fix: result.maybeOk() should constraint T to NonNullable<T> (#156)

Author: patrickmichalina

src/result/result.interface.ts

@@ -10,7 +10,7 @@ export interface IResultMatchPattern<T, E, U> {
 export interface IResult<T, E> {
   isOk(): boolean
   isFail(): boolean
-  maybeOk(): IMaybe<T>
+  maybeOk(): IMaybe<NonNullable<T>>
   maybeFail(): IMaybe<E>
   unwrap(): T | never
   unwrapOr(opt: T): T

src/result/result.spec.ts

@@ -27,8 +27,9 @@ describe('result', () => {
     })
 
     it('should ...', () => {
-      const _sut = ok('Test')
+      const _sut = ok<string | undefined, string>('Test')
         .maybeOk()
+        .map(b => b)
         .valueOr('Some Other')
 
       expect(_sut).toEqual('Test')

src/result/result.ts

@@ -17,7 +17,7 @@ export abstract class Result<TOk, TFail> implements IResult<TOk, TFail> {
   abstract unwrap(): TOk | never
   abstract unwrapOr(opt: TOk): TOk
   abstract unwrapFail(): TFail | never
-  abstract maybeOk(): IMaybe<TOk>
+  abstract maybeOk(): IMaybe<NonNullable<TOk>>
   abstract maybeFail(): IMaybe<TFail>
   abstract match<M>(fn: IResultMatchPattern<TOk, TFail, M>): M
   abstract map<M>(fn: (val: TOk) => M): IResult<M, TFail>
@@ -50,8 +50,8 @@ export class OkResult<TOk, TFail> extends Result<TOk, TFail> {
     throw new ReferenceError('Cannot unwrap a success as a failure')
   }
 
-  maybeOk(): IMaybe<TOk> {
-    return maybe(this.successValue)
+  maybeOk(): IMaybe<NonNullable<TOk>> {
+    return maybe(this.successValue as NonNullable<TOk>)
   }
 
   maybeFail(): IMaybe<TFail> {
@@ -101,7 +101,7 @@ export class FailResult<TOk, TFail> extends Result<TOk, TFail> implements IResul
     return this.value
   }
 
-  maybeOk(): IMaybe<TOk> {
+  maybeOk(): IMaybe<NonNullable<TOk>> {
     return none()
   }