added support for try-catch exception handlers

README.md

@@ -223,6 +223,26 @@ For another example, lets combine `if` and `not` into a macro named `unless`, th
 
 The `quasiquote` form described above is essential for declaring macros. Internally macros are just functions with a special flag.
 
+## Exceptions
+
+The language has support for `try-catch` style exception handlers. The syntax is `(try A (catch B C))` where A is evaluated first and if exception is thrown, then C is evaluated with the symbol B bound to the value of the exception. Exceptions are thrown using the `throw` core function. You can give any data structure as argument to `throw` and it is passed along to `catch`. This way exceptions can contain more data than just a string that represents an error message.
+
+Simple example of throwing and catching an exception:
+
+```
+> (try (throw {"msg":"message"}) (catch ex (str "error: " (get ex "msg"))))
+"error: message"
+```
+
+Exceptions generated by PHP are catched as well. Their value will be a hash-map with keys `type`, `file`, `line` and `message`:
+
+```
+> (try (get "wrong" "type") (catch ex (get ex "type")))
+"TypeError"
+```
+
+The Repl contains its own exception handler defined in PHP that will catch any exceptions thrown outside of `try-catch` form.
+
 ## Reflection
 
 You can use the `meta` special form to retrieve the arguments, body, code or full definition of user-defined functions:
@@ -276,6 +296,7 @@ or    | yes | `(or false 0 1)` | `1` | Return the first value that evaluates to
 quote | yes | | | See the section Quoting.
 quasiquote | yes | | | See the section Quoting.
 quasiquote-expand | yes | | | See the section Quoting.
+try   | yes | | | See the section Exceptions.
 undef | yes | `(undef myFn)` | `<function>` | Remove a definition from the current environment. Return the removed value.
 
 ## Functions
@@ -293,7 +314,7 @@ print | no  | `(print "hello world")` | `hello world` | Print expression on the
 pstr  | yes | `(pstr {"a":"b"})` | `"{\"a\":\"b\"}"` | Print expression to string.
 read  | yes | `(read "(+ 1 2 3)")` | `(+ 1 2 3)` | Read a string as code and return the expression.
 sleep | no  | `(sleep 2000)` | `null` | Sleep for the given period given in milliseconds using [usleep](https://www.php.net/manual/en/function.usleep).
-throw | yes | `(throw "invalid value")` | `error: "invalid value"` | Throw an exception. The given value is passed to catch.
+throw | yes | `(throw "invalid value")` | `error: "invalid value"` | Throw an exception. The given value is passed to catch. See the section Exceptions.
 timer | no  | `(timer (fn (d) (sleep d)) 200)` | `0.20010209` | Measure the execution time of a function and return it in seconds.
 
 ### Collection functions

src/Evaller.php

@@ -116,9 +116,7 @@ class Evaller
                 } elseif ($symbolName == 'def') {
                     if ($astLength != 3) {
                         throw new MadLispException("def requires exactly 2 arguments");
-                    }
-
-                    if (!($astData[1] instanceof Symbol)) {
+                    } elseif (!($astData[1] instanceof Symbol)) {
                         throw new MadLispException("first argument to def is not symbol");
                     }
 
@@ -170,9 +168,7 @@ class Evaller
                 } elseif ($symbolName == 'fn' || $symbolName == 'macro') {
                     if ($astLength != 3) {
                         throw new MadLispException("$symbolName requires exactly 2 arguments");
-                    }
-
-                    if (!($astData[1] instanceof Seq)) {
+                    } elseif (!($astData[1] instanceof Seq)) {
                         throw new MadLispException("first argument to $symbolName is not seq");
                     }
 
@@ -213,9 +209,7 @@ class Evaller
                 } elseif ($symbolName == 'let') {
                     if ($astLength != 3) {
                         throw new MadLispException("let requires exactly 2 arguments");
-                    }
-
-                    if (!($astData[1] instanceof MList)) {
+                    } elseif (!($astData[1] instanceof MList)) {
                         throw new MadLispException("first argument to let is not list");
                     }
 
@@ -367,12 +361,47 @@ class Evaller
                     }
 
                     return $this->quasiquote($astData[1]);
+                } elseif ($symbolName == 'try') {
+                    if ($astLength != 3) {
+                        throw new MadLispException("try requires exactly 2 arguments");
+                    } elseif (!($astData[2] instanceof MList)) {
+                        throw new MadLispException("second argument to try is not list");
+                    }
+
+                    $catch = $astData[2]->getData();
+
+                    if (count($catch) == 3 && $catch[0] instanceof Symbol &&
+                        $catch[0]->getName() == 'catch' && $catch[1] instanceof Symbol) {
+
+                        try {
+                            return $this->eval($astData[1], $env, $depth + 1);
+                        } catch (\Throwable $ex) {
+                            if ($ex instanceof MadLispUserException) {
+                                $exVal = $ex->getValue();
+                            } else {
+                                // Return a hash-map for PHP exceptions
+                                $exVal = new Hash([
+                                    'type' => get_class($ex),
+                                    'file' => $ex->getFile(),
+                                    'line' => $ex->getLine(),
+                                    'message' => $ex->getMessage()
+                                ]);
+                            }
+
+                            $newEnv = new Env('catch', $env);
+                            $newEnv->set($catch[1]->getName(), $exVal);
+
+                            $env = $newEnv;
+                            $ast = $catch[2];
+                            continue; // tco
+                        }
+                    } else {
+                        throw new MadLispException("invalid form for catch");
+                    }
                 } elseif ($symbolName == 'undef') {
                     if ($astLength != 2) {
                         throw new MadLispException("undef requires exactly 1 argument");
-                    }
-
-                    if (!($astData[1] instanceof Symbol)) {
+                    } elseif (!($astData[1] instanceof Symbol)) {
                         throw new MadLispException("first argument to undef is not symbol");
                     }