diff --git a/src/dune_vars.mli b/src/dune_vars.mli
new file mode 100644
index 00000000..29190372
--- /dev/null
+++ b/src/dune_vars.mli
@@ -0,0 +1 @@
+val deps : string -> int option
diff --git a/src/dune_vars.mll b/src/dune_vars.mll
new file mode 100644
index 00000000..8481d9b5
--- /dev/null
+++ b/src/dune_vars.mll
@@ -0,0 +1,13 @@
+{
+  open Stdune
+}
+
+let digit     = ['0'-'9']
+
+rule deps = parse
+  | "deps[" (digit+ as i) "]" eof { Some (Int.of_string_exn i) }
+  | _  { None }
+
+{
+let deps s = deps (Lexing.from_string s)
+}