Seven at one blow – New & polished features in Scalaz 7

Seven at one blow
New & polished features in Scalaz 7
Lars Hupel
Februrary 8th, 2013

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À-la-carte implicits Unapply Laws Type-level programming Community involvement
About @larsr h
▶ studies Computer Science
▶ first contribution December 2011
▶ committer since February 2012
▶ active (co)maintainer since October 2012
▶ did not invent any of the following
Lars Hupel Seven at one blow Februrary 8th, 2013 2 / 35

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About Scalaz
Scalaz is a Scala library for functional programming.
▶ purely functional data structures
▶ type classes known from Haskell
▶ current stable version: 6.0.4
▶ successor under development for over a year
compatible with 2.9.x, 2.10.x (and probably 2.11.x)

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À-la-carte implicits
Unapply
Laws
Type-level programming
Community involvement

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Unapply
Laws

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Previously, in Scalaz 6
all-you-can-eat object provides most of the implicits

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Previously, in Scalaz 6
Example
// import everything
import scalaz._
import Scalaz._
// get all instances
implicitly[Monad[Option]]
// get all syntax
(10.some |@| none) { (_: Int) |+| (_: Int) }

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Now, in Scalaz 7
Distinction between different kinds of implicits
1. type class instances
▶ ... of our own data types
▶ ... of standard library types
2. syntax: infix operators
▶ ... for type class instances
▶ ... for standard library types
3. functions on our own data types

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Now, in Scalaz 7
Example
import scalaz._
{
// import the instance
import std.option._
Monad[Option]
}

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Now, in Scalaz 7
Example
import scalaz._
{
import std.option._, std.anyVal._
import syntax.apply._
import syntax.semigroup._
import syntax.std.option._
(10.some |@| none) { (_: Int) |+| (_: Int) }
}

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Now, in Scalaz 7
Example
import scalaz._
{
import std.option._, std.anyVal._
import syntax.all._, syntax.std.all._
(10.some |@| none) { (_: Int) |+| (_: Int) }
}

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Why is this better?
Disadvantages
▶ more import statements
▶ makes compile-time debugging harder

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Why is this better?
Advantages
▶ you can use syntax, but you don’t have to
▶ improves readability of the code
▶ makes compile-time debugging easier
▶ increases compilation speed

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Unapply
Laws

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The problem
def f[M[_], A](x: M[A]): M[A] = x
f { (x: Int) => x }

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The problem
def f[M[_], A](x: M[A]): M[A] = x
f { (x: Int) => x }
Prints a type error:
found : Int => Int
required: ?M[?A]

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The problem
Candidates
▶ M[X] = Int ⇒ X
▶ M[X] = X ⇒ Int
▶ M[X] = X ⇒ X
▶ M[X] = Int ⇒ Int
▶ ...
This is a known limitation (SI-2712).

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Let’s use math!
▶ known as Higher-order unification
▶ Scala adds subtyping
▶ everything is subtype of Any
▶ Any is always well-kinded
▶ ambiguous
▶ undecidable

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Let’s approximate!
def f[M[_], A](x: M[A]): M[A] = x
f { (x: Int) => x }
A type constructor without a type class is rarely useful.

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Let’s approximate!
def f[M[_] : Monad, A](x: M[A]): M[B] = ???
f { (x: Int) => x }
Now: more information which can guide type inference

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Dependent types to the rescue
Define a proof object containing:
▶ the type constructor M[_]
▶ the inner type A
▶ the type class instance M[_] : Monad
... and have lots of implicits guided by:
▶ prioritisation
▶ availability of type class instances

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Demo

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Does it work?
Yes!
▶ Scalaz 6: conversions for each concrete type
▶ now: general mechanism for common shapes
▶ in many cases, type parameters can be omitted
List(1, 2, 3) traverseU { a =>
State { x: Int => (x + 1, a) }
}

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Does it work?
Well, sort of.
▶ 500 lines of boilerplate
▶ one implicit per type shape (there are infinitely many)
▶ compile errors can get a little funky
▶ needs dependent types

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Unapply
Laws

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Contracts
.
.
“ The equals method implements an equivalence relation on non-null
object references:
▶ It is reflexive: for any non-null reference value x,
x.equals(x) should return true.
▶ It is symmetric: for any non-null reference values x and y,
x.equals(y) should return true if and only if y.equals(x)
returns true.
▶ ...
Java API: Object#equals .
.
”

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Type classes & Lavs
▶ type classes abstract over types
▶ offer common functionality
▶ users expect similar behaviour
▶ ... which needs to be formalised

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Legal problems
.
.
“ I have a legal question: Is this a lawful monoid to you?
Tony Morris .
.
”

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Laws in Scalaz
▶ embedded in type classes, mirroring their hierarchy
▶ one method per law
▶ ScalaCheck tests instances
▶ ... by producing random input
▶ Not a theorem prover!
▶ ... but better than usual unit tests
▶ our bindings can be used by other projects
▶ 332 occurences of checkAll in our tests

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Demo

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Unapply
Laws

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It’s in the types
Scalaz has heterogeneous lists too!
▶ ... but not as general as in Shapeless
▶ used for:
▶ Applicative syntax
▶ String formatting
▶ type class composition

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Heterogeneous lists
Everybody loves tuples.
▶ fixed arity, different element types
▶ maximum arity 22, one class per arity
▶ but: no useful manipulations (reverse, append, access, ...)

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Demo

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Unapply
Laws

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What happened in the past year?
We got tons of contributions (≈ 200 issues & pull requests).
▶ folding non-empty data structures
▶ Lens families
▶ documentation & test fixes
▶ interoperability with other libraries
▶ new data types
▶ less lawless classes
▶ ...

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Validation DSL
check(”123456789012345”).
checkThat(strLength(16, ”wrong length”)).
checkThat(luhn(”wrong checksum”)).
convertTo(int(”not an integer”)).
toValidation
// Result:
// Failure(NonEmptyList(wrong length, wrong checksum))

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What can you do?
... if you’re using Scalaz 6
▶ migrate to the latest milestone
▶ Tell us about it!
... if you’re not using Scalaz
▶ adopt Scalaz 7
▶ Ask questions!

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What are we going to do?
▶ release 7.0.0-final soon ... probably this year
▶ not break too much until then
▶ stay binary compatible in the 7.0.x series
▶ revamp our web site(s)

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Q & A
larsrh.github.com
@larsr h

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Seven at one blow – New & polished features in Scalaz 7

Seven at one blow – New & polished features in Scalaz 7

Lars Hupel

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