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-rw-r--r--src/CHAD/AST.hs3
1 files changed, 3 insertions, 0 deletions
diff --git a/src/CHAD/AST.hs b/src/CHAD/AST.hs
index 2f4b5c2..be7f95e 100644
--- a/src/CHAD/AST.hs
+++ b/src/CHAD/AST.hs
@@ -139,6 +139,9 @@ data Expr x env t where
EError :: x a -> STy a -> String -> Expr x env a
deriving instance (forall ty. Show (x ty)) => Show (Expr x env t)
+-- | A (well-typed, well-scoped) expression using De Bruijn indices. The full
+-- 'Expr' type is parametrised on an indexed type of "additional info" (@x@);
+-- 'Ex' sets this to nothing.
type Ex = Expr (Const ())
ext :: Const () a