expr c a where
+    Pure :: a -> Expr c a
+    App :: Expr c (a -> b) -> Expr c a -> Expr c b
+    GetContext :: Expr c c
+    AddDependency :: ExprContext c => ExprDependency c -> Expr c a -> Expr c a
+    ExprIO :: IO a -> Expr c a
+
+instance Functor (Expr c) where
+    fmap f x = Pure f <*> x
+
+instance Applicative (Expr c) where
+    pure = Pure
+    (<*>) = App
+
+
+class Monoid (ExprDependency c) => ExprContext c where
+    type ExprDependency c :: Type
+
+
+getContext :: Expr c c
+getContext = GetContext
+
+
+addDependency :: ExprContext c => ExprDependency c -> Expr c a -> Expr c a
+addDependency = AddDependency
+
+collectDependencies :: ExprContext c => Expr c a -> ExprDependency c
+collectDependencies = \case
+    Pure {} -> mempty
+    App f x -> collectDependencies f <> collectDependencies x
+    GetContext {} -> mempty
+    AddDependency d x -> d <> collectDependencies x
+    ExprIO {} -> mempty
+
+
+exprIO :: IO a -> Expr c a
+exprIO = ExprIO