From ddf063973c1c38ce22c662147a18438bc9c0876e Mon Sep 17 00:00:00 2001
From: Simon Forman <sforman@hushmail.com>
Date: Tue, 28 Apr 2020 15:28:43 -0700
Subject: [PATCH] Trace command.

Don't print trace by default, instead a new `trace` combinator wirks like `i` and prints a trace.
---
 joy/__main__.py           |  2 ++
 joy/joy.py                |  8 +-------
 joy/utils/pretty_print.py | 29 +++++++++++++++++++++++++++++
 3 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/joy/__main__.py b/joy/__main__.py
index c461ef8..29017c5 100644
--- a/joy/__main__.py
+++ b/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
diff --git a/joy/joy.py b/joy/joy.py
index 684a305..6a47a1b 100644
--- a/joy/joy.py
+++ b/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()
diff --git a/joy/utils/pretty_print.py b/joy/utils/pretty_print.py
index 1d636be..217e294 100644
--- a/joy/utils/pretty_print.py
+++ b/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):