diff --git a/lib/generics/generics.js b/lib/generics/generics.js
index a722a1f..110b2d4 100644
--- a/lib/generics/generics.js
+++ b/lib/generics/generics.js
@@ -187,6 +187,12 @@ export const substitute = (type, substitutions, stack=[]) => {
 };
 
 export const assignFn = (funType, paramType) => {
+  const [result] = assignFnSubstitutions(funType, paramType);
+  return result;
+};
+
+// same as above, but also returns the substitutions that took place
+export const assignFnSubstitutions = (funType, paramType) => {
   if (getSymbol(funType) !== symbolFunction) {
     throw new NotAFunctionError(`${prettyT(funType)} is not a function type!`);
   }
@@ -196,8 +202,8 @@ export const assignFn = (funType, paramType) => {
   const {substitutions} = __unify(inType, paramType);
   // console.log(substitutions, prettyT(outType));
   const substitutedFnType = substitute(outType, substitutions);
-  return recomputeTypeVars([substitutedFnType])[0];
-};
+  return [recomputeTypeVars([substitutedFnType])[0], substitutions];
+}
 
 // Ensures that no type variables overlap
 export const recomputeTypeVars = types => {