diff --git a/generic.js b/generic.js
new file mode 100644
index 0000000..259065a
--- /dev/null
+++ b/generic.js
@@ -0,0 +1,172 @@
+import { Bool, Int } from "./primitives/symbols.js";
+import { fnType, lsType } from "./type_registry.js";
+import { deepEqual } from "./util.js";
+
+// let nextSymbol = 'a';
+
+// export const makeGeneric = callback => {
+//   const typeParam = Symbol(nextSymbol);
+//   nextSymbol = String.fromCharCode(nextSymbol.charCodeAt(0) + 1);
+//   return {
+//     typeParam,
+//     type: callback(typeParam),
+//   };
+// };
+
+
+import { inspect } from 'node:util';
+
+function pretty(obj) {
+  return inspect(obj, {colors: true});
+}
+
+export const matchGeneric = (
+  {typeVars: formalTypeVars, type: formalType},
+  {typeVars: actualTypeVars, type: actualType},
+) => {
+  if (deepEqual(formalType, actualType)) {
+    return {
+      substitutions: new Map(),
+      typeVars: new Set([
+        ...actualTypeVars,
+        ...formalTypeVars]),
+      type: actualType,
+    }
+  }
+
+  if (formalTypeVars.has(formalType)) {
+    // simplest case: substitute formal type param
+    return {
+      substitutions: new Map([[formalType, actualType]]),
+      typeVars: new Set([
+        ...actualTypeVars,
+        ...formalTypeVars].filter(a => a !== formalType)),
+      type: actualType,
+    };
+  }
+
+  if (formalType.in !== undefined) {
+    // function type
+    if (actualType.in === undefined) {
+      throw new Error(`cannot assign ${pretty(actualType)} to ${pretty(formalType)}`);
+    }
+    else {
+      // both are function type
+      const inType = matchGeneric({typeVars: formalTypeVars, type: formalType.in}, {typeVars: actualTypeVars, type: actualType.in});
+      const outType = matchGeneric({typeVars: formalTypeVars, type: formalType.out}, {typeVars: actualTypeVars, type: actualType.out});
+      // check for conflicts between 'in' and 'out' subsitutions
+      for (const [typeVar, actual] of inType.substitutions) {
+        if (outType.substitutions.has(typeVar)) {
+          if (!deepEqual(actual, outType.substitutions.get(typeVar))) {
+            throw new Error(`conflicting assignment for ${pretty(typeVar)}: ${pretty(a)}`);
+          }
+        }
+      }
+      // merge substitutions
+      const newSubstitutions = new Map([
+        ...inType.substitutions,
+        ...outType.substitutions,
+      ]);
+      const newTypeVars = new Set([
+        ...actualTypeVars,
+        ...formalTypeVars].filter(a => !newSubstitutions.has(a)));
+      return {
+        substitutions: newSubstitutions,
+        typeVars: newTypeVars,
+        type: fnType({in: inType.type, out: outType.type}),
+      };
+    }
+  }
+
+  if (formalType.listOf !== undefined) {
+    // list type
+    if (actualType.listOf === undefined) {
+      throw new Error(`cannot assign ${pretty(actualType)} to ${pretty(formalType)}`);
+    }
+    else {
+      // both are list type
+      const elementType = matchGeneric(
+        {typeVars: formalTypeVars, type: formalType.listOf},
+        {typeVars: actualTypeVars, type: actualType.listOf});
+      return {
+        substitutions: elementType.substitutions,
+        typeVars: new Set([
+          ...actualTypeVars,
+          ...formalTypeVars].filter(a => !elementType.substitutions.has(a))),
+        type: lsType(elementType.type),
+      };
+    }
+  }
+  throw new Error("i don't know what to do :(")
+};
+
+export const matchConcrete = ({typeVars, type: formalType}, actualType) => {
+  return matchGeneric({typeVars, type: formalType}, {typeVars: new Set(), type: actualType});
+};
+
+export const substitute = (type, substitutions) => {
+  if (substitutions.has(type)) {
+    return substitutions.get(type);
+  }
+  if (type.listOf !== undefined) {
+    // list type
+    return lsType(substitute(type.listOf, substitutions));
+  }
+  if (type.in !== undefined) {
+    // function type
+    return fnType({
+      in: substitute(type.in, substitutions),
+      out: substitute(type.out, substitutions),
+    })
+  }
+  throw new Error("i don't know what to do :(");
+}
+
+export const assign = (genFnType, paramType) => {
+  const matchedInType = matchGeneric({
+    typeVars: genFnType.typeVars,
+    type: genFnType.type.in,
+  }, paramType);
+  const substitutedOutType = substitute(genFnType.type.out, matchedInType.substitutions);
+  return {
+    typeVars: matchedInType.typeVars,
+    type: substitutedOutType,
+  };
+}
+
+const a = Symbol('a');
+const formal = {
+  typeVars: new Set([a]),
+  type: fnType({in: a, out: Int}),
+};
+const actual = fnType({in: lsType(Bool), out: Int});
+console.log(matchConcrete(formal, actual));
+
+
+const b = Symbol('b');
+const c = Symbol('c');
+const formal2 = {
+  typeVars: new Set([a, b]),
+  type: fnType({in: fnType({in: a, out: a}), out: b}),
+}
+const actual2 = {
+  typeVars: new Set([c]),
+  type: fnType({in: fnType({in: Bool, out: Bool}), out: c}),
+}
+console.log(matchGeneric(formal2, actual2));
+
+
+const mapFnType = {
+  typeVars: new Set([a, b]),
+  type: fnType({
+    in: fnType({in: a, out: b}),
+    out: fnType({in: lsType(a), out: lsType(b)}),
+  }),
+};
+const idFnType = {
+  typeVars: new Set([c]),
+  type: fnType({in: c, out: c}),
+};
+
+// should be: listOf(c) -> listOf(c)
+console.log(assign(mapFnType, idFnType));