diff --git a/lib/generics/generics.js b/lib/generics/generics.js
index f5688b1..343c765 100644
--- a/lib/generics/generics.js
+++ b/lib/generics/generics.js
@@ -88,7 +88,7 @@ export class NotAFunctionError extends Error {}
 //   typeVars: all the type variables in both fType and aType
 //   fType, aType: generic types to unify
 //   fStack, aStack: internal use.
-const __unify = (fType, aType, fStack=[], aStack=[]) => {
+export const __unify = (fType, aType, fStack=[], aStack=[]) => {
   // console.log("__unify", {typeVars, fType: prettyT(fType), aType: prettyT(aType), fStack, aStack});
   if (isTypeVar(fType)) {
     // simplest case: formalType is a type paramater
@@ -196,18 +196,22 @@ export const assignFnSubstitutions = (funType, paramType) => {
   if (getSymbol(funType) !== symbolFunction) {
     throw new NotAFunctionError(`${prettyT(funType)} is not a function type!`);
   }
-  [funType, paramType] = recomputeTypeVars([funType, paramType]);
-  // console.log(prettyT(funType), prettyT(paramType));
-  const [inType, outType] = funType.params.map(p => p(funType));
-  const {substitutions} = __unify(inType, paramType);
+  const [[refunType, funS], [reparamType, paramS]] = recomputeTypeVarsSubstitutions([funType, paramType]);
+  const [inType, outType] = refunType.params.map(p => p(refunType));
+  const {substitutions} = __unify(inType, reparamType);
   // console.log(substitutions, prettyT(outType));
   const substitutedFnType = substitute(outType, substitutions);
   const computedOutType = recomputeTypeVars([substitutedFnType])[0];
   return [computedOutType, substitutions];
-}
+};
 
 // Ensures that no type variables overlap
 export const recomputeTypeVars = types => {
+  return recomputeTypeVarsSubstitutions(types)
+    .map(([newType, _subst]) => newType);
+};
+
+export const recomputeTypeVarsSubstitutions = types => {
   let nextIdx = 0;
   return types.map(type => {
     const substitutions = new Map();
@@ -215,6 +219,6 @@ export const recomputeTypeVars = types => {
     for (const typeVar of typeVars) {
       substitutions.set(typeVar, TYPE_VARS[nextIdx++]);
     }
-    return substitute(type, substitutions);
+    return [substitute(type, substitutions), substitutions];
   });
-}
+};