diff --git a/packages/squiggle-lang/src/js/index.ts b/packages/squiggle-lang/src/js/index.ts
index 4892c2f3..a9f2080c 100644
--- a/packages/squiggle-lang/src/js/index.ts
+++ b/packages/squiggle-lang/src/js/index.ts
@@ -10,9 +10,7 @@ export type { t as DistPlus } from "../rescript/OldInterpreter/DistPlus.gen";
 import {
   genericDist,
   env,
-  resultDist,
-  resultFloat,
-  resultString,
+  error,
 } from "../rescript/TypescriptInterface.gen";
 export {makeSampleSetDist} from "../rescript/TypescriptInterface.gen";
 import {
@@ -52,7 +50,7 @@ export function run(
   squiggleString: string,
   samplingInputs?: SamplingInputs,
   environment?: exportEnv
-): { tag: "Ok"; value: exportType } | { tag: "Error"; value: string } {
+): result<exportType, string> {
   let si: SamplingInputs = samplingInputs
     ? samplingInputs
     : defaultSamplingInputs;
@@ -60,19 +58,17 @@ export function run(
   return runAll(squiggleString, si, env);
 }
 
-//This is clearly not fully typed. I think later we should use a functional library to
-// provide a better Either type and corresponding functions.
-type result =
+type result<a, b> =
   | {
       tag: "Ok";
-      value: any;
+      value: a;
     }
   | {
       tag: "Error";
-      value: any;
+      value: b;
     };
 
-export function resultMap(r: result, mapFn: any): result {
+export function resultMap<a,b,c>(r: result<a,c>, mapFn: ((x: a) => b)): result<b,c> {
   if (r.tag === "Ok") {
     return { tag: "Ok", value: mapFn(r.value) };
   } else {
@@ -80,11 +76,11 @@ export function resultMap(r: result, mapFn: any): result {
   }
 }
 
-export function resultExn(r: result): any {
-  r.value
+export function resultExn<a,c>(r: result<a,c>): a | c {
+  return r.value
 }
 
-export class GenericDist {
+export class Distribution {
   t: genericDist;
   env: env;
 
@@ -94,133 +90,133 @@ export class GenericDist {
     return this;
   }
 
-  mapResultDist(r: resultDist) {
-    return resultMap(r, (v: genericDist) => new GenericDist(v, this.env));
+  mapResultDist(r: result<genericDist, error>) : result<Distribution, error>{
+    return resultMap(r, (v: genericDist) => new Distribution(v, this.env));
   }
 
-  mean() {
+  mean() : result<number,error> {
     return Constructors_mean({ env: this.env }, this.t);
   }
 
-  sample(): resultFloat {
+  sample(): result<number,error> {
     return Constructors_sample({ env: this.env }, this.t);
   }
 
-  pdf(n: number): resultFloat {
+  pdf(n: number): result<number,error> {
     return Constructors_pdf({ env: this.env }, this.t, n);
   }
 
-  cdf(n: number): resultFloat {
+  cdf(n: number): result<number,error> {
     return Constructors_cdf({ env: this.env }, this.t, n);
   }
 
-  inv(n: number): resultFloat {
+  inv(n: number): result<number,error> {
     return Constructors_inv({ env: this.env }, this.t, n);
   }
 
-  normalize() {
+  normalize() : result<Distribution, error> {
     return this.mapResultDist(
       Constructors_normalize({ env: this.env }, this.t)
     );
   }
 
-  toPointSet() {
+  toPointSet() : result<Distribution, error>{
     return this.mapResultDist(
       Constructors_toPointSet({ env: this.env }, this.t)
     );
   }
 
-  toSampleSet(n: number) {
+  toSampleSet(n: number) : result<Distribution, error>{
     return this.mapResultDist(
       Constructors_toSampleSet({ env: this.env }, this.t, n)
     );
   }
 
-  truncate(left: number, right: number) {
+  truncate(left: number, right: number) : result<Distribution, error> {
     return this.mapResultDist(
       Constructors_truncate({ env: this.env }, this.t, left, right)
     );
   }
 
-  inspect() {
+  inspect() : result<Distribution, error> {
     return this.mapResultDist(Constructors_inspect({ env: this.env }, this.t));
   }
 
-  toString(): resultString {
+  toString(): result<string, error> {
     return Constructors_toString({ env: this.env }, this.t);
   }
 
-  toSparkline(n: number): resultString {
+  toSparkline(n: number): result<string, error> {
     return Constructors_toSparkline({ env: this.env }, this.t, n);
   }
 
-  algebraicAdd(d2: GenericDist) {
+  algebraicAdd(d2: Distribution) : result<Distribution, error> {
     return this.mapResultDist(
       Constructors_algebraicAdd({ env: this.env }, this.t, d2.t)
     );
   }
 
-  algebraicMultiply(d2: GenericDist) {
+  algebraicMultiply(d2: Distribution) : result<Distribution, error> {
     return this.mapResultDist(
       Constructors_algebraicMultiply({ env: this.env }, this.t, d2.t)
     );
   }
 
-  algebraicDivide(d2: GenericDist) {
+  algebraicDivide(d2: Distribution) : result<Distribution, error> {
     return this.mapResultDist(
       Constructors_algebraicDivide({ env: this.env }, this.t, d2.t)
     );
   }
 
-  algebraicSubtract(d2: GenericDist) {
+  algebraicSubtract(d2: Distribution) : result<Distribution, error> {
     return this.mapResultDist(
       Constructors_algebraicSubtract({ env: this.env }, this.t, d2.t)
     );
   }
 
-  algebraicLogarithm(d2: GenericDist) {
+  algebraicLogarithm(d2: Distribution) : result<Distribution, error> {
     return this.mapResultDist(
       Constructors_algebraicLogarithm({ env: this.env }, this.t, d2.t)
     );
   }
 
-  algebraicPower(d2: GenericDist) {
+  algebraicPower(d2: Distribution) : result<Distribution, error>{
     return this.mapResultDist(
       Constructors_algebraicPower({ env: this.env }, this.t, d2.t)
     );
   }
 
-  pointwiseAdd(d2: GenericDist) {
+  pointwiseAdd(d2: Distribution) {
     return this.mapResultDist(
       Constructors_pointwiseAdd({ env: this.env }, this.t, d2.t)
     );
   }
 
-  pointwiseMultiply(d2: GenericDist) {
+  pointwiseMultiply(d2: Distribution) : result<Distribution, error>{
     return this.mapResultDist(
       Constructors_pointwiseMultiply({ env: this.env }, this.t, d2.t)
     );
   }
 
-  pointwiseDivide(d2: GenericDist) {
+  pointwiseDivide(d2: Distribution) : result<Distribution, error>{
     return this.mapResultDist(
       Constructors_pointwiseDivide({ env: this.env }, this.t, d2.t)
     );
   }
 
-  pointwiseSubtract(d2: GenericDist) {
+  pointwiseSubtract(d2: Distribution) : result<Distribution, error>{
     return this.mapResultDist(
       Constructors_pointwiseSubtract({ env: this.env }, this.t, d2.t)
     );
   }
 
-  pointwiseLogarithm(d2: GenericDist) {
+  pointwiseLogarithm(d2: Distribution) : result<Distribution, error>{
     return this.mapResultDist(
       Constructors_pointwiseLogarithm({ env: this.env }, this.t, d2.t)
     );
   }
 
-  pointwisePower(d2: GenericDist) {
+  pointwisePower(d2: Distribution) : result<Distribution, error>{
     return this.mapResultDist(
       Constructors_pointwisePower({ env: this.env }, this.t, d2.t)
     );