feat: introduce maybe.filter (#39)

Author: patrickmichalina

src/interfaces/maybe.interface.ts

@@ -61,4 +61,6 @@ export interface IMaybe<T> extends IMonad<T> {
 
   // tslint:disable-next-line:readonly-array
   of(x?: T, ...args: any[]): IMaybe<T>
+  
+  filter(fn: (t: T) => boolean): IMaybe<T>
 }

src/monads/maybe.ts

@@ -5,11 +5,16 @@ export function maybe<T>(value?: T): IMaybe<NonNullable<T>> {
     of: maybe,
     valueOr: (val: NonNullable<T>) => value === null || value === undefined ? val : value as NonNullable<T>,
     valueOrCompute: (f: () => NonNullable<T>) => value === null || value === undefined ? f() : value as NonNullable<T>,
-    tap: (obj: Partial<IMaybePattern<T, void>>) => value === null || value === undefined ? obj.none && obj.none() : obj.some && obj.some(value as  NonNullable<T>),
+    tap: (obj: Partial<IMaybePattern<T, void>>) => value === null || value === undefined ? obj.none && obj.none() : obj.some && obj.some(value as NonNullable<T>),
     tapNone: (f: () => void) => (value === null || value === undefined) && f(),
     tapSome: (f: (val: NonNullable<T>) => void) => value !== null && value !== undefined && f(value as NonNullable<T>),
-    match: <R>(pattern: IMaybePattern<T, R>) => value === null || value === undefined ? pattern.none() : pattern.some(value as  NonNullable<T>),
+    match: <R>(pattern: IMaybePattern<T, R>) => value === null || value === undefined ? pattern.none() : pattern.some(value as NonNullable<T>),
     map: <R>(f: (t: NonNullable<T>) => R) => value === null || value === undefined ? maybe<R>() : maybe<R>(f(value as NonNullable<T>)),
-    flatMap: <R>(f: (d:  NonNullable<T>) => IMaybe<R>) => value === null || value === undefined ? maybe<R>() : f(value as  NonNullable<T>)
+    flatMap: <R>(f: (d: NonNullable<T>) => IMaybe<R>) => value === null || value === undefined ? maybe<R>() : f(value as NonNullable<T>),
+    filter: (fn: (d: NonNullable<T>) => boolean) => value === null || value === undefined
+      ? maybe()
+      : fn(value as NonNullable<T>)
+        ? maybe(value as NonNullable<T>)
+        : maybe()
   }
 }

test/monads/maybe.spec.ts

@@ -290,5 +290,43 @@ describe('Maybe', () => {
       maybe(sut).tapSome(() => expect(1).toEqual(1))
     })
   })
+
+  describe('when filtering', () => {
+    it('pass value through if predicate is resolves true', () => {
+      const thing: { readonly isGreen: boolean } | undefined = { isGreen: true }
+
+      expect.assertions(1)
+      maybe(thing)
+        .filter(a => a.isGreen === true)
+        .tap({
+          some: _thing => expect(_thing).toEqual(thing),
+          none: () => expect(1).toEqual(1)
+        })
+    })
+
+    it('should not pass value through if predicate is resolves false', () => {
+      const thing: { readonly isGreen: boolean } | undefined = { isGreen: false }
+
+      expect.assertions(1)
+      maybe(thing)
+        .filter(a => a.isGreen === true)
+        .tap({
+          some: _thing => expect(true).toBe(false),
+          none: () => expect(1).toEqual(1)
+        })
+    })
+
+    it('should handle undefineds correctly', () => {
+      const thing = undefined as { readonly isGreen: boolean } | undefined
+
+      expect.assertions(1)
+      maybe(thing)
+        .filter(a => a.isGreen === true)
+        .tap({
+          some: _thing => expect(true).toBe(false),
+          none: () => expect(1).toEqual(1)
+        })
+    })
+  })
 })