feat: nil/null/empty/blank literals, resolves #102

src/drop/blank-drop.ts

@@ -0,0 +1,12 @@
+import { isNil, isString } from 'src/util/underscore'
+import { isDrop } from 'src/drop/idrop'
+import { EmptyDrop } from 'src/drop/empty-drop'
+
+export class BlankDrop extends EmptyDrop {
+  equals (value: any) {
+    if (value === false) return true
+    if (isNil(isDrop(value) ? value.value() : value)) return true
+    if (isString(value)) return /^\s*$/.test(value)
+    return super.equals(value)
+  }
+}

src/drop/drop.ts

@@ -0,0 +1,2 @@
+export abstract class Drop {
+}

src/drop/empty-drop.ts

@@ -0,0 +1,27 @@
+import { Drop } from './drop'
+import { IComparable } from './icomparable'
+import { isObject, isString, isArray } from 'src/util/underscore'
+import { IDrop } from 'src/drop/idrop'
+
+export class EmptyDrop extends Drop implements IDrop, IComparable {
+  equals (value: any) {
+    if (isString(value) || isArray(value)) return value.length === 0
+    if (isObject(value)) return Object.keys(value).length === 0
+    return false
+  }
+  gt () {
+    return false
+  }
+  geq () {
+    return false
+  }
+  lt () {
+    return false
+  }
+  leq () {
+    return false
+  }
+  value () {
+    return ''
+  }
+}

src/drop/icomparable.ts

@@ -0,0 +1,13 @@
+import { isFunction } from 'src/util/underscore'
+
+export interface IComparable {
+  equals: (rhs: any) => boolean
+  gt: (rhs: any) => boolean
+  geq: (rhs: any) => boolean
+  lt: (rhs: any) => boolean
+  leq: (rhs: any) => boolean
+}
+
+export function isComparable (arg: any): arg is IComparable {
+  return arg && isFunction(arg.equals)
+}

src/drop/idrop.ts

@@ -0,0 +1,10 @@
+import { Drop } from './drop'
+import { isFunction } from 'src/util/underscore'
+
+export interface IDrop {
+  value(): any
+}
+
+export function isDrop (value: any): value is IDrop {
+  return value instanceof Drop && isFunction((value as any).value)
+}

src/drop/null-drop.ts

@@ -0,0 +1,26 @@
+import { Drop } from './drop'
+import { IComparable } from './icomparable'
+import { isNil } from 'src/util/underscore'
+import { IDrop, isDrop } from 'src/drop/idrop'
+import { BlankDrop } from 'src/drop/blank-drop'
+
+export class NullDrop extends Drop implements IDrop, IComparable {
+  equals (value: any) {
+    return isNil(isDrop(value) ? value.value() : value) || value instanceof BlankDrop
+  }
+  gt () {
+    return false
+  }
+  geq () {
+    return false
+  }
+  lt () {
+    return false
+  }
+  leq () {
+    return false
+  }
+  value () {
+    return null
+  }
+}