Merge pull request #14413 from dotty-staging/draft-higher-order-expressions

Add draft as regression tests

Author: nicolasstucki

tests/neg/higher-order-quoted-expressions.scala

@@ -0,0 +1,44 @@
+// Draft of higher order expressions
+// ---------------------------------
+// A higher order expression is a an expression that may contain unbound variables.
+// Unbound variables are listed as arguments of the type lambda of the expression.
+// Splicing can only be done on Expr[_ <: Any].
+// `apply methods on higher order expression provide a way to replace the unbound variables.
+
+// This can be used for HOAS quoted patterns to not return a lambda (see HOAS patterns section in https://infoscience.epfl.ch/record/288718?ln=en).
+// The use would be the same as each higher order expression would have an apply method.
+// But as it would be an expression, the expression would showable.
+// The expression could also be directly transformed into a Term of the reflection API.
+
+// Question: How to extend this to allow unbound type (Type) in the expression?
+
+class Expr[+T <: AnyKind]
+object Expr:
+  extension [T, R](hoe: Expr[[_ >: T <: T] =>> R]) def apply(x1: Expr[T]): Expr[R] = ???
+  extension [T1, T2, R](hoe: Expr[[_ >: T1 <: T1, _ >: T2 <: T2] =>> R]) def apply(x1: Expr[T1], x2: Expr[T2]): Expr[R] = ???
+  // Are lower bounds in lambda parameters necessary?
+  // How could this be generalized to n arguments?
+
+
+def `'`[T](e: T): Expr[T] = ???
+def `$`[T](e: Expr[T]): T = ???
+
+def f(): Unit =
+  val e: Expr[Int] = ???
+  val hoe1: Expr[[T >: Int <: Int] =>> Int] = ??? // assumed to come from HOAS pattern with one unbound variable of type Int
+  val hoe2: Expr[[T1 >: Int <: Int, T2 >: Int <: Int] =>> Int] = ??? // assumed to come from HOAS pattern with 2 unbound variables of type Int
+  val e2: Expr[Int] = hoe1(e)
+  val e3: Expr[Int] = hoe2(e, e)
+
+  {
+    `$`{e}
+    `$`{e2}
+    `$`{e3}
+    `$`{hoe1(e)}
+    `$`{hoe2(e, e)}
+    `$`{hoe1} // error: Found: Expr[[T >: Int <: Int] =>> Int]), Required: Expr[Any] // may contain references to 1 unbound variable
+    `$`{hoe2} // error // may contain references to 2 unbound variables
+  }
+  `'`{1}
+  `'`{`$`{e}}
+  `'`{??? : ([T >: Int <: Int] =>> Int)} // error: Missing type parameter for [T >: Int <: Int] =>> Int // not a valid quote literal expression