diff --git a/default.nix b/default.nix
index 7c536a03..ea7b2557 100644
--- a/default.nix
+++ b/default.nix
@@ -9,6 +9,8 @@ let
     git-hooks
     gitignore
     ;
+  inherit (import sources.flake-inputs) import-flake;
+  inputs = (import-flake { src = ./.; }).inputs;
   inherit (pkgs) lib;
   pre-commit-check =
     (import "${git-hooks}/nix" {
@@ -67,6 +69,7 @@ in
   # re-export inputs so they can be overridden granularly
   # (they can't be accessed from the outside any other way)
   inherit
+    inputs
     sources
     system
     pkgs
diff --git a/deployment/data-model-test.nix b/deployment/data-model-test.nix
index 16a5caaf..3f24df94 100644
--- a/deployment/data-model-test.nix
+++ b/deployment/data-model-test.nix
@@ -1,9 +1,12 @@
 let
-  inherit (import ../default.nix { }) pkgs;
+  inherit (import ../default.nix { }) pkgs inputs;
   inherit (pkgs) lib;
   eval =
     module:
     (lib.evalModules {
+      specialArgs = {
+        inherit inputs;
+      };
       modules = [
         module
         ./data-model.nix
@@ -11,8 +14,6 @@ let
     }).config;
 in
 {
-  _class = "nix-unit";
-
   test-eval = {
     expr =
       let
diff --git a/deployment/data-model.nix b/deployment/data-model.nix
index 81ce96c2..af6eda78 100644
--- a/deployment/data-model.nix
+++ b/deployment/data-model.nix
@@ -1,6 +1,7 @@
 {
   lib,
   config,
+  inputs,
   ...
 }:
 let
@@ -15,6 +16,20 @@ let
     submodule
     str
     ;
+
+  functionType = import ./interface.nix;
+  application-resources = {
+    options.resources = mkOption {
+      type = attrsOf (
+        attrTag (
+          lib.mapAttrs' (name: resource: {
+            ${name} = mkOption { type = resource.consumer; };
+          }) config.resources
+        )
+      );
+    };
+  };
+  nixops4Deployment = inputs.nixops4.modules.nixops4Deployment.default;
 in
 {
   options = {
@@ -51,7 +66,19 @@ in
             };
             config-mapping = mkOption {
               description = "Mapping of application configuration to deployment resources, a description of what an application needs to run";
-              # TODO: type = (submodule application.config.module) -> (attrsOf (attrTag (lib.mapAttrs' (name: resource: { ${name} = mkOption { type = resource.consumer; }; }) config.resources))) /* something like that */
+              type = application.config.function.implementation;
+            };
+            # TODO: somewhere we still need to
+            # - apply = { implementation = config-mapping; input = <application configuration value from operator>; }
+            # - apply.output
+            function = mkOption {
+              description = "Function type for the mapping from application configuration to required resources";
+              type = submodule functionType;
+              readOnly = true;
+              default = {
+                input-type = application.config.module;
+                output-type = application-resources;
+              };
             };
           };
         })
@@ -75,8 +102,18 @@ in
             };
             resource-mapping = mkOption {
               description = "Mapping of resources required by applications to available resources; the result can be deployed";
+              type = environment.config.function.implementation;
               # TODO: type = consumer-resources /* same as the output of application.config-mapping, should be in a `let` */ -> nixops4Deployment
             };
+            function = mkOption {
+              description = "Function type for the mapping from resources to a (NixOps4) deployment";
+              type = submodule functionType;
+              readOnly = true;
+              default = {
+                input-type = application-resources;
+                output-type = submodule nixops4Deployment;
+              };
+            };
           };
         })
       );
diff --git a/deployment/interface.nix b/deployment/interface.nix
new file mode 100644
index 00000000..225526d1
--- /dev/null
+++ b/deployment/interface.nix
@@ -0,0 +1,49 @@
+/**
+  Modular function type
+*/
+{ config, ... }:
+{
+  options = {
+    input-type = mkOption {
+      type = deferredModule;
+    };
+    output-type = mkOption {
+      type = deferredModule;
+    };
+    implementation = mkOption {
+      type = optionType;
+      readOnly = true;
+      default = implementationTo (
+        submodule (implementation: {
+          options = {
+            input = mkOption {
+              type = submodule config.input-type;
+            };
+            output = mkOption {
+              type = submodule config.output-type;
+            };
+          };
+        })
+      );
+    };
+    apply = mkOption {
+      type = optionType;
+      readOnly = true;
+      default = submodule (apply: {
+        options = {
+          implementation = mkOption {
+            type = config.implementation;
+          };
+          input = mkOption {
+            type = submodule config.input-type;
+          };
+          output = mkOption {
+            type = submodule config.output-type;
+            readOnly = true;
+            default = (apply.config.implementation apply.config.input).output;
+          };
+        };
+      });
+    };
+  };
+}