fix: Remove type information discarding. (#332)

* chore: Add tests regarding type narrowing expectations. * fix: Remove type information discarding. This lead to probably unfixable type collisions, so the feature must go.
thenativeweb · Jul 23, 2021 · bfa3e56 · bfa3e56
1 parent 8903bd5
commit bfa3e56
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Showing 2 changed files with 22 additions and 57 deletions.
diff --git a/lib/Result.ts b/lib/Result.ts
@@ -1,17 +1,17 @@
 interface ResultBase<TValue, TError extends Error> {
- hasError: () => this is ResultError<TError> & ResultBase<any, TError>;
- hasValue: () => this is ResultValue<TValue> & ResultBase<TValue, any>;
+ hasError: () => this is ResultError<TValue, TError>;
+ hasValue: () => this is ResultValue<TValue, TError>;
 
  unwrapOrThrow: (errorTransformer?: (err: TError) => Error) => TValue;
  unwrapOrElse: (handleError: (error: Error) => TValue) => TValue;
  unwrapOrDefault: (defaultValue: TValue) => TValue;
 }
 
-interface ResultError<TError extends Error> {
+interface ResultError<TValue, TError extends Error> extends ResultBase<TValue, TError> {
  error: TError;
 }
 
-const error = function <TError extends Error>(err: TError): ResultError<TError> & ResultBase<any, TError> {
+const error = function <TValue, TError extends Error>(err: TError): ResultError<TValue, TError> {
  return {
  hasError (): boolean {
  return true;
@@ -26,24 +26,24 @@ const error = function <TError extends Error>(err: TError): ResultError<TError>
  // eslint-disable-next-line @typescript-eslint/no-throw-literal
  throw err;
  },
- unwrapOrElse<TValue> (handleError: (error: TError) => TValue): TValue {
+ unwrapOrElse (handleError: (error: TError) => TValue): TValue {
  return handleError(err);
  },
- unwrapOrDefault<TValue> (defaultValue: TValue): TValue {
+ unwrapOrDefault (defaultValue: TValue): TValue {
  return defaultValue;
  },
  error: err
  };
 };
 
-interface ResultValue<TValue> {
+interface ResultValue<TValue, TError extends Error> extends ResultBase<TValue, TError> {
  value: TValue;
 }
 
 const value: {
- <TValue extends undefined>(): ResultValue<TValue> & ResultBase<TValue, any>;
- <TValue>(value: TValue): ResultValue<TValue> & ResultBase<TValue, any>;
-} = function <TValue>(val?: TValue): ResultValue<TValue | undefined> & ResultBase<TValue | undefined, any> {
+ <TValue extends undefined, TError extends Error>(): ResultValue<TValue, TError>;
+ <TValue, TError extends Error>(value: TValue): ResultValue<TValue, TError>;
+} = function <TValue, TError extends Error>(val?: TValue): ResultValue<TValue | undefined, TError> {
  return {
  hasError (): boolean {
  return false;
@@ -64,7 +64,7 @@ const value: {
  };
 };
 
-type Result<TValue, TError extends Error> = ResultBase<TValue, TError>;
+type Result<TValue, TError extends Error> = ResultValue<TValue, TError> | ResultError<TValue, TError>;
 
 export type {
  ResultValue,

diff --git a/test/unit/ResultTests.ts b/test/unit/ResultTests.ts
@@ -76,13 +76,15 @@ suite('Result', (): void => {
  assert.that(resultHasError).is.false();
  });
 
- test(`narrows the type to the error case and discards value type information.`, async (): Promise<void> => {
+ test('extrapolates value type correctly.', async (): Promise<void> => {
  const result = getValue();
 
  if (result.hasError()) {
- // eslint-disable-next-line @typescript-eslint/no-unused-vars
- const newResult: Result<{ something: 'elseEntirely' }, Error> = result;
+ return;
  }
+
+ // eslint-disable-next-line @typescript-eslint/no-unused-vars
+ const containedValue: Value = result.value;
  });
  });
 
@@ -103,17 +105,15 @@ suite('Result', (): void => {
  assert.that(resultHasValue).is.false();
  });
 
- test(`narrows the type to the value case and discards error type information.`, async (): Promise<void> => {
- const result = getValue();
-
- interface CustomError extends Error {
- bar: string;
- }
+ test('extrapolates error type correctly.', async (): Promise<void> => {
+ const result = getError();
 
  if (result.hasValue()) {
- // eslint-disable-next-line @typescript-eslint/no-unused-vars
- const newResult: Result<Value, CustomError> = result;
+ return;
  }
+
+ // eslint-disable-next-line @typescript-eslint/no-unused-vars
+ const containedError = result.error;
  });
  });
 
@@ -246,39 +246,4 @@ suite('Result', (): void => {
  }
  });
  });
-
- test('is assignable to a result with the same error type if the result is known to be an error.', async (): Promise<void> => {
- class CustomError extends Error {
- public someProp = 0;
- }
-
- // This function compiling it enough for this test.
- // eslint-disable-next-line @typescript-eslint/no-unused-vars
- const someFunction = function (someResult: Result<number, CustomError>): Result<string, CustomError> {
- if (someResult.hasError()) {
- return someResult;
- }
-
- return error(new CustomError());
- };
- });
-
- test('is assignable to a result with the same value type if the result is known to be an value.', async (): Promise<void> => {
- class CustomError1 extends Error {
- public someProp = 0;
- }
- class CustomError2 extends Error {
- public someOtherProp = 'some string';
- }
-
- // This function compiling it enough for this test.
- // eslint-disable-next-line @typescript-eslint/no-unused-vars
- const someFunction = function (someResult: Result<string, CustomError1>): Result<string, CustomError2> {
- if (someResult.hasValue()) {
- return someResult;
- }
-
- return value('');
- };
- });
 });