Diogo Castro dcastro

A type variable can be either universally ("for all x, then ...") or existentially quantified ("there exists some x, ...").

(If you're a maths persons, this corresponds to ∀x. x and ∃x. x, respectively)

In haskell, universally quantified type vars are represented with a forall:

head :: forall a. [a] -> Maybe a

The problem

These programs are safe to refactor:

object p1 {
  val x = 1 + 123
  val y = 1 + 123
}

This assumes familiarity with kinds and GADTs. Here's the extensions you're gonna need:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE ExplicitForAll #-}

	import Language.Haskell.TH.Lib
	import Language.Haskell.TH.Syntax

	module Problem where

	data PP a where
	PP :: forall a. Typeable a => PP a

	instance Lift (PP a) where
	lift PP = [\|PP\|]

	Calling approve
	[Debug] [Morley.Client.TezosClient.Impl.getAlias#254] Getting an alias for tz1TfLF61NboKZYbU7hxX3C1jXw45im6d8Kt
	[Debug] [Morley.Client.TezosClient.Impl.callTezosClient#447] Running: tezos-client -A carthage.testnet.tezos.serokell.team -P 8732 --mode mockup list known contracts
	[Debug] [Morley.Client.TezosClient.Impl.callTezosClient#453] stdout:
	nettest.Stablecoin.Stablecoin FA1.2: KT1VuoysRj6tWFXAjrbQd67YMnTMi3SpNTdC
	nettest.Stablecoin.natConsumer: KT1GnUjbtLqQgeupcDCReYmzTa86mkvwgoRa
	nettest.Stablecoin.nettest.transferlist_internal: KT1QQAymdeefTj3RwTkvjvGWTDWJPenoiG7k
	nettest.Stablecoin.nettest.Stablecoin: KT1Q4DccWWaM15nc9iaMDcwLNj3m85k8euVF
	helper: KT1DW3Fq5m7q1bn8QgjhSX9iEwcBAK82bva6
	nettest.Stablecoin.chainIdContract: KT1KUhbZCatB5X6svDpXJ4gAX5a9X1LyU2yv

	Calling approve
	[Debug] [Morley.Client.TezosClient.Impl.getAlias#254] Getting an alias for tz1TfLF61NboKZYbU7hxX3C1jXw45im6d8Kt
	[Debug] [Morley.Client.TezosClient.Impl.callTezosClient#447] Running: tezos-client -A carthage.testnet.tezos.serokell.team -P 8732 --mode mockup list known contracts
	[Debug] [Morley.Client.TezosClient.Impl.callTezosClient#453] stdout:
	nettest.Stablecoin.Stablecoin FA1.2: KT1GrQC4nGZckcfeCtLu2sKphsNsoxUoLNtK
	nettest.Stablecoin.natConsumer: KT1GdZ9EoAY2o5jHN2x4rACwTDAwcCGZV7bn
	helper: KT1DW3Fq5m7q1bn8QgjhSX9iEwcBAK82bva6
	nettest.Stablecoin.chainIdContract: KT1KUhbZCatB5X6svDpXJ4gAX5a9X1LyU2yv
	nettest.Stablecoin.permitCounterConsumer: KT1AihogGMjD3R8y5TK31Z6iJKXwCThK4i5D
	nettest.Stablecoin.nettest.Stablecoin: KT1HBxv2cjKjY1nWALMbKKMdsymrdTf8hxtG

	Loading...
	Welcome to the Ammonite Repl 1.3.2
	(Scala 2.12.7 Java 1.8.0_212)
	If you like Ammonite, please support our development at www.patreon.com/lihaoyi
	@ import $ivy.`org.scala-lang.modules::scala-parser-combinators:1.1.2`
	import $ivy.$

	@ {
	sealed trait Expr
	case class Term(term: String) extends Expr

	import cats.implicits._
	import org.joda.time.DateTime
	import tv.spotx.ironbark.druid.{DimensionMetadata, MeasurementMetadata, Schema}
	import tv.spotx.ironbark.syntaxtree.druid.DruidQuery
	import tv.spotx.ironbark.validation.Err.{Errs, MaybeValid}

	object Test extends App {

	// case class AuthCheck[+A](authorize: UserEntities => Either[Errs, (A, UserEntities)])

	import eu.timepit.refined._
	import eu.timepit.refined.api._
	import eu.timepit.refined.auto._

	import eu.timepit.refined.numeric._
	import eu.timepit.refined.boolean._
	import eu.timepit.refined.char._
	import eu.timepit.refined.collection._
	import eu.timepit.refined.generic._
	import eu.timepit.refined.string._

	import org.scalacheck.cats.implicits._
	import org.scalacheck.Gen
	import io.circe.{Decoder, Encoder}

	object Test extends App {
	case class X(i: Int)

	implicit val x: Encoder[X] = Wrapper[X].deriveInstance[Encoder]
	implicit val y: Decoder[X] = Wrapper[X].deriveInstance[Decoder].withErrorMessage("some custom error msg")
	val z: Gen[X] = Wrapper[X].lift(Gen.choose(0, 10))