Add docs and use foldLeft

Author: nicolasstucki

compiler/src/dotty/tools/dotc/tastyreflect/KernelImpl.scala

@@ -1896,12 +1896,12 @@ class KernelImpl(val rootContext: core.Contexts.Context) extends Kernel {
   }
 
   def betaReduce(fn: Term, args: List[Term]) given (ctx: Context): Term = {
-    val (argVals, argRefs) = args.map(arg => arg.tpe match {
-      case tpe: SingletonType if isIdempotentExpr(arg) => (Nil, arg)
+    val (argVals, argRefs) = args.foldLeft((List.empty[ValDef], List.empty[Tree])) { case ((acc1, acc2), arg) => arg.tpe match {
+      case tpe: SingletonType if isIdempotentExpr(arg) => (acc1, arg :: acc2)
       case _ =>
         val argVal = SyntheticValDef(NameKinds.UniqueName.fresh("x".toTermName), arg).withSpan(arg.span)
-        (argVal :: Nil, ref(argVal.symbol))
-    }).unzip
+        (argVal :: acc1, ref(argVal.symbol) :: acc2)
+    }}
     def rec(fn: Tree): Tree = fn match {
       case Inlined(call, bindings, expansion) =>
         // this case must go before closureDef to avoid dropping the inline node
@@ -1919,7 +1919,7 @@ class KernelImpl(val rootContext: core.Contexts.Context) extends Kernel {
       case _ =>
         fn.select(nme.apply).appliedToArgs(argRefs).withSpan(fn.span)
     }
-    seq(argVals.flatten, rec(fn))
+    seq(argVals, rec(fn))
   }
 
   //

library/src/scala/tasty/reflect/Kernel.scala

@@ -1538,6 +1538,9 @@ trait Kernel {
    */
   def searchImplicit(tpe: Type) given (ctx: Context): ImplicitSearchResult
 
+  /** Inline fn if it is an explicit closure possibly nested inside the expression of a block.
+   *  Otherwise apply the arguments to the closure.
+   */
   def betaReduce(f: Term, args: List[Term]) given (ctx: Context): Term
 
 }