cleanup unification a bit

lib/generics/unify.js

@@ -29,6 +29,7 @@ export class SubstitutionCycle extends Error {
   }
 }
 
+// pure
 export const subsitutionsEqual = (m1,m2) => {
   if (m1.size !== m2.size ) return false;
   for (const [key1,type1] of m1) {
@@ -37,6 +38,7 @@ export const subsitutionsEqual = (m1,m2) => {
   return true;
 };
 
+// pure
 // Partial ordering between types
 //  - deep-equal types are equal (e.g., Int == Int)
 //  - non-typevars are smaller than typevars (e.g., Int < a)
@@ -68,17 +70,20 @@ const partialCompareTypes = (typeA, typeB) => {
   }
 }
 
+// pure
 const checkCycle = (typevar, type) => {
   if (occurring(type).has(typevar)) {
     throw new SubstitutionCycle(typevar, type);
   }
 }
 
+// impure, modifies 'substitutions'
 const addReduce = (substitutions, typevar, type) => {
   // console.log('add ', prettyS(typevar, type));
   substitutions.set(typevar, type);
 }
 
+// impure, modifies 'substitutions'
 const attemptReduce = (substitutions, typevar, type) => {
   // assuming 'substitutions' is already reduced as much as possible,
   // substitute all typevars in our type with the existing substitutions
@@ -108,8 +113,24 @@ const attemptReduce = (substitutions, typevar, type) => {
   }
 }
 
+// pure
+export const mergeSubstitutionsN = (substs) => {
+  return substs.reduce((acc, cur) => {
+    return mergeSubstitutions2(acc, cur);
+  }, new Map());
+}
+
+// pure
+const mergeSubstitutions2 = (substA, substB) => {
+  const result = new Map(substA);
+  for (const [typevarB, typeB] of substB) {
+    attemptReduce(result, typevarB, typeB);
+  }
+  return result;
+}
+
+// pure
 export const unify = (typeA, typeB) => {
-  const substitutions = new Map();
   try {
     if (isTypeVar(typeA)) {
       if (isTypeVar(typeB)) {
@@ -139,20 +160,14 @@ export const unify = (typeA, typeB) => {
       throw new IncompabibleTypesError(typeA, typeB);
     }
 
-    const unifiedParams = zip(typeA.params, typeB.params)
+    const subs = zip(typeA.params, typeB.params)
     .map(([getParamA, getParamB]) => {
       const paramA = getParamA(typeA);
       const paramB = getParamB(typeB);
       // console.log('request...');
       return unify(paramA, paramB);
     });
-
-    // merge substitutions
-    unifiedParams.forEach(subst => {
-      for (const [typevar, type] of subst) {
-        attemptReduce(substitutions, typevar, type);
-      }
-    });
+    return mergeSubstitutionsN(subs);
   }
   catch (e) {
     if (e instanceof SubstitutionCycle) {
@@ -164,5 +179,4 @@ export const unify = (typeA, typeB) => {
     }
     throw e;
   }
-  return substitutions;
 };