Feat/added validation in either and result

docs/either/index.md

@@ -98,8 +98,24 @@ const result2 = calculate(10, 2)
   .map(n => n + 1)
   .mapLeft(e => console.log(`Error: ${e}`)) // Prints "Error: Result is too small"
   .unwrapOr(0);
+```
+
+### `validate(predicate: (value: R) => boolean, leftValue: L): Either<L, R>`
+Validates the `Right` value based on a predicate. If the predicate returns `true`, keeps the value. If it returns `false`, converts to `Left` with the provided error. Does nothing for `Left`.
+
+```ts
+import { Left, Right } from 'holo-fn/either';
+
+const result1 = new Right<string, number>(25).validate((n) => n >= 18, 'Must be 18+');
+console.log(result1.unwrapOr(0)); // 25
+
+const result2 = new Right<string, number>(15).validate((n) => n >= 18, 'Must be 18+');
+console.log(result2.isLeft()); // true
+console.log(result2.unwrapOr(0)); // 0
+
+const result3 = new Left<string, number>('Already failed').validate((n) => n >= 18, 'Must be 18+');
+console.log(result3.isLeft()); // true (keeps original error)
 
-console.log(result2); // 0
 ```
 
 ### `unwrapOr(defaultValue: R): R`
@@ -387,6 +403,69 @@ console.log(result); // 10
 
 ---
 
+### `validate`
+
+Curried version of `validate` for `Either`. This allows filtering/validating values in a functional pipeline with custom error values.
+
+```ts
+import { right, validate, unwrapOr } from 'holo-fn/either';
+
+const validateAge = (age: number) =>
+  pipe(
+    right<string, number>(age),
+    validate((x: number) => x >= 0, 'Age cannot be negative'),
+    validate((x: number) => x <= 150, 'Age too high'),
+    validate((x: number) => x >= 18, 'Must be 18+'),
+    unwrapOr(0)
+  );
+
+console.log(validateAge(25)); // 25
+console.log(validateAge(15)); // 0 (fails validation)
+```
+
+**Common use cases:**
+
+```ts
+import { right, tryCatch, validate } from 'holo-fn/either';
+
+// Validate email format
+const validateEmail = (email: string) =>
+  pipe(
+    right<string, string>(email),
+    validate((s: string) => s.length > 0, 'Email is required'),
+    validate((s: string) => s.includes('@'), 'Must contain @'),
+    validate((s: string) => s.includes('.'), 'Invalid domain')
+  );
+
+console.log(validateEmail('[email protected]').unwrapOr('Invalid email'));
+
+// Parse and validate numbers
+const parsePositive = (input: string) =>
+  pipe(
+    tryCatch(
+      () => parseInt(input, 10),
+      () => 'Invalid number'
+    ),
+    validate((n: number) => !isNaN(n), 'Not a number'),
+    validate((n: number) => n > 0, 'Must be positive')
+  );
+
+console.log(parsePositive('42').unwrapOr(0));
+
+// Validate with structured errors
+type ValidationError = { code: string; message: string };
+const validateUser = (age: number) =>
+  pipe(
+    right<ValidationError, number>(age),
+    validate((x: number) => x >= 18, { code: 'AGE_ERROR', message: 'Must be 18+' })
+  );
+
+console.log(validateUser(20));
+console.log(validateUser(15));
+```
+
+---
+
 ### `unwrapOr`
 
 Curried version of `unwrapOr` for `Either`. This provides a cleaner way to unwrap the value in a `Either`, returning a default value if it's `Left`.

docs/result/index.md

@@ -57,6 +57,22 @@ const result2 = new Err<number, string>("Error")
 console.log(result2); // 0
 ```
 
+### `validate(predicate: (value: T) => boolean, error: E): Result<T, E>`
+Validates the `Ok` value based on a predicate. If the predicate returns `true`, keeps the value. If it returns `false`, converts to `Err` with the provided error. Does nothing for `Err`.
+
+```ts
+import { Ok, Err } from "holo-fn/result";
+
+const result1 = new Ok(25).validate((n) => n >= 18, 'Must be 18+');
+console.log(result1.unwrapOr(0)); // 25
+
+const result2 = new Ok(15).validate((n) => n >= 18, 'Must be 18+');
+console.log(result2.isErr()); // true
+
+const result3 = new Err<number, string>('Already failed').validate((n) => n >= 18, 'Must be 18+');
+console.log(result3.isErr()); // true (keeps original error)
+```
+
 ### `unwrapOr(defaultValue: T): T`
 Returns the value of `Ok`, or the default value for `Err`.
 
@@ -277,6 +293,69 @@ console.log(result); // 15
 
 ---
 
+### `validate`
+
+Curried version of `validate` for `Result`. This allows filtering/validating values in a functional pipeline with custom error messages.
+
+```ts
+import { ok, validate, unwrapOr } from 'holo-fn/result';
+
+const validateAge = (age: number) =>
+  pipe(
+    ok<number, string>(age),
+    validate((x) => x >= 0, 'Age cannot be negative'),
+    validate((x) => x <= 150, 'Age too high'),
+    validate((x) => x >= 18, 'Must be 18+'),
+    unwrapOr(0)
+  );
+
+console.log(validateAge(25)); // 25
+console.log(validateAge(15)); // 0 (fails validation)
+```
+
+**Common use cases:**
+
+```ts
+// Validate email format
+const validateEmail = (email: string) =>
+  pipe(
+    ok<string, string>(email),
+    validate((s) => s.length > 0, 'Email is required'),
+    validate((s) => s.includes('@'), 'Must contain @'),
+    validate((s) => s.includes('.'), 'Invalid domain')
+  );
+
+console.log(validateEmail('[email protected]').unwrapOr('Invalid'));
+
+// Parse and validate numbers
+const parsePositive = (input: string) =>
+  pipe(
+    fromThrowable(
+      () => parseInt(input, 10),
+      () => 'Invalid number'
+    ),
+    validate((n) => !isNaN(n), 'Not a number'),
+    validate((n) => n > 0, 'Must be positive')
+  );
+
+console.log(parsePositive('42').unwrapOr(0)); // 42
+console.log(parsePositive('-5').unwrapOr(0));
+
+// Validate objects
+type User = { name: string; age: number };
+const validateUser = (user: User) =>
+  pipe(
+    ok<User, string>(user),
+    validate((u) => u.name.length > 0, 'Name required'),
+    validate((u) => u.age >= 18, 'Must be adult')
+  );
+
+console.log(validateUser({ name: 'Alice', age: 30 }).unwrapOr({ name: '', age: 0 }));
+console.log(validateUser({ name: '', age: 16 }).unwrapOr({ name: '', age: 0 }));
+```
+
+---
+
 ### `unwrapOr`
 
 Curried version of `unwrapOr` for `Result`. This provides a cleaner way to unwrap the value in a `Result`, returning a default value if it's `Err`.

src/either/index.ts

@@ -5,6 +5,7 @@ export interface Either<L, R> {
   map<U>(fn: (value: R) => U): Either<L, U>;
   mapLeft<M>(fn: (err: L) => M): Either<M, R>;
   chain<U>(fn: (value: R) => Either<L, U>): Either<L, U>;
+  validate(predicate: (value: R) => boolean, leftValue: L): Either<L, R>;
   unwrapOr(defaultValue: R): R;
   match<T>(cases: { left: (left: L) => T; right: (right: R) => T }): T;
   equals(other: Either<L, R>): boolean;
@@ -35,6 +36,10 @@ export class Right<L, R> implements Either<L, R> {
     return fn(this.value);
   }
 
+  validate(predicate: (value: R) => boolean, leftValue: L): Either<L, R> {
+    return predicate(this.value) ? this : new Left<L, R>(leftValue);
+  }
+
   unwrapOr(_: R): R {
     return this.value;
   }
@@ -75,6 +80,10 @@ export class Left<L, R = never> implements Either<L, R> {
     return new Left<L, U>(this.value);
   }
 
+  validate(_predicate: (value: R) => boolean, _leftValue: L): Either<L, R> {
+    return this;
+  }
+
   unwrapOr(defaultValue: R): R {
     return defaultValue;
   }
@@ -136,6 +145,12 @@ export const chain =
     return either.chain(fn);
   };
 
+export const validate =
+  <L, R>(predicate: (value: R) => boolean, leftValue: L) =>
+  (either: Either<L, R>): Either<L, R> => {
+    return either.validate(predicate, leftValue);
+  };
+
 export const unwrapOr =
   <L, R>(defaultValue: R) =>
   (either: Either<L, R>): R => {

src/result/index.ts

@@ -5,6 +5,7 @@ export interface Result<T, E> {
   map<U>(fn: (value: T) => U): Result<U, E>;
   mapErr<F>(fn: (err: E) => F): Result<T, F>;
   chain<U>(fn: (value: T) => Result<U, E>): Result<U, E>;
+  validate(predicate: (value: T) => boolean, error: E): Result<T, E>;
   unwrapOr(defaultValue: T): T;
   match<U>(cases: { ok: (value: T) => U; err: (err: E) => U }): U;
   equals(other: Result<T, E>): boolean;
@@ -35,6 +36,10 @@ export class Ok<T, E> implements Result<T, E> {
     return fn(this.value);
   }
 
+  validate(predicate: (value: T) => boolean, error: E): Result<T, E> {
+    return predicate(this.value) ? this : new Err<T, E>(error);
+  }
+
   unwrapOr(_: T): T {
     return this.value;
   }
@@ -75,6 +80,10 @@ export class Err<T, E> implements Result<T, E> {
     return new Err<U, E>(this.error);
   }
 
+  validate(_predicate: (value: T) => boolean, _error: E): Result<T, E> {
+    return this;
+  }
+
   unwrapOr(defaultValue: T): T {
     return defaultValue;
   }
@@ -142,6 +151,12 @@ export const chain =
     return result.chain(fn);
   };
 
+export const validate =
+  <T, E>(predicate: (value: T) => boolean, error: E) =>
+  (result: Result<T, E>): Result<T, E> => {
+    return result.validate(predicate, error);
+  };
+
 export const unwrapOr =
   <T, E>(defaultValue: T) =>
   (result: Result<T, E>): T => {

tests/either.test.ts

@@ -14,6 +14,7 @@ import {
   right,
   tryCatch,
   unwrapOr,
+  validate,
 } from '../src/either';
 
 describe('Either', () => {
@@ -59,6 +60,31 @@ describe('Either', () => {
     expect(result.unwrapOr(0)).toBe(0);
   });
 
+  it('Right.validate should keep value when predicate returns true', () => {
+    const result = new Right<string, number>(25).validate((x) => x >= 18, 'Must be 18+');
+    expect(result.isRight()).toBe(true);
+    expect(result.unwrapOr(0)).toBe(25);
+  });
+
+  it('Right.validate should convert to Left when predicate returns false', () => {
+    const result = new Right<string, number>(15).validate((x) => x >= 18, 'Must be 18+');
+    expect(result.isLeft()).toBe(true);
+    expect(result.match({ left: (e) => e, right: (_) => '' })).toBe('Must be 18+');
+  });
+
+  it('Right.validate should support chaining multiple validations', () => {
+    const validateAge = (age: number) =>
+      right<string, number>(age)
+        .validate((x) => x >= 0, 'Age cannot be negative')
+        .validate((x) => x <= 150, 'Age too high')
+        .validate((x) => x >= 18, 'Must be 18+');
+
+    expect(validateAge(25).unwrapOr(0)).toBe(25);
+    expect(validateAge(15).match({ left: (e) => e, right: (_) => '' })).toBe('Must be 18+');
+    expect(validateAge(-5).match({ left: (e) => e, right: (_) => '' })).toBe('Age cannot be negative');
+    expect(validateAge(200).match({ left: (e) => e, right: (_) => '' })).toBe('Age too high');
+  });
+
   it('Right.unwrapOr returns the value', () => {
     const result = new Right(42);
     expect(result.unwrapOr(0)).toBe(42);
@@ -249,6 +275,24 @@ describe('Either - Curried Helpers', () => {
     expect(result).toBe(0);
   });
 
+  it('Left.validate should not run predicate (short-circuit)', () => {
+    const result = right<string, number>(15)
+      .validate((x) => x >= 18, 'Must be 18+')
+      .validate((x) => x <= 100, 'Too old');
+
+    expect(result.match({ left: (e) => e, right: (_) => '' })).toBe('Must be 18+');
+  });
+
+  it('curried validate should work with pipe', () => {
+    const result = pipe(
+      right<string, number>(42),
+      validate((x: number) => x > 0, 'Must be positive'),
+      validate((x: number) => x % 2 === 0, 'Must be even'),
+      unwrapOr(0)
+    );
+    expect(result).toBe(42);
+  });
+
   it('should unwrapOr with default value using curried unwrapOr', () => {
     const result = pipe(new Right(10), unwrapOr(42));
     expect(result).toBe(10);