Experimental support for use of expressions in custom variables

README.md

@@ -39,7 +39,7 @@ You can use [Composer](https://getcomposer.org/) to install the bundle to your p
 composer require --dev theofidry/psysh-bundle
 ```
 
-Then, enable the bundle by updating your `app/AppKernel.php` file to enable the bundle:  
+Then, enable the bundle by updating your `app/AppKernel.php` file to enable the bundle:
 (not needed on symfony 5, bundle is automaticaly registred in `config/bundles.php`)
 ```php
 <?php
@@ -115,17 +115,21 @@ services:
 ### Adding custom variables
 It is possible to add custom variables to the shell via configuration.
 Variables can be of any type, container parameters references (e.g. `%kernel.debug%`) or even services
-(prefixed with `@`, e.g. `"@my_service"`).
+(prefixed with `@`, e.g. `"@my_service"`). An arbitrary evaluated expression can be specified as a
+string enclosed with `{` and `}`, though keep in mind that malformed expressions will prevent Psysh
+from loading. In an evaluated expression, the special token `%containerId` can be used to access the
+container itself, by using `service('%containerId')`.
 
 ```yaml
-# app/config/config_dev.yml
+# config/packages/psysh.yaml
 
 psysh:
     variables:
         foo: bar
         router: "@router"
         some: [thing, else]
         debug: "%kernel.debug%"
+        doctrine: "{service('%containerId').get('doctrine')}"
 ```
 
 Now if you run `php app/console psysh` and then `ls`, you will see the variables `$foo`, `$router`, `$some` and `$debug`,

src/DependencyInjection/PsyshExtension.php

@@ -20,6 +20,7 @@
 use Symfony\Component\HttpKernel\DependencyInjection\Extension;
 use function array_merge;
 use function is_string;
+use function preg_replace;
 use function sprintf;
 use function strpos;
 use function substr;
@@ -50,16 +51,21 @@ public function load(array $configs, ContainerBuilder $container): void
 
         $config = $this->processConfiguration(new Configuration(), $configs);
 
+        $containerId = 'test.service_container';
+
         foreach ($config['variables'] as $name => $value) {
-            if (is_string($value) && strpos($value, '@') === 0) {
-                $value = new Reference(substr($value, 1));
+            if (is_string($value)) {
+                if (strpos($value, '@') === 0) {
+                    $value = new Reference(substr($value, 1));
+                } else if (strpos($value, '{') === 0) {
+                    $expression = preg_replace('/%containerId\b/', $containerId, substr($value, 1, -1));
+                    $value = new Expression($expression);
+                }
             }
 
             $config['variables'][$name] = $value;
         }
 
-        $containerId = 'test.service_container';
-
         $container
             ->findDefinition('psysh.shell')
             ->addMethodCall(