specialize compiled partials with one placeholder

src/Utils/PartiallyAppliedFunction.php

@@ -82,6 +82,24 @@ class PartiallyAppliedFunction implements Decorator, Compilable
             return static fn (...$args) => $callable(...$arguments, ...array_slice(array_values($args), 0, $extra));
         }
 
+        // The common shape, e.g. partial(str_pad(...), _, 8): one placeholder and no room for more arguments. An
+        // unfilled placeholder stays in place, as below.
+        if (count($placeholders) === 1 && $extra === 0) {
+            $position = $placeholders[0];
+
+            if ($position === 0) {
+                $rest = array_slice($arguments, 1);
+
+                return static fn ($value = PLACEHOLDER) => $callable($value, ...$rest);
+            }
+
+            return static function ($value = PLACEHOLDER) use ($callable, $arguments, $position) {
+                $arguments[$position] = $value;
+
+                return $callable(...$arguments);
+            };
+        }
+
         return static function (...$args) use ($callable, $arguments, $placeholders, $extra) {
             $args  = array_values($args);
             $count = min(count($placeholders), count($args));

tests/CompileTest.php

@@ -107,6 +107,17 @@ class CompileTest extends \PHPUnit\Framework\TestCase
         $this->assertSame('aa', f\compile(f\curry(str_repeat(...), null, 'a'))(2, 'extra'));
     }
 
+    public function testCompileSinglePlaceholderOfInternalTarget()
+    {
+        $leading = f\partial(str_pad(...), _, 3, '0', STR_PAD_LEFT);
+        $middle  = f\partial(substr(...), 'abcdef', _, 2);
+
+        $this->assertSame('007', f\compile($leading)('7', 'extra'));
+        $this->assertSame('cd', f\compile($middle)(2, 'extra'));
+        $this->assertSame($this->outcome(fn () => $leading()), $this->outcome(fn () => f\compile($leading)()));
+        $this->assertSame($this->outcome(fn () => $middle()), $this->outcome(fn () => f\compile($middle)()));
+    }
+
     public function testCompileWithFewerArgumentsThanPlaceholdersMatchesDecorator()
     {
         $partial = f\partial(str_repeat(...), _, _);