Conservative Language Use For Fun and Profit

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Conservative Mark Hibberd Scala

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You can make better use of tools by having deep knowledge of a tool, but only applying a specialised subset of its features. Claim #1

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You can’t effectively control your use of tools to a specialised subset unless you have very deep knowledge of the tool. Claim #2

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Programming languages are just tools. Claim #3

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Conservative usage patterns generalise to lots of tools not just languages. Claim #4

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SCALA a primer

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1 scala> val x = List("1", "2", "3", "4").toSet() + "5" 2 3 scala> println(x) 4 5 output: 6 7 ???

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1 scala> val x = List("1", "2", "3", "4").toSet() + "5" 2 3 scala> println(x) 4 5 output: 6 7 false5

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1 scala> println({} + "") 2 3 output: 4 5 ???

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1 scala> println({} + "") 2 3 output: 4 5 ()

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1 scala> val x = Option(1).zip(Option(1)) == 2 Option((1, 1)) 3 4 scala> println(x) 5 6 output: 7 8 ???

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1 scala> val x = Option(1).zip(Option(1)) == 2 Option((1, 1)) 3 4 scala> println(x) 5 6 output: 7 8 false

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1 scala> val x = Option(1).zip(Option(1)) == 2 List((1, 1)) 3 4 scala> println(x) 5 6 output: 7 8 ???

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1 scala> val x = Option(1).zip(Option(1)) == 2 List((1, 1)) 3 4 scala> println(x) 5 6 output: 7 8 true

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1 scala> val x = Option(1).zip(Option(1)) == 2 Vector((1, 1)) 3 4 scala> println(x) 5 6 output: 7 8 true

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scalac thinks your code is bad

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1 List("1", "2", "3", "4").toSet() + “5"

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1 List("1", "2", "3", "4").toSet() + "5" 2 3 // example.scala:1: error: not enough arguments for 4 // method apply: (elem: Any)Boolean in trait GenSet. 5 // Unspecified value parameter elem. -Yno-adapted-args

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1 def update(data: String): Future[Unit] = for { 2 n <- get 3 _ <- save(n, data) 4 } yield set(n + 1) 5 6 def get: Future[Int] = 7 ??? 8 9 def set(n: Int): Future[Int] = 10 ??? 11 12 def save(n: Int, data: String): Future[Unit] = 13 ???

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1 def update(data: String): Future[Unit] = for { 2 n <- get 3 _ <- save(n, data) 4 } yield set(n + 1) 5 6 //example.scala:4: warning: discarded non-Unit value -Ywarn-value-discard

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-Xlint / -Ywarn-all https://github.com/scala/scala/blob/v2.10.3/src/compiler/scala/tools/nsc/settings/ Warnings.scala#L18-L44

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-Xfatal-warnings

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-Yno-predef

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scala scala vs

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Lets reduce the surface area of the language we have to worry about

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Lets not save characters by risking correctness

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Lets not make it “easy” by building APIs using ever scala feature ever

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Lets do more with less

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Syntax

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Types & Inference

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Closed better than Open

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Don’t confuse Types and Data

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Scala does not have Type Inference

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Variance or Not

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Any is the same as not having Types

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Implicits

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Type Classes vs Global Mutable HashMaps

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Syntactic Extensions

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Scala is just a Tool

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Performance vs Correctness

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Concurrent Scala is not a Win

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Integration not Ecosystem

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End Game