Instantly share code, notes, and snippets.

Embed
What would you like to do?
/***
scalaVersion := "2.11.6"
libraryDependencies ++= Seq("org.scalaz" %% "scalaz-core" % "7.1.0")
*/
// Code listing for http://devth.com/2015/thrush-cond-is-not-a-monad/
object Main extends App {
import scalaz._, Scalaz._
type Step[A] = (A => Boolean, A => A)
case class Request(
target: String,
params: Map[String, String] = Map.empty,
headers: Map[String, String] = Map.empty) {
// validation and helper functions
val acceptMap = Map("html" -> "text/html", "json" -> "application/json")
val isValidAccept: (String => Boolean) = acceptMap.isDefinedAt _
def addParam(k: String, v: String) = this.copy(params=params.updated(k, v))
def addHeader(k: String, v: String) = this.copy(headers=headers.updated(k, v))
}
case class ThrushCond[A](steps: Step[A]*) {
/** Perform a pipeline step only if the value meets a predicate */
def guard[A](pred: (A => Boolean), fn: (A => A)): (A => A) =
(a: A) => if (pred(a)) fn(a) else a
/** Compose the steps into a single function */
def comp = Function.chain(steps.map { step => guard(step._1, step._2) })
/** Run a value through the pipeline */
def run(a: A) = comp(a)
}
case object ThrushCond {
/** Evidence of a PlusEmpty */
implicit def thrushCondPlusEmpty: PlusEmpty[ThrushCond] =
new PlusEmpty[ThrushCond] {
def plus[A](a: ThrushCond[A], b: => ThrushCond[A]): ThrushCond[A] =
ThrushCond[A]((Function.const(true), b.comp compose a.comp))
def empty[A]: ThrushCond[A] = ThrushCond[A]()
}
/** Use PlusEmpty to provide evidence of a Monoid[Request] */
implicit def requestMonoid: Monoid[ThrushCond[Request]] =
thrushCondPlusEmpty.monoid[Request]
/** Evidence of a Semigroup */
implicit def thrushCondSemigroup[A]: Semigroup[ThrushCond[A]] =
new Semigroup[ThrushCond[A]] {
def append(t1: ThrushCond[A], t2: => ThrushCond[A]): ThrushCond[A] =
ThrushCond[A]((Function.const(true), t2.comp compose t1.comp))
}
}
import ThrushCond._ // evidence
import scala.language.postfixOps
//
// Examples
//
// sample values from user input
val userId: Int = 1
val userName: Option[String] = Some("devth")
val address: Option[String] = None
val accept = "json"
val shouldCache = false
val req = Request("/users")
val userPipeline = ThrushCond[Request](
({_ => userName.isDefined}, {_.addParam("userName", userName.get)}),
({_ => address.isDefined}, {_.addParam("address", address.get)}))
val headerPipeline = ThrushCond[Request](
({_.isValidAccept(accept)}, {req =>
req.addHeader("accept", req.acceptMap(accept))}))
//
// Semigroup
//
println("Semigroup:")
println(userPipeline |+| headerPipeline run req)
//
// PlusEmpty
//
println("PlusEmpty:")
println(userPipeline <+> headerPipeline run req)
//
// Monoid
//
val cachePipeline = ThrushCond[Request](
({_ => !shouldCache}, {_.addHeader("cache-control", "no-cache")}))
val pipeline = List(userPipeline, headerPipeline, cachePipeline) suml
println("Monoid:")
println(pipeline run req)
//
// Output
//
// Semigroup:
// Request(/users,Map(userName -> devth),Map(accept -> application/json))
// PlusEmpty:
// Request(/users,Map(userName -> devth),Map(accept -> application/json))
// Monoid:
// Request(/users,Map(userName -> devth),Map(accept -> application/json,
// cache-control -> no-cache))
}
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment