Added the turtle dsl example

examples/turtle_dsl.py

@@ -0,0 +1,80 @@
+# This example implements a LOGO-like toy language for Python's turtle, with interpreter.
+
+import turtle
+
+from lark import Lark
+
+turtle_grammar = """
+    start: instruction+
+
+    instruction: MOVEMENT NUMBER            -> movement
+               | "c" COLOR [COLOR]          -> change_color
+               | "fill" code_block          -> fill
+               | "repeat" NUMBER code_block -> repeat
+
+    code_block: "{" instruction+ "}"
+
+    MOVEMENT: "f"|"b"|"l"|"r"
+    COLOR: LETTER+
+
+    %import common.LETTER
+    %import common.INT -> NUMBER
+    %import common.WS
+    %ignore WS
+"""
+
+parser = Lark(turtle_grammar)
+
+def run_instruction(t):
+    if t.data == 'change_color':
+        turtle.color(*t.children)   # We just pass the color names as-is
+
+    elif t.data == 'movement':
+        name, number = t.children
+        { 'f': turtle.fd,
+          'b': turtle.bk,
+          'l': turtle.lt,
+          'r': turtle.rt, }[name](int(number))
+
+    elif t.data == 'repeat':
+        count, block = t.children
+        for i in range(int(count)):
+            run_instruction(block)
+
+    elif t.data == 'fill':
+        turtle.begin_fill()
+        run_instruction(t.children[0])
+        turtle.end_fill()
+
+    elif t.data == 'code_block':
+        for cmd in t.children:
+            run_instruction(cmd)
+    else:
+        raise SyntaxError('Unknown instruction: %s' % t.data)
+
+
+def run_turtle(program):
+    parse_tree = parser.parse(program)
+    for inst in parse_tree.children:
+        run_instruction(inst)
+
+def main():
+    while True:
+        code = input('> ')
+        try:
+            run_turtle(code)
+        except Exception as e:
+            print(e)
+
+def test():
+    text = """
+        c red yellow
+        fill { repeat 36 {
+            f200 l170
+        }}
+    """
+    run_turtle(text)
+
+if __name__ == '__main__':
+    #test
+    main()