A basic, still descriptive, example that demonstrates some possible uses for Http4s Extend can be found here
Note: this project is in early alpha stage. Not usable in production.
Http4s Extend is available for Scala 2.11.x and 2.12.x. To use it create the dependency by adding the following to the sbt build
libraryDependencies += "com.github.barambani" %% "http4s-extend" % "0.0.43"a sample configuration can be found here where the dependencies are taken from here
For its own purpose is integration, Http4s Extend will bring into a using project some dependencies. Those are:
| Http4s | Monix | Scalaz Concurrent | |
|---|---|---|---|
| 0.0.43 | 0.18.19 | 3.0.0-RC1 | 7.3.0-M25 |
The NewType trait is a building block to help create zero allocation new types like
object MkAndBoolean extends newtype[Boolean]
val AndBoolean = MkAndBoolean
type AndBoolean = AndBoolean.TThis tecnique is a great help when trying to avoid orphan type class instances for, creating a newtype, allows to have eventual instances in the implicit scope even when the type of interest and the type class itself are owned by someone else and their companion objects cannot be changed. Having this possibility without paying an allocation cost per use is very desirable and cannot be achieved with the language's Value Classes. Considering this example in fact
class ValueClass(val v: Boolean) extends AnyVal
class testNewType {
val ntA1 = AndBoolean(true)
val ntA2 = AndBoolean(false)
val ntTuple = (ntA1, ntA2)
val ntLs = List(ntA1, ntA2)
val ntId = identity(ntA1)
}
class testValueClass {
val vcA1 = new ValueClass(true)
val vcA2 = new ValueClass(false)
val vcTuple = (vcA1, vcA2)
val vcLs = List(vcA1, vcA2)
val vcId = identity(vcA1)
}and giving a look at its disassembled code we can see how NewType's approach differs from the Value Classes in terms of allocations (see how the occurrences of new differ).
Tuple
public http4s.extend.testValueClass();
descriptor: ()V
Code:
37: new #54 // class scala/Tuple2
40: dup
41: new #81 // class http4s/extend/ValueClass
44: dup
45: aload_0
46: invokevirtual #83 // Method vcA1:()Z
49: invokespecial #86 // Method http4s/extend/ValueClass."<init>":(Z)V
52: new #81 // class http4s/extend/ValueClass
55: dup
56: aload_0
57: invokevirtual #88 // Method vcA2:()Z
60: invokespecial #86 // Method http4s/extend/ValueClass."<init>":(Z)V
63: invokespecial #91 // Method scala/Tuple2."<init>":(Ljava/lang/Object;Ljava/lang/Object;)V
66: putfield #50 // Field vcTuple:Lscala/Tuple2;
public http4s.extend.testNewType();
descriptor: ()V
Code:
55: new #59 // class scala/Tuple2
58: dup
59: aload_0
60: invokevirtual #116 // Method ntA1:()Ljava/lang/Object;
63: aload_0
64: invokevirtual #118 // Method ntA2:()Ljava/lang/Object;
67: invokespecial #121 // Method scala/Tuple2."<init>":(Ljava/lang/Object;Ljava/lang/Object;)V
70: putfield #55 // Field ntTuple:Lscala/Tuple2;List
public http4s.extend.testValueClass();
descriptor: ()V
Code:
81: getstatic #97 // Field scala/collection/immutable/List$.MODULE$:Lscala/collection/immutable/List$;
84: getstatic #102 // Field scala/Predef$.MODULE$:Lscala/Predef$;
87: iconst_2
88: anewarray #81 // class http4s/extend/ValueClass
91: dup
92: iconst_0
93: new #81 // class http4s/extend/ValueClass
96: dup
97: aload_0
98: invokevirtual #83 // Method vcA1:()Z
101: invokespecial #86 // Method http4s/extend/ValueClass."<init>":(Z)V
104: aastore
105: dup
106: iconst_1
107: new #81 // class http4s/extend/ValueClass
110: dup
111: aload_0
112: invokevirtual #88 // Method vcA2:()Z
115: invokespecial #86 // Method http4s/extend/ValueClass."<init>":(Z)V
118: aastore
119: invokevirtual #106 // Method scala/Predef$.genericWrapArray:(Ljava/lang/Object;)Lscala/collection/mutable/WrappedArray;
122: invokevirtual #110 // Method scala/collection/immutable/List$.apply:(Lscala/collection/Seq;)Lscala/collection/immutable/List;
125: putfield #57 // Field vcLs:Lscala/collection/immutable/List;
public http4s.extend.testNewType();
descriptor: ()V
Code:
85: getstatic #126 // Field scala/collection/immutable/List$.MODULE$:Lscala/collection/immutable/List$;
88: getstatic #131 // Field scala/Predef$.MODULE$:Lscala/Predef$;
91: iconst_2
92: anewarray #4 // class java/lang/Object
95: dup
96: iconst_0
97: aload_0
98: invokevirtual #116 // Method ntA1:()Ljava/lang/Object;
101: aastore
102: dup
103: iconst_1
104: aload_0
105: invokevirtual #118 // Method ntA2:()Ljava/lang/Object;
108: aastore
109: invokevirtual #135 // Method scala/Predef$.genericWrapArray:(Ljava/lang/Object;)Lscala/collection/mutable/WrappedArray;
112: invokevirtual #138 // Method scala/collection/immutable/List$.apply:(Lscala/collection/Seq;)Lscala/collection/immutable/List;
115: putfield #62 // Field ntLs:Lscala/collection/immutable/List;Identity
public http4s.extend.testValueClass();
descriptor: ()V
Code:
141: getstatic #102 // Field scala/Predef$.MODULE$:Lscala/Predef$;
144: new #81 // class http4s/extend/ValueClass
147: dup
148: aload_0
149: invokevirtual #83 // Method vcA1:()Z
152: invokespecial #86 // Method http4s/extend/ValueClass."<init>":(Z)V
155: invokevirtual #114 // Method scala/Predef$.identity:(Ljava/lang/Object;)Ljava/lang/Object;
158: checkcast #81 // class http4s/extend/ValueClass
161: invokevirtual #117 // Method http4s/extend/ValueClass.v:()Z
164: putfield #63 // Field vcId:Z
public http4s.extend.testNewType();
descriptor: ()V
Code:
131: getstatic #131 // Field scala/Predef$.MODULE$:Lscala/Predef$;
134: aload_0
135: invokevirtual #116 // Method ntA1:()Ljava/lang/Object;
138: invokevirtual #141 // Method scala/Predef$.identity:(Ljava/lang/Object;)Ljava/lang/Object;
141: putfield #68 // Field ntId:Ljava/lang/Object;Even if this approach is understandably the best compromise available in Scala at the moment, it comes at a cost. When the newtypes are used on value types in fact, they are not erased to the original base type as it's the case for Value Classes, but they are erased to Object as can be seen from the disassembled code above and from the snippet below
public boolean vcA1();
descriptor: ()Z
public boolean vcA2();
descriptor: ()Zpublic java.lang.Object ntA1();
descriptor: ()Ljava/lang/Object;
public java.lang.Object ntA2();
descriptor: ()Ljava/lang/Object;