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
+  }
+}

src/render/syntax.ts

@@ -2,14 +2,43 @@ import * as lexical from '../parser/lexical'
 import assert from '../util/assert'
 import Scope from 'src/scope/scope'
 import { range, last } from 'src/util/underscore'
+import { isComparable } from 'src/drop/icomparable'
+import { NullDrop } from 'src/drop/null-drop'
+import { EmptyDrop } from 'src/drop/empty-drop'
+import { BlankDrop } from 'src/drop/blank-drop'
+import { isDrop } from 'src/drop/idrop'
 
-const operators = {
-  '==': (l: any, r: any) => l === r,
-  '!=': (l: any, r: any) => l !== r,
-  '>': (l: any, r: any) => l !== null && r !== null && l > r,
-  '<': (l: any, r: any) => l !== null && r !== null && l < r,
-  '>=': (l: any, r: any) => l !== null && r !== null && l >= r,
-  '<=': (l: any, r: any) => l !== null && r !== null && l <= r,
+const binaryOperators: {[key: string]: (lhs: any, rhs: any) => boolean} = {
+  '==': (l: any, r: any) => {
+    if (isComparable(l)) return l.equals(r)
+    if (isComparable(r)) return r.equals(l)
+    return l === r
+  },
+  '!=': (l: any, r: any) => {
+    if (isComparable(l)) return !l.equals(r)
+    if (isComparable(r)) return !r.equals(l)
+    return l !== r
+  },
+  '>': (l: any, r: any) => {
+    if (isComparable(l)) return l.gt(r)
+    if (isComparable(r)) return r.lt(l)
+    return l > r
+  },
+  '<': (l: any, r: any) => {
+    if (isComparable(l)) return l.lt(r)
+    if (isComparable(r)) return r.gt(l)
+    return l < r
+  },
+  '>=': (l: any, r: any) => {
+    if (isComparable(l)) return l.geq(r)
+    if (isComparable(r)) return r.leq(l)
+    return l >= r
+  },
+  '<=': (l: any, r: any) => {
+    if (isComparable(l)) return l.leq(r)
+    if (isComparable(r)) return r.geq(l)
+    return l <= r
+  },
   'contains': (l: any, r: any) => {
     if (!l) return false
     if (typeof l.indexOf !== 'function') return false
@@ -19,41 +48,54 @@ const operators = {
   'or': (l: any, r: any) => isTruthy(l) || isTruthy(r)
 }
 
-export function evalExp (exp: string, scope: Scope): any {
-  assert(scope, 'unable to evalExp: scope undefined')
+export function parseExp (exp: string, scope: Scope): any {
+  assert(scope, 'unable to parseExp: scope undefined')
   const operatorREs = lexical.operators
   let match
   for (let i = 0; i < operatorREs.length; i++) {
     const operatorRE = operatorREs[i]
     const expRE = new RegExp(`^(${lexical.quoteBalanced.source})(${operatorRE.source})(${lexical.quoteBalanced.source})$`)
     if ((match = exp.match(expRE))) {
-      const l = evalExp(match[1], scope)
-      const op = operators[match[2].trim()]
-      const r = evalExp(match[3], scope)
+      const l = parseExp(match[1], scope)
+      const op = binaryOperators[match[2].trim()]
+      const r = parseExp(match[3], scope)
       return op(l, r)
     }
   }
 
   if ((match = exp.match(lexical.rangeLine))) {
-    const low = evalValue(match[1], scope)
-    const high = evalValue(match[2], scope)
+    const low = parseValue(match[1], scope)
+    const high = parseValue(match[2], scope)
     return range(low, high + 1)
   }
 
-  return evalValue(exp, scope)
+  return parseValue(exp, scope)
+}
+
+export function evalExp (str: string, scope: Scope): any {
+  const value = parseExp(str, scope)
+  return isDrop(value) ? value.value() : value
 }
 
-export function evalValue (str: string, scope: Scope) {
+function parseValue (str: string, scope: Scope): any {
   if (!str) return null
   str = str.trim()
 
   if (str === 'true') return true
   if (str === 'false') return false
+  if (str === 'nil' || str === 'null') return new NullDrop()
+  if (str === 'empty') return new EmptyDrop()
+  if (str === 'blank') return new BlankDrop()
   if (!isNaN(Number(str))) return Number(str)
   if ((str[0] === '"' || str[0] === "'") && str[0] === last(str)) return str.slice(1, -1)
   return scope.get(str)
 }
 
+export function evalValue (str: string, scope: Scope): any {
+  const value = parseValue(str, scope)
+  return isDrop(value) ? value.value() : value
+}
+
 export function isTruthy (val: any): boolean {
   return !isFalsy(val)
 }

src/scope/scope.ts

@@ -3,11 +3,17 @@ import { __assign } from 'tslib'
 import assert from '../util/assert'
 import { NormalizedFullOptions, applyDefault } from '../liquid-options'
 import BlockMode from './block-mode'
-import IContext from './icontext'
+
+export type Context = {
+  [key: string]: any
+  liquid_method_missing?: (key: string) => any // eslint-disable-line
+  to_liquid?: () => any // eslint-disable-line
+  toLiquid?: () => any  // eslint-disable-line
+}
 
 export default class Scope {
   opts: NormalizedFullOptions
-  contexts: Array<IContext>
+  contexts: Array<Context>
   blocks: object = {}
   groups: {[key: string]: number} = {}
   blockMode: BlockMode = BlockMode.OUTPUT
@@ -67,10 +73,10 @@ export default class Scope {
     }
     return null
   }
-  private readProperty (obj: IContext, key: string) {
+  private readProperty (obj: Context, key: string) {
     let val
     if (_.isNil(obj)) {
-      val = undefined
+      val = obj
     } else {
       obj = toLiquid(obj)
       val = key === 'size' ? readSize(obj) : obj[key]
@@ -144,7 +150,7 @@ export default class Scope {
   }
 }
 
-function toLiquid (obj: IContext) {
+function toLiquid (obj: Context) {
   if (_.isFunction(obj.to_liquid)) {
     return obj.to_liquid()
   }
@@ -154,7 +160,7 @@ function toLiquid (obj: IContext) {
   return obj
 }
 
-function readSize (obj: IContext) {
+function readSize (obj: Context) {
   if (!_.isNil(obj.size)) return obj.size
   if (_.isArray(obj) || _.isString(obj)) return obj.length
   return obj.size

src/util/underscore.ts

@@ -6,11 +6,11 @@ const arrToStr = Array.prototype.toString
  * @param {any} value The value to check.
  * @return {Boolean} Returns true if value is a string, else false.
  */
-export function isString (value: any) {
+export function isString (value: any): value is string {
   return toStr.call(value) === '[object String]'
 }
 
-export function isFunction (value: any) {
+export function isFunction (value: any): value is Function {
   return typeof value === 'function'
 }
 
@@ -37,7 +37,7 @@ export function stringify (value: any): string {
 }
 
 function defaultToString (value: any): string {
-  const cache: string[] = []
+  const cache: any[] = []
   return JSON.stringify(value, (key, value) => {
     if (isObject(value)) {
       if (cache.indexOf(value) !== -1) {
@@ -57,12 +57,12 @@ export function isNil (value: any): boolean {
   return value === null || value === undefined
 }
 
-export function isArray (value: any): boolean {
+export function isArray (value: any): value is any[] {
   // be compatible with IE 8
   return toStr.call(value) === '[object Array]'
 }
 
-export function isError (value: any): boolean {
+export function isError (value: any): value is Error {
   const signature = toStr.call(value)
   // [object XXXError]
   return signature.substr(-6, 5) === 'Error' ||
@@ -102,7 +102,7 @@ export function last (arr: any[] | string): any | string {
  * @param {any} value The value to check.
  * @return {Boolean} Returns true if value is an object, else false.
  */
-export function isObject (value: any): boolean {
+export function isObject (value: any): value is object {
   const type = typeof value
   return value !== null && (type === 'object' || type === 'function')
 }

test/e2e/drop.ts

@@ -0,0 +1,21 @@
+import Liquid from '../..'
+import { expect, use } from 'chai'
+import * as chaiAsPromised from 'chai-as-promised'
+
+use(chaiAsPromised)
+
+describe('drop', function () {
+  var engine: Liquid
+  beforeEach(function () {
+    engine = new Liquid()
+  })
+  it('should test blank strings', async function () {
+    const src = `
+    {% unless settings.fp_heading == blank %}
+        <h1>{{ settings.fp_heading }}</h1>
+    {% endunless %}`
+    var ctx = { settings: { fp_heading: '' } }
+    const html = await engine.parseAndRender(src, ctx)
+    return expect(html).to.match(/^\s+$/)
+  })
+})

test/e2e/parse-and-render.ts

@@ -62,4 +62,9 @@ describe('.parseAndRender()', function () {
     const html = await engine.parseAndRender(src)
     return expect(html).to.equal('apples')
   })
+  it('should support nil(null, undefined) literal', async function () {
+    const src = '{% if notexist == nil %}true{% endif %}'
+    const html = await engine.parseAndRender(src)
+    expect(html).to.equal('true')
+  })
 })

test/unit/builtin/tags/case.ts

@@ -19,22 +19,20 @@ describe('tags/case', function () {
     const html = await liquid.parseAndRender(src)
     return expect(html).to.equal('foo')
   })
-  it('should resolve empty string if not hit', async function () {
-    const src = '{% case empty %}' +
-            '{% when "foo" %}foo{% when ""%}bar' +
-            '{%endcase%}'
-    const ctx = {
-      empty: ''
-    }
-    const html = await liquid.parseAndRender(src, ctx)
+  it('should resolve blank as empty string', async function () {
+    const src = '{% case blank %}{% when ""%}bar{%endcase%}'
+    const html = await liquid.parseAndRender(src)
+    return expect(html).to.equal('bar')
+  })
+  it('should resolve empty as empty string', async function () {
+    const src = '{% case empty %}{% when ""%}bar{%endcase%}'
+    const html = await liquid.parseAndRender(src)
     return expect(html).to.equal('bar')
   })
   it('should accept empty string as branch name', async function () {
-    const src = '{% case false %}' +
-            '{% when "foo" %}foo{% when ""%}bar' +
-            '{%endcase%}'
+    const src = '{% case "" %}{% when ""%}bar{%endcase%}'
     const html = await liquid.parseAndRender(src)
-    return expect(html).to.equal('')
+    return expect(html).to.equal('bar')
   })
   it('should support boolean case', async function () {
     const src = '{% case false %}' +

test/unit/builtin/tags/for.ts

@@ -1,12 +1,12 @@
 import Liquid from 'src/liquid'
 import { expect, use } from 'chai'
 import * as chaiAsPromised from 'chai-as-promised'
-import IContext from 'src/scope/icontext'
+import { Context } from 'src/scope/scope'
 
 use(chaiAsPromised)
 
 describe('tags/for', function () {
-  let liquid: Liquid, ctx: IContext
+  let liquid: Liquid, ctx: Context
   before(function () {
     liquid = new Liquid()
     liquid.registerTag('throwingTag', {