Created
September 14, 2018 14:58
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object HigherOrderAbstractSyntax { | |
/** | |
* Let's write a language which supports: | |
* | |
* let i = 0 | |
* in while (i < 10) { i = i + 1 } | |
*/ | |
trait Expr[F[_]] { | |
def intLit(value: Int): F[Int] | |
def add(l: F[Int], r: F[Int]): F[Int] | |
// Uses scala lambdas in input type | |
def let[A, B](name: Symbol, value: F[A], body: F[A] => F[B]): F[B] | |
def updateVar[A](name: Symbol, value: F[A]): F[A] | |
def lessThan(l: F[Int], Right: F[Int]): F[Boolean] | |
def while0[A](condition: F[Boolean], body: F[A]): F[Unit] | |
} | |
object Expr { | |
def apply[F[_]](implicit F: Expr[F]): Expr[F] = F | |
} | |
implicit class IntExprSyntax[F[_]](left: F[Int]) { | |
def + (right: F[Int])(implicit F: Expr[F]): F[Int] = F.add(left, right) | |
def < (right: F[Int])(implicit F: Expr[F]): F[Boolean] = F.lessThan(left, right) | |
} | |
def int[F[_]: Expr](i: Int): F[Int] = Expr[F].intLit(i) | |
def let[F[_]: Expr, A, B](name: Symbol, value: F[A])(body: F[A] => F[B]): F[B] = | |
Expr[F].let(name, value, body) | |
def while0[F[_]: Expr, A](condition: F[Boolean])(body: F[A]): F[Unit] = | |
Expr[F].while0(condition, body) | |
case class IState(value: Map[Symbol, Any]) { | |
def addVariable(name: Symbol, v: Any): IState = | |
copy(value = value + (name -> v)) | |
def removeVariable(name: Symbol): IState = | |
copy(value = value - name) | |
} | |
import scalaz.zio._ | |
// Look how in this program, 'i is defined in let, but than becomes a real | |
// scala variable in the places where you want to use it. | |
// | |
// Variables in the DSL, have now become variables in scala as well. This | |
// doesn't hold for the updateVar yet. This can be worked around, but if | |
def program[F[_]: Expr]: F[Unit] = | |
let('i, int(0))(i => | |
while0(i < int(10))( | |
Expr[F].updateVar('i, i + int(1)) | |
) | |
) | |
} |
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