add naive Terminal type

nt/primitives/bootstrap/default.nix

@@ -39,6 +39,7 @@ in
       parse = ./parse/bootstrap.nix;
 
       maybe = ./maybe.nix;
+      terminal = ./terminal.nix;
       trapdoor = ./trapdoor.nix;
     }
   ]

nt/primitives/bootstrap/terminal.nix

@@ -0,0 +1,38 @@
+{...}: let
+  inherit
+    (builtins)
+    attrNames
+    concatStringsSep
+    isAttrs
+    typeOf
+    ;
+in rec {
+  # Naive Terminal Type
+  # NOTE: preserves lazy eval for _value
+  Terminal = value: {
+    _value = value;
+  };
+
+  # Type Checking
+  isTerminal = T: isAttrs T && attrNames T == ["_value"];
+  # XXX: TODO: make a pretty toString function
+  # XXX: TODO: make a pretty toString function
+  # XXX: TODO: make a pretty toString function
+  # XXX: TODO: make a pretty toString function
+  enfIsTerminal = T: msg: let
+    throw' = got: throw "${msg}: expected naive type Terminal but got ${got}";
+    attrs =
+      attrNames T
+      |> map (name: "\"${name}\"")
+      |> concatStringsSep ", ";
+  in
+    if isAttrs T
+    then isTerminal T || throw' "attribute set with structure [${attrs}]"
+    else throw' "value \"${toString T}\" of primitive type \"${typeOf T}\"";
+
+  # Unwrap Operation
+  # Lift a value out of the Terminal context.
+  unwrapTerminal = T:
+    assert enfIsTerminal T "unwrapTerminal";
+      T._value_;
+}