• Closure VS. Method
• break from closure
• curry
• Memoization
• composition
• methods
• delegate
• tricky
• example

[!NOTE] references:

• Closures
• Closures in Groovy
• Groovy Goodness: Passing Closures to Methods
• Groovy Goodness: Closure Arguments
• Groovy Goodness: Identity Closure
• 实战 Groovy: 用 curry 过的闭包进行函数式编程

Closure VS. Method

closures have benefits over regular methods and are a powerful feature of Groovy:

• We can pass a Closure as an argument to a method
• Unary closures can use the implicit it parameter
• We can assign a Closure to a variable and execute it later, either as a method or with call
• Groovy determines the return type of the closures at runtime
• We can declare and invoke closures inside a closure
• Closures always return a value

• method

  def formatToLowerCase( String name ) {
    name.toLowerCase()
  }
  

• closure

  def formatToLowerCaseClosure = { name ->
    name.toLowerCase()
  }
  

break from closure

def list = [1, 2, 3, 4, 5]
list.any { element ->
  if (element == 2)
    return // continue

  println element

  if (element == 3)
    return true // break
}

curry

• left curry

  def multiply = { x, y -> return x * y }
  def triple = multiply.curry(3)           // triple = { y -> return 3 * y }
  

• example

  def nCopies = { int n, String... str -> str.join('')*n }
  def twice = nCopies.curry(2)
  println twice('|', '\\', '|', '/' )
  assert twice('|', '\\', '|', '/' ) == nCopies( 2, '|', '\\', '|', '/' )
  

  • result

    |\|/|\|/
    

• others left curry

  def multiConcat = { int n, String... args ->
    args.join('')*n
  }
  multiConcat( 3, '*', '-', '=' )
  

  • result

    *-=*-=*-=
    

• right curry

  def nCopies = { int n, String str -> str*n }
  def twice = nCopies.rcurry( '*-=*=-*' )
  println twice(2)
  assert twice(2) == nCopies( 2, '*-=*=-*' )
  

  • result

    *-=*=-**-=*=-*
    

  [!TIP]

  def nCopies = { int n, String str -> str*n }
  
  def twice   = nCopies.rcurry( '*-=*=-*' )
  def divider = nCopies.curry( 2 )
  
  assert nCopies( 2, '*-=*=-*' ) == twice( 2 )                // right curry
  assert nCopies( 2, '-.-.-.-' ) == divider( '-.-.-.-' )      // left  curry
  

• index with curry

  def volume = { int l, int w, int h -> "l: ${l}\nw: ${w}\nh: ${h}" }
  def fixedWidthVolume = volume.ncurry(1, 2)
  println fixedWidthVolume( 3, 4 )
  

  • result

    l: 3
    w: 2
    h: 4
    

Memoization

• slow

  def fib
  fib = { long n -> n<2 ? n : fib(n-1) + fib(n-2) }
  println fib(10)
  
  // result
  55
  

• fast

  tips: Closures.memoize()

  def fib
  fib = { long n -> n<2 ? n : fib(n-1) + fib(n-2) }.memoize()
  fib(50)
  
  // result
  12586269025
  

composition

double composition

def plus2  = { it + 2 }
def times3 = { it * 3 }
def times3plus2 = plus2 << times3
//                  |        + execute first
//                  + execute second
// result
assert times3plus2(3) == 3*3+2
assert times3plus2(3) == plus2(times3(3))

def plus2times3 = plus2 >> times3
//                  |        + execute last
//                  + execute first
// result
assert plus2times3(3) == (3+2)*3
assert plus2times3(3) == times3(plus2(3))

• example for recursive in List

  def map1 = [ a: 10 , b: 2 , c: 3 ]
  def map2 = [ b: 3  , c: 2 , d: 5 ]
  def maps = [ map1  , map2 ]
  
  def process( def maps, Closure myLambda ) {
    maps.sum { it.keySet() }.collectEntries { key ->
      [ key,
        { x ->
          x.subList(1, x.size()).inject(x[0], myLambda)
        }( maps.findResults { it[key] } )
      ]
    }
  }
  
  def sumResult  = process(maps) { a, b -> a + b }
  def prodResult = process(maps) { a, b -> a * b }
  def minResult  = process(maps) { a, b -> a < b ? a : b }
  
  assert sumResult  == [ a:10, b:5, c:5, d:5 ]
  assert prodResult == [ a:10, b:6, c:6, d:5 ]
  assert minResult  == [ a:10, b:2, c:2, d:5 ]
  

  • resolution

    assert [2,4,5].inject(1){ a, b -> a + b  }   == 12
    assert [2,4,5].inject(1, { a, b -> a + b }) == 12
    

triple composition

def multiply    = { x, y -> return x * y }
def triple      = multiply.curry(3)
def quadruple   = multiply.curry(4)
def composition = { f, g, x -> return f(g(x)) }
def twelveTimes = composition.curry(triple, quadruple)      //  twelveTimes = { y -> composition { y -> 3*(4*y) } }
def threeDozen  = twelveTimes(3)

methods

• various method to call closure

  def work( String input, Closure cl ) {
    cl(input)
  }
  
  Closure assertJava = {
    it == 'Java'
  }
  
  println work( "Java", assertJava )
  println work("Java", { it == 'Java' })   // ==> work 'Java', { it == 'Java' }
  println work( 'Java' ){
    it == 'Java'
  }
  

• frequent usage

  def on( String name, String dString = 'is' ) {
    [
      skip: { -> println "no params. skip" },
  
      foo: { String f, Map fmap ->
        foo( name, dString, f, fmap )
        [
          bar: { Map map -> bar( '', map ) }
        ]
      } ,
  
      bar: { Map map -> bar( name, map ) }
    ]
  }
  
  def reset( Map m ) {
    m.collect{ k, v -> "${k} : ${v}" }.join('\n')
  }
  
  def foo( String n, String ds, String f, Map m ) {
    println "${n} ${ds} ${f} !\ndetails :\n\t\t${reset(m)}"
  }
  
  def bar( String n = '', Map m ) {
    println "${n ? "${n}\n": ''}\t\t${reset(m)}"
  }
  

  • result

    on('marslo')
      .foo( 'awesome', [ 'age' : 34 ] )
      .bar( ['gender' : 'female' ] )
    
    // result
    marslo is awesome !
    details :
        age : 34
        gender : female
    

  or

  on('marslo').skip()
  
  // result
  no params. skip
  

delegate

[!NOTE] references:

• Convert list to enumerated list in Groovy

List.metaClass.collectWithIndex = { yield ->
  def collected = []
  delegate.eachWithIndex { listItem, index ->
    collected << yield(listItem, index)
  }

  return collected
}

result = list.collectWithIndex { it, index -> "${index + 1}. ${it}" }

• .collect with an index

  List.metaClass.collectWithIndex = { body->
    def i=0
    delegate.collect { body(it, i++) }
  }
  
  // or even
  List.metaClass.collectWithIndex = { body->
    [ delegate, 0..<delegate.size() ].transpose().collect(body)
  }
  

tricky

this

class Enclosing {
  void run() {
    def whatIsThisObject = { getThisObject() }
    assert whatIsThisObject() == this

    def whatIsThis = { this }
    assert whatIsThis() == this
  }
}

Enclosing e = new Enclosing()
e.run()

example

Closure return Closure

• simple

  def withClosure( Object object, Closure body ) {
    Closure bar = { obj -> obj.toString() }
    Closure foo = { cid ->
      [
        (java.lang.String)    : { body([ claz: cid.class.simpleName,  str: bar(cid) ]) } ,
        (java.util.ArrayList) : { body([ claz: cid.class.simpleName, list: bar(cid) ]) }
      ].find { it.key.isAssignableFrom( cid.getClass() ) }.value.call()
    }
    foo object
  }
  
  withClosure('s')   { b -> println "${b.claz} -> ${b.str}" }
  withClosure(['a']) { b -> println "${b.claz} -> ${b.list}" }
  
  -- result --
  String -> s
  ArrayList -> [a]
  

• working with Jenkins Credential Class

  import com.datapipe.jenkins.vault.credentials.common.VaultSSHUserPrivateKeyImpl
  import com.datapipe.jenkins.vault.credentials.common.VaultUsernamePasswordCredentialImpl
  import com.cloudbees.plugins.credentials.impl.UsernamePasswordCredentialsImpl
  import com.cloudbees.jenkins.plugins.sshcredentials.impl.BasicSSHUserPrivateKey
  import com.cloudbees.plugins.credentials.CredentialsProvider
  import com.cloudbees.plugins.credentials.common.StandardCredentials
  import groovy.transform.Field
  
  def withCredential( String credentialsId, Closure body ) {
    Closure foo = { cid ->
      [
        ( VaultSSHUserPrivateKeyImpl )          : { body([ username: cid.username, id: cid.id ]) },
        ( VaultUsernamePasswordCredentialImpl ) : { body([ username: cid.username, id: cid.id ]) },
        ( UsernamePasswordCredentialsImpl )     : { body([ username: cid.username, id: cid.id ]) },
        ( BasicSSHUserPrivateKey )              : { body([ username: cid.username, id: cid.id ]) }
      ].find { it.key.isAssignableFrom( cid.getClass() ) }.value.call()
    }
  
    foo CredentialsProvider.lookupCredentials( StandardCredentials.class, jenkins.model.Jenkins.instance).find { credentialsId == it.id }
  }
  
  withCredential( 'BASIC_CREDENTIAL' ) { account ->
    println "${account.id.getClass()} -> ${account.username}"
  }
  
  withCredential( 'SSH_CREDENTIAL' ) { account ->
    println "${account.id.getClass()} -> ${account.username}"
  }
  
  -- result --
  [Pipeline] Start of Pipeline
  [Pipeline] echo
  class java.lang.String -> functional-account
  [Pipeline] echo
  class java.lang.String -> functional-account
  [Pipeline] End of Pipeline
  Finished: SUCCESS
  

• Closure return Closure with DSL

  def withCredential( String credentialsId, Closure body ) {
    Closure foo = { cid ->
      [
        ( VaultSSHUserPrivateKeyImpl )          : {
            withCredentials([[ $class: 'VaultSSHUserPrivateKeyBinding', credentialsId: credentialsId, privateKeyVariable : 'SSHKEY', usernameVariable: 'USERNAME' ]]) {
              body([ username : '${USERNAME}', sshkey : '${SSHKEY}' ])
            }
        },
  
        ( VaultUsernamePasswordCredentialImpl ) : {
            withCredentials([[ $class: 'VaultUsernamePasswordCredentialBinding', credentialsId: credentialsId, passwordVariable: 'PASSWORD', usernameVariable: 'USERNAME' ]]) {
              body([ username : '${USERNAME}', password : '${PASSWORD}' ])
            }
        },
  
        ( UsernamePasswordCredentialsImpl )     : {
            withCredentials([ usernamePassword( credentialsId: credentialsId, usernameVariable: 'USERNAME', passwordVariable: 'PASSWORD' ) ]) {
              body([ username : '${USERNAME}', password : '${PASSWORD}' ])
            }
        },
  
        ( BasicSSHUserPrivateKey )              : {
            withCredentials([ sshUserPrivateKey( credentialsId: credentialsId, keyFileVariable: 'SSHKEY', usernameVariable: 'USERNAME' ) ]) {
              body([ username : '${USERNAME}', sshkey : '${SSHKEY}' ])
            }
        }
      ].find { it.key.isAssignableFrom( cid.getClass() ) }.value.call()
    }
  
    foo CredentialsProvider.lookupCredentials( StandardCredentials.class, jenkins.model.Jenkins.instance).find { credentialsId == it.id }
  }
  
  def download( String url, String credential = 'BASIC_CREDENTIAL' ) {
    withCredential( credential ) { account ->
      sh """
        set +x
        [ ! -z '${name}' ] && [ ! -d \$(dirname '${name}') ] && mkdir -p \$(dirname '${name}')
        bash -c "/usr/bin/curl -u${account.username}:${account.password} -skg ${url}"
      """
    } // withCredential
  }