6
我指的是下面列出的Ken Scambler的源代碼，也請參閱GitHub source。免費單子和AST的關係
package kenbot.free
import scalaz._
import Scalaz._
import Free._
import scala.collection.mutable
// This example is based off the one in Runar Bjarnason's "Dead Simple Dependency Injection" talk.
// http://www.youtube.com/watch?v=ZasXwtTRkio
// 0. Fantasy API
// def put(key: String, value: String): Unit
// def get(key: String): String
// def delete(key: String): Unit
// 1. ADT
sealed trait KVS[+Next]
case class Put[Next](key: String, value: String, next: Next) extends KVS[Next] // <---- def put(key: String, value: String): Unit
case class Get[Next](key: String, onResult: String => Next) extends KVS[Next] // <---- def get(key: String): String
case class Delete[Next](key: String, next: Next) extends KVS[Next] // <---- def delete(key: String): Unit
object KVS {
type Script[A] = Free[KVS, A]
// 2. Functor definition
implicit val functor: Functor[KVS] = new Functor[KVS] {
def map[A,B](kvs: KVS[A])(f: A => B): KVS[B] = kvs match {
case Put(key, value, next) => Put(key, value, f(next))
case Get(key, onResult) => Get(key, onResult andThen f)
case Delete(key, next) => Delete(key, f(next))
}
}
// 3. Lifting functions
def put(key: String, value: String): Script[Unit] = liftF(Put(key, value,()))
def get(key: String): Script[String] = liftF(Get(key, identity))
def delete(key: String): Script[Unit] = liftF(Delete(key,()))
// 4. Composite functions
def modify(key: String, f: String => String): Free[KVS, Unit] = for {
v <- get(key)
_ <- put(key, f(v))
} yield()
// 5. Write scripts
val script: Free[KVS, Unit] = for {
id <- get("swiss-bank-account-id")
_ <- modify(id, (_ + 1000000))
_ <- put("bermuda-airport", "getaway car")
_ <- delete("tax-records")
} yield()
// 6. Interpreters
// Building an immutable structure
def interpretPure(kvs: Script[Unit], table: Map[String, String] = Map.empty): Map[String, String] = kvs.resume.fold({
case Get(key, onResult) => interpretPure(onResult(table(key)), table)
case Put(key, value, next) => interpretPure(next, table + (key -> value))
case Delete(key, next) => interpretPure(next, table - key)
}, _ => table)
// Directly running effects
def interpretImpure(kvs: Script[Unit], table: mutable.Map[String, String]): Unit = kvs.go {
case Get(key, onResult) => onResult(table(key))
case Put(key, value, next) => table += (key -> value); next
case Delete(key, next) => table -= key; next
}
}
如果按照下列slides，任何腳本（見源代碼5）被「轉化」成免費的單子中類似Suspend(Op(Suspend(Op(......(Result(Op))..))。
不幸的是，5下的腳本只是一個線性的命令序列。
即使搜索了幾個小時的網在，我無法找到任何的例子，給了以下的問題更深入的瞭解：
- 線性操作的順序（這也是樹當然）由
Suspend(Op(Suspend(Op(......(Result(Op))..))表示，並且因此是AST的表示。這個假設是正確的嗎？ - AST如何表示真實表示免費單子內的AST？我認爲，發生這種情況時，包括控制結構？ （例如根據條件左右樹枝）。有人可以舉例說明一個真正的AST起作用的例子嗎？也許，舉例說明如何在給定的例子中實現「if」。 （？在源代碼下5給出）
- 什麼是一般的方法，包括控制結構成腳本
PS：請嘗試堅持斯卡拉/ ScalaZ，因爲Haskell是（還）沒有我的領域的專業知識。