Trace command.

Don't print trace by default, instead a new `trace` combinator wirks like `i` and prints a trace.

joy/__main__.py

@@ -20,6 +20,8 @@
 from __future__ import print_function
 from .library import initialize
 from .joy import repl
+from .utils import pretty_print  # Inscribe trace command.
+
 
 print('''\
 Thun - Copyright © 2017 Simon Forman

joy/joy.py

@@ -28,7 +28,6 @@ from builtins import input
 from traceback import print_exc, format_exc
 from .parser import text_to_expression, ParseError, Symbol
 from .utils.stack import stack_to_string
-from .utils.pretty_print import TracePrinter
 
 
 def joy(stack, expression, dictionary, viewer=None):
@@ -101,16 +100,11 @@ def repl(stack=(), dictionary=None):
 				text = input('joy? ')
 			except (EOFError, KeyboardInterrupt):
 				break
-			viewer = TracePrinter()
 			try:
-				stack, _, dictionary = run(text, stack, dictionary, viewer.viewer)
+				stack, _, dictionary = run(text, stack, dictionary)
 			except:
 				exc = format_exc() # Capture the exception.
-				viewer.print_() # Print the Joy trace.
-				print('-' * 73)
 				print(exc) # Print the original exception.
-			else:
-				viewer.print_()
 	except:
 		print_exc()
 	print()

joy/utils/pretty_print.py

@@ -43,6 +43,35 @@ from __future__ import print_function
 from builtins import object
 from traceback import print_exc
 from .stack import expression_to_string, stack_to_string
+from ..joy import joy
+from ..library import inscribe, FunctionWrapper
+
+
+@inscribe
+@FunctionWrapper
+def trace(stack, expression, dictionary):
+	'''Evaluate a Joy expression on a stack and print a trace.
+
+	This function is just like the `i` combinator but it also prints a
+	trace of the evaluation
+
+	:param stack stack: The stack.
+	:param stack expression: The expression to evaluate.
+	:param dict dictionary: A ``dict`` mapping names to Joy functions.
+	:rtype: (stack, (), dictionary)
+
+	'''
+	tp = TracePrinter()
+	quote, stack = stack
+	try:
+		s, _, d = joy(stack, quote, dictionary, tp.viewer)
+	except:
+		tp.print_()
+		print('-' * 73)
+		raise
+	else:
+		tp.print_()
+	return s, expression, d
 
 
 class TracePrinter(object):