Implement Psalm (https://github.com/vimeo/psalm)

CHANGELOG.md

@@ -11,6 +11,7 @@ Common and useful classes, methods, exceptions etc.
 5. Fix integration with [Coveralls](https://www.coveralls.io) (available as the badge in [README.md](README.md))
 6. Implement [PHPStan](https://github.com/phpstan/phpstan)
 7. PHPUnit > execute tests in random order
+8. Implement [Psalm](https://github.com/vimeo/psalm)
 
 # 1.0.1
 

composer.json

@@ -27,7 +27,8 @@
         "phpstan/phpstan": "^0.11.1",
         "phpunit/phpunit": "^8.0",
         "sebastian/phpcpd": "^4.1",
-        "squizlabs/php_codesniffer": " ^2.9"
+        "squizlabs/php_codesniffer": " ^2.9",
+        "vimeo/psalm": "^3.0"
     },
     "autoload": {
         "psr-4": {

psalm.xml

@@ -0,0 +1,53 @@
+<?xml version="1.0"?>
+<psalm
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    totallyTyped="false"
+    xmlns="https://getpsalm.org/schema/config"
+    xsi:schemaLocation="https://getpsalm.org/schema/config file:///project/vendor/vimeo/psalm/config.xsd"
+>
+    <projectFiles>
+        <directory name="src" />
+        <ignoreFiles>
+            <directory name="vendor" />
+        </ignoreFiles>
+    </projectFiles>
+
+    <issueHandlers>
+        <LessSpecificReturnType errorLevel="info" />
+
+        <!-- level 3 issues - slightly lazy code writing, but provably low false-negatives -->
+
+        <DeprecatedMethod errorLevel="info" />
+        <DeprecatedProperty errorLevel="info" />
+        <DeprecatedClass errorLevel="info" />
+        <DeprecatedConstant errorLevel="info" />
+        <DeprecatedInterface errorLevel="info" />
+        <DeprecatedTrait errorLevel="info" />
+
+        <InternalMethod errorLevel="info" />
+        <InternalProperty errorLevel="info" />
+        <InternalClass errorLevel="info" />
+
+        <MissingClosureReturnType errorLevel="info" />
+        <MissingReturnType errorLevel="info" />
+        <MissingPropertyType errorLevel="info" />
+        <InvalidDocblock errorLevel="info" />
+        <MisplacedRequiredParam errorLevel="info" />
+
+        <PropertyNotSetInConstructor errorLevel="info" />
+        <MissingConstructor errorLevel="info" />
+        <MissingClosureParamType errorLevel="info" />
+        <MissingParamType errorLevel="info" />
+
+        <RedundantCondition errorLevel="info" />
+
+        <DocblockTypeContradiction errorLevel="info" />
+        <RedundantConditionGivenDocblockType errorLevel="info" />
+
+        <UnresolvableInclude errorLevel="info" />
+
+        <RawObjectIteration errorLevel="info" />
+
+        <InvalidStringClass errorLevel="info" />
+    </issueHandlers>
+</psalm>