diff --git a/generics/generics.js b/generics/generics.js
index 58cd914..8a4754f 100644
--- a/generics/generics.js
+++ b/generics/generics.js
@@ -1,8 +1,9 @@
 import { lsType } from "../structures/list_common.js";
 import { fnType } from "../metacircular.js";
-import { deepEqual, pretty } from "../util.js";
+import { deepEqual, DefaultMap, pretty } from "../util.js";
+import { numDictType } from "../typeclasses/num_type.js";
 
-const genericTypeRegistry = new DefaultMap(underlyingType => ({generic: underlyingType}));
+const genericTypeRegistry = new DefaultMap(underlyingType => ({ generic: underlyingType }));
 
 // type constructor for generic kinds,
 //   for instance:
@@ -23,15 +24,15 @@ export const makeGeneric = callback => {
   const c = Symbol('c');
   const d = Symbol('d');
   const e = Symbol('e');
-  const type = callback(a,b,c,d,e);
+  const type = callback(a, b, c, d, e);
   return {
-    typeVars: occurring(type, new Set([a,b,c,d,e])),
+    typeVars: occurring(type, new Set([a, b, c, d, e])),
     type,
   };
 };
 
 // From the given set of type variables, return only those that occur in the given type.
-const occurring = (type, typeVars) => {
+export const occurring = (type, typeVars) => {
   if (typeVars.has(type)) {
     return new Set([type]);
   }
@@ -130,9 +131,24 @@ export const unify = (
   }
 
   if (formalType.numDict !== undefined) {
-
+    if (actualType.numDict === undefined) {
+      throw new Error(`cannot assign ${pretty(actualType)} to ${pretty(formalType)}`);
+    }
+    else {
+      // both are NumDict type
+      const underlyingType = unify(
+        {typeVars: formalTypeVars, type: formalType.numDict},
+        {typeVars: actualTypeVars, type: actualType.numDict});
+      return {
+        substitutions: underlyingType.substitutions,
+        typeVars: new Set([
+          ...actualTypeVars,
+          ...formalTypeVars].filter(a => !underlyingType.substitutions.has(a))),
+        type: numDictType(underlyingType.type),
+      };
+    }
   }
-  throw new Error("i don't know what to do :(")
+  throw new Error("i don't know what to do :(");
 };
 
 // export const matchConcrete = ({typeVars, type: formalType}, actualType) => {
@@ -167,4 +183,4 @@ export const assign = (genFnType, paramType) => {
     typeVars: matchedInType.typeVars,
     type: substitutedOutType,
   };
-}
+};
diff --git a/generics/types.js b/generics/types.js
new file mode 100644
index 0000000..139597f
--- /dev/null
+++ b/generics/types.js
@@ -0,0 +1,2 @@
+
+
diff --git a/metacircular.js b/metacircular.js
index 5070987..5601f13 100644
--- a/metacircular.js
+++ b/metacircular.js
@@ -53,7 +53,6 @@ export const typedFnType = (instance, callback) => {
 export const typedFnType2 = callback => {
   const fnTs = [];
   const wrappedFnType = ({ in: inType, out: outType }) => {
-    console.log('wrappedFnType called');
     const fnT = fnType({ in: inType, out: outType });
     fnTs.push(fnT);
     return fnT;
diff --git a/package.json b/package.json
index d76b85f..c0a0cfb 100644
--- a/package.json
+++ b/package.json
@@ -1,4 +1,5 @@
 {
+	"type": "module",
 	"dependencies": {
 		"@inquirer/prompts": "^7.4.0"
 	}
diff --git a/structures/sum.js b/structures/sum.js
index eca45d6..0c1dcf1 100644
--- a/structures/sum.js
+++ b/structures/sum.js
@@ -1,8 +1,8 @@
-import { sumType } from "../type_registry.js";
 import { prodType } from "./product.js";
 import { fnType } from "../metacircular.js";
 import { Function, Type } from "../metacircular.js";
 import { Module } from "./list_types/module.js";
+import { DefaultMap } from "../util.js";
 
 const sumTypeRegistry = new DefaultMap(leftType => new DefaultMap(rightType => ({ operator: "sum", leftType, rightType })));
 
diff --git a/typeclasses/eq.js b/typeclasses/eq.js
new file mode 100644
index 0000000..e69de29
diff --git a/typeclasses/num.js b/typeclasses/num.js
index 8996933..c68cd60 100644
--- a/typeclasses/num.js
+++ b/typeclasses/num.js
@@ -1,10 +1,9 @@
-import { makeGeneric } from "../generics/generics";
-import { addDouble, mulDouble } from "../primitives/double";
-import { addInt, mulInt } from "../primitives/int";
-import { typedFnType, typedFnType2 } from "../metacircular";
-
-const numDictTypeRegistry = new DefaultMap(a => ({numDict: a}));
-export const numDictType = a => numDictTypeRegistry.getdefault(a, true);
+import { makeGeneric } from "../generics/generics.js";
+import { addDouble, mulDouble } from "../primitives/double.js";
+import { addInt, mulInt } from "../primitives/int.js";
+import { Type, typedFnType, typedFnType2 } from "../metacircular.js";
+import { Double, Int } from "../primitives/symbols.js";
+import { numDictType } from "./num_type.js";
 
 export const getAdd  = numDict => numDict.add;
 export const getMul  = numDict => numDict.mul;
@@ -40,8 +39,8 @@ const DoubleNumDict = {
 }
 
 export const ModuleNumInstances = {l:[
-  {i: IntNumDict   , t: NumDict},
-  {i: DoubleNumDict, t: NumDict},
+  {i: IntNumDict   , t: numDictType(Int)},
+  {i: DoubleNumDict, t: numDictType(Double)},
 ]};
 
 // mapping from type to type class implementation
diff --git a/typeclasses/num.test.js b/typeclasses/num.test.js
new file mode 100644
index 0000000..95c3f19
--- /dev/null
+++ b/typeclasses/num.test.js
@@ -0,0 +1,23 @@
+import { assign } from "../generics/generics.js";
+import { makeGeneric } from "../generics/generics.js";
+import { fnType } from "../metacircular.js";
+import { Double, Int } from "../primitives/symbols.js";
+import { numDictType } from "./num_type.js";
+
+const square = numDict => x => getMul(numDict)(x)(x);
+
+const squareFnType = makeGeneric(a => 
+  fnType({
+    in: numDictType(a),
+    out: fnType({
+      in: a,
+      out: a,
+    }),
+  }));
+
+// should be: Int -> Int
+console.log(assign(squareFnType, makeGeneric(() => numDictType(Int))));
+
+// should be: Double -> Double
+console.log(assign(squareFnType, makeGeneric(() => numDictType(Double))));
+
diff --git a/typeclasses/num_type.js b/typeclasses/num_type.js
new file mode 100644
index 0000000..54537c9
--- /dev/null
+++ b/typeclasses/num_type.js
@@ -0,0 +1,4 @@
+import { DefaultMap } from "../util.js";
+
+const numDictTypeRegistry = new DefaultMap(a => ({ numDict: a }));
+export const numDictType = a => numDictTypeRegistry.getdefault(a, true);
diff --git a/typeclasses/resolve.js b/typeclasses/resolve.js
new file mode 100644
index 0000000..8aaf7c0
--- /dev/null
+++ b/typeclasses/resolve.js
@@ -0,0 +1 @@
+// const resolveTypeClass = (mapping, )
\ No newline at end of file