Fix PFA in combination with pipeline operator optimizations

Author: thekid

src/main/php/lang/ast/emit/EmulatePipelines.class.php

@@ -1,22 +1,36 @@
 <?php namespace lang\ast\emit;
 
-use lang\ast\nodes\{CallableExpression, CallableNewExpression, Variable};
+use lang\ast\nodes\{CallableExpression, CallableNewExpression, Variable, Placeholder};
 
 /**
  * Emulates pipelines / the pipe operator, including a null-safe version.
  *
+ * ```php
+ * // Enclose expressions as follows:
+ * $in |> $expr;
+ * ($expr)($in);
+ *
+ * // Optimize for first-class callables with single placeholder argument:
+ * $in |> strlen(...);
+ * strlen($in);
+ * ```
+ *
  * @see  https://wiki.php.net/rfc/pipe-operator-v3
  * @see  https://externals.io/message/107661#107670
  * @test lang.ast.unittest.emit.PipelinesTest
  */
 trait EmulatePipelines {
 
+  private function singlePlaceholder($arguments) {
+    return 1 === sizeof($arguments) && $arguments[0] instanceof Placeholder;
+  }
+
   protected function emitPipeTarget($result, $target, $arg) {
-    if ($target instanceof CallableNewExpression) {
+    if ($target instanceof CallableNewExpression && $this->singlePlaceholder($target->arguments)) {
       $target->type->arguments= [new Variable(substr($arg, 1))];
       $this->emitOne($result, $target->type);
       $target->type->arguments= null;
-    } else if ($target instanceof CallableExpression) {
+    } else if ($target instanceof CallableExpression && $this->singlePlaceholder($target->arguments)) {
       $this->emitOne($result, $target->expression);
       $result->out->write('('.$arg.')');
     } else {

src/test/php/lang/ast/unittest/emit/CallableSyntaxTest.class.php

@@ -315,4 +315,14 @@ public function run() {
     }');
     Assert::equals('cba', $f('abc'));
   }
+
+  #[Test]
+  public function partial_function_application_with_pipe() {
+    $r= $this->run('class %T {
+      public function run() {
+        return ["hello world"] |> array_map(str_replace("hello", "hi", ?), ?);
+      }
+    }');
+    Assert::equals(['hi world'], $r);
+  }
 }