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April 30, 2017 22:20
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I wrote a monad
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/* __ *\ | |
** ________ ___ / / ___ __ ____ Scala.js tools ** | |
** / __/ __// _ | / / / _ | __ / // __/ (c) 2013-2017, LAMP/EPFL ** | |
** __\ \/ /__/ __ |/ /__/ __ |/_// /_\ \ http://scala-js.org/ ** | |
** /____/\___/_/ |_/____/_/ | |__/ /____/ ** | |
** |/____/ ** | |
\* */ | |
package org.scalajs.core.tools.linker.backend.emitter | |
/** A monad that associates a set of global variable names to a value. | |
* | |
* Definition: | |
* {{{ | |
* WithGlobals.apply(a) === WithGlobals(a, ∅) | |
* | |
* WithGlobals(v, w).flatMap(f) === WithGlobals(f(v).value, w ∪ f(v).globalVarNames) | |
* }}} | |
* | |
* Left identity: | |
* {{{ | |
* WithGlobals.apply(a).flatMap(f) =?= f(a) | |
* -------------------- | |
* WithGlobals(a, ∅).flatMap(f) =?= f(a) | |
* ---------------------------- | |
* WithGlobals(f(a).value, ∅ ∪ f(a).globalVarNames) =?= f(a) | |
* ----------------------- | |
* WithGlobals(f(a).value, f(a).globalVarNames ) =?= f(a) | |
* }}} | |
* | |
* Right identity: | |
* {{{ | |
* WithGlobals(v, w).flatMap(WithGlobals.apply) =?= WithGlobals(v, w) | |
* -------------------------------------------- | |
* WithGlobals(WithGlobals.apply(v).value, w ∪ WithGlobals.apply(v).globalVarNames) =?= WithGlobals(v, w) | |
* -------------------------- ----------------------------------- | |
* WithGlobals( v , w ∪ ∅ =?= WithGlobals(v, w) | |
* }}} | |
* | |
* Associativity: | |
* {{{ | |
* WithGlobals(v, w).flatMap(f).flatMap(g) =?= WithGlobals(v, w).flatMap(x => f(x).flatMap(g)) | |
* ------------ | |
* WithGlobals(f(v).value, w ∪ f(v).globalVarNames).flatMap(g) =?= ... | |
* ----------------------------------------------------------- | |
* WithGlobals(g(f(v).value).value, (w ∪ f(v).globalVarNames) ∪ g(f(v).value).globalVarNames) =?= ... | |
* | |
* ... =?= WithGlobals(v, w).flatMap(x => f(x).flatMap(g)) | |
* ----------------------------------------------- | |
* ... =?= WithGlobals((x => f(x).flatMap(g))(v).value, w ∪ (x => f(x).flatMap(g))(v).globalVarNames) | |
* ------------------------------- ---------------------------------------- | |
* | |
* Aside: | |
* (x => f(x).flatMap(g))(v) | |
* === f(v).flatMap(g) | |
* === WithGlobals(g(f(v).value).value, f(v).globalVarNames ∪ g(f(v).value).globalVarNames) | |
* | |
* ... =?= WithGlobals(g(f(v).value).value, w ∪ (f(v).globalVarNames ∪ g(f(v).value).globalVarNames)) | |
* }}} | |
*/ | |
private[emitter] final case class WithGlobals[+A]( | |
value: A, globalVarNames: Set[String]) { | |
import WithGlobals._ | |
def map[B](f: A => B): WithGlobals[B] = | |
WithGlobals(f(value), globalVarNames) | |
def flatMap[B](f: A => WithGlobals[B]): WithGlobals[B] = { | |
val t = f(value) | |
WithGlobals(t.value, unionPreserveEmpty(globalVarNames, t.globalVarNames)) | |
} | |
} | |
private[emitter] object WithGlobals { | |
/** Constructs a `WithGlobals` with an empty set `globalVarNames`. */ | |
def apply[A](value: A): WithGlobals[A] = | |
new WithGlobals(value, Set.empty) | |
def list[A](xs: List[WithGlobals[A]]): WithGlobals[List[A]] = { | |
/* This could be a cascade of flatMap's, but the following should be more | |
* efficient. | |
*/ | |
val values = xs.map(_.value) | |
val globalVarNames = xs.foldLeft(Set.empty[String]) { (prev, x) => | |
unionPreserveEmpty(prev, x.globalVarNames) | |
} | |
WithGlobals(values, globalVarNames) | |
} | |
def option[A](xs: Option[WithGlobals[A]]): WithGlobals[Option[A]] = | |
xs.fold[WithGlobals[Option[A]]](WithGlobals(None))(_.map(Some(_))) | |
/** Same as `a ++ b` but tries to preserve `Set.EmptySet`. | |
* | |
* Using this method over `a ++ b` is meaningful is there is a strong | |
* likelihood that both parameters are empty, or at least one of them. | |
*/ | |
private def unionPreserveEmpty(a: Set[String], b: Set[String]): Set[String] = { | |
if (a.isEmpty) b | |
else if (b.isEmpty) a | |
else a ++ b | |
} | |
} |
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