Leveraging Scala Macros for Better Validation

Leveraging Scala
Macros for Better
Validation
Tomer Gabel, Wix
September 2014

I Have a Dream
• Definition:
case class Person(
firstName: String,
lastName: String
)
implicit val personValidator =
validator[Person] { p ⇒
p.firstName is notEmpty
p.lastName is notEmpty
}

I Have a Dream
• Usage:
validate(Person("Wernher", "von Braun”))
== Success
validate(Person("", "No First Name”))
== Failure(Set(RuleViolation(
value = "",
constraint = "must not be empty",
description = "firstName"
)))

Basic Architecture
API
Combinator Library
DSL
Macro Transformation

The Accord API
• Validation can succeed or fail
• A failure comprises one or more violations
sealed trait Result
case object Success extends Result
case class Failure(violations: Set[Violation])
extends Result
• The validator typeclass:
trait Validator[-T] extends (T ⇒ Result)

Why Macros?
• Quick refresher:
}
Implicit “and”
Automatic description
generation

Full Disclosure
Macros are experimental
Macros are hard
I will gloss over a lot of details
… and simplify a lot of things

Abstract Syntax Trees
• An intermediate representation of code
– Structure (semantics)
– Metadata (e.g. types) – optional!
• Provided by the reflection API
• Alas, mutable
– Until Dotty comes along

def method(param: String) = param.toUpperCase

Apply(
Select(
Ident(newTermName("param")),
newTermName("toUpperCase")
),
List()
)

ValDef(
Modifiers(PARAM),
newTermName("param"),
Select(
Ident(scala.Predef),
newTypeName("String")
),
EmptyTree // Value
)

DefDef(
Modifiers(),
newTermName("method"),
List(), // Type parameters
List( // Parameter lists
List(parameter)
),
TypeTree(), // Return type
implementation
)

Def Macro 101
• Looks and acts like a normal function
def radix(s: String, base: Int): Long
val result = radix("2710", 16)
// result == 10000L
• Two fundamental differences:
– Invoked at compile time instead of runtime
– Operates on ASTs instead of values

Def Macro 101
• Needs a signature & implementation
def radix(s: String, base: Int): Long =
macro radixImpl
def radixImpl
Values
(c: Context)
(s: c.Expr[String], base: c.Expr[Int]):
c.Expr[Long]
ASTs

Def Macro 101
• What’s in a context?
– Enclosures (position)
– Error handling
– Logging
– Infrastructure

Overview
}
• The validator macro:
Macro Application
Validation Rules
– Rewrites each rule by addition a description
– Aggregates rules with an and combinator

Signature
def validator[T](v: T ⇒ Unit): Validator[T] =
macro ValidationTransform.apply[T]
def apply[T : c.WeakTypeTag]
(c: Context)
(v: c.Expr[T ⇒ Unit]):
c.Expr[Validator[T]]

Brace
yourselves
Here be dragons

Walkthrough
Search for rule
Process rule
Generate description
Rewrite rule

Search for Rule
• A rule is an expression of type Validator[_]
• We search by:
– Recursively pattern matching over an AST
– On match, apply a function on the subtree
– Encoded as a partial function from Tree to R

Search for Rule
def collectFromPattern[R]
(tree: Tree)
(pattern: PartialFunction[Tree, R]): List[R] = {
var found: Vector[R] = Vector.empty
new Traverser {
override def traverse(subtree: Tree) {
if (pattern isDefinedAt subtree)
found = found :+ pattern(subtree)
else
super.traverse(subtree)
}
}.traverse(tree)
found.toList
}

Search for Rule
• Putting it together:
case class Rule(ouv: Tree, validation: Tree)
def processRule(subtree: Tree): Rule = ???
def findRules(body: Tree): Seq[Rule] = {
val validatorType = typeOf[Validator[_]]
collectFromPattern(body) {
case subtree if subtree.tpe <:< validatorType ⇒
processRule(subtree)
}
}

Process Rule
• The user writes:
• The compiler emits:
Type: Validator[_]
Contextualizer(p.firstName).is(notEmpty)
Object Under Validation
(OUV)
Validation

Process Rule
• This is effectively an Apply AST node
• The left-hand side is the OUV
• The right-hand side is the validation
– But we can use the entire expression!
• Contextualizer is our entry point

Process Rule
Apply
Select
Apply
TypeApply
Contextualizer
String
Select
Ident(“p”)
firstName
is
notEmpty

Process Rule
Apply
TypeApply
Contextualizer
String
Select
Ident(“p”)
firstName

Process Rule
Apply
TypeApply
Contextualizer
Φ
Select
Ident(“p”)
firstName

Process Rule
Apply
TypeApply
Contextualizer
Φ
OUV
Φ
Φ
case Apply(TypeApply(Select(_, `term`), _), ouv :: Nil) ⇒

Process Rule
• Putting it together:
val term = newTermName("Contextualizer")
def processRule(subtree: Tree): Rule =
extractFromPattern(subtree) {
case Apply(TypeApply(Select(_, `term`), _), ouv :: Nil) ⇒
Rule(ouv, subtree)
} getOrElse abort(subtree.pos, "Not a valid rule")

Generate Description
• Consider the object under validation
• In this example, it is a field accessor
• The function prototype is the entry point
Select
Ident(“p”)
firstName
...
}

• How to get at the prototype?
• The macro signature includes the rule block:
(c: Context)
(v: c.Expr[T ⇒ Unit]):
c.Expr[Validator[T]]
• To extract the prototype:
val Function(prototype :: Nil, body) =
v.tree // prototype: ValDef

• Putting it all together:
def describeRule(rule: ValidationRule) = {
val para = prototype.name
val Select(Ident(`para`), description) =
rule.ouv
description.toString
}

Rewrite Rule
• We’re constructing a Validator[Person]
• A rule is itself a Validator[T]. For example:
• We need to:
– Lift the rule to validate the enclosing type
– Apply the description to the result

Quasiquotes
• Provide an easy way to construct ASTs:
Apply(
Select(
Ident(newTermName"x"),
newTermName("$plus")
),
List(
Ident(newTermName("y"))
)
)
q"x + y"

Quasiquotes
• Quasiquotes also let you splice trees:
def greeting(whom: c.Expr[String]) =
q"Hello "$whom"!"
• And can be used in pattern matching:
val q"$x + $y" = tree

Rewrite Rule
new Validator[Person] {
def apply(p: Person) = {
val validation =
validation(p.firstName) withDescription "firstName"
}
}

Rewrite Rule
• Putting it all together:
def rewriteRule(rule: ValidationRule) = {
val desc = describeRule(rule)
val tree = Literal(Constant(desc))
q"""
new com.wix.accord.Validator[${weakTypeOf[T]}] {
def apply($prototype) = {
val validation = ${rule.validation}
validation(${rule.ouv}) withDescription $tree
}
}
"""
}

Epilogue
• The finishing touch: and combinator
(c: Context)
(v: c.Expr[T ⇒ Unit]): c.Expr[Validator[T]] = {
val Function(prototype :: Nil, body) = v.tree
// ... all the stuff we just discussed
val rules = findRules(body) map rewriteRule
val result =
q"new com.wix.accord.combinators.And(..$rules)"
c.Expr[Validator[T]](result)
}

tomer@tomergabel.com
@tomerg
http://il.linkedin.com/in/tomergabel
Check out Accord at:
http://github.com/wix/accord
Thank you for listening
WE’RE DONE
HERE!

Leveraging Scala Macros for Better Validation

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