From f640f437f1f05c89d06bd5735f0cc18a567662af Mon Sep 17 00:00:00 2001
From: Simon Forman <sforman@hushmail.com>
Date: Mon, 12 Sep 2022 13:50:40 -0700
Subject: [PATCH] cond

---
 implementations/Python/joy/library.py | 93 ---------------------------
 implementations/Python/simplejoy.py   | 57 +++++++++++++++-
 2 files changed, 56 insertions(+), 94 deletions(-)

diff --git a/implementations/Python/joy/library.py b/implementations/Python/joy/library.py
index c39c164..2c9d0f9 100644
--- a/implementations/Python/joy/library.py
+++ b/implementations/Python/joy/library.py
@@ -75,99 +75,6 @@ S_times = Symbol('times')
 #  return (q, (p, stack)), expression, dictionary
 
 
-@inscribe
-@FunctionWrapper
-def branch(stack, expression, dictionary):
-    '''
-    Use a Boolean value to select one of two quoted programs to run.
-
-    ::
-
-        branch == roll< choice i
-
-    ::
-
-           False [F] [T] branch
-        --------------------------
-              F
-
-           True [F] [T] branch
-        -------------------------
-                 T
-
-    '''
-    (then, (else_, (flag, stack))) = stack
-    return stack, concat(then if flag else else_, expression), dictionary
-
-
-@inscribe
-@FunctionWrapper
-def ifte(stack, expression, dictionary):
-  '''
-  If-Then-Else Combinator
-  ::
-
-                  ... [if] [then] [else] ifte
-       ---------------------------------------------------
-          ... [[else] [then]] [...] [if] infra select i
-
-
-
-
-                ... [if] [then] [else] ifte
-       -------------------------------------------------------
-          ... [else] [then] [...] [if] infra first choice i
-
-
-  Has the effect of grabbing a copy of the stack on which to run the
-  if-part using infra.
-  '''
-  (else_, (then, (if_, stack))) = stack
-  expression = (S_infra, (S_first, (S_choice, (S_i, expression))))
-  stack = (if_, (stack, (then, (else_, stack))))
-  return stack, expression, dictionary
-
-
-@inscribe
-@FunctionWrapper
-def cond(stack, expression, dictionary):
-    '''
-    This combinator works like a case statement.  It expects a single quote
-    on the stack that must contain zero or more condition quotes and a 
-    default quote.  Each condition clause should contain a quoted predicate
-    followed by the function expression to run if that predicate returns
-    true.  If no predicates return true the default function runs.
-
-    It works by rewriting into a chain of nested `ifte` expressions, e.g.::
-
-              [[[B0] T0] [[B1] T1] [D]] cond
-        -----------------------------------------
-           [B0] [T0] [[B1] [T1] [D] ifte] ifte
-
-    '''
-    conditions, stack = stack
-    if conditions:
-        expression = _cond(conditions, expression)
-        try:
-            # Attempt to preload the args to first ifte.
-            (P, (T, (E, expression))) = expression
-        except ValueError:
-            # If, for any reason, the argument to cond should happen to contain
-            # only the default clause then this optimization will fail.
-            pass
-        else:
-            stack = (E, (T, (P, stack)))
-    return stack, expression, dictionary
-
-
-def _cond(conditions, expression):
-    (clause, rest) = conditions
-    if not rest:  # clause is [D]
-        return clause
-    P, T = clause
-    return (P, (T, (_cond(rest, ()), (S_ifte, expression))))
-
-
 @inscribe
 @FunctionWrapper
 def dip(stack, expression, dictionary):
diff --git a/implementations/Python/simplejoy.py b/implementations/Python/simplejoy.py
index 5453bba..79c4015 100755
--- a/implementations/Python/simplejoy.py
+++ b/implementations/Python/simplejoy.py
@@ -2031,6 +2031,56 @@ def ifte(stack, expr, dictionary):
     return stack, expr, dictionary
 
 
+S_cond = Symbol('cond')
+
+
+@inscribe
+def cond(stack, expr, dictionary):
+    '''
+    This combinator works like a case statement.  It expects a single quote
+    on the stack that must contain zero or more condition quotes and a 
+    default quote.  Each condition clause should contain a quoted predicate
+    followed by the function expression to run if that predicate returns
+    true.  If no predicates return true the default function runs.
+
+    It works by rewriting into a chain of nested `ifte` expressions, e.g.::
+
+           [[D]] cond
+        ----------------  (Kind of pointless)
+             D
+
+           [[[IF] THEN] [D]] cond
+        ---------------------------- (with single condition, same as ifte)
+            [IF] [THEN] [D] ifte
+
+
+              [[[IF] THEN] ...] cond
+        ----------------------------------- (multiple conditions)
+           [IF] [THEN] [[...] cond] ifte
+
+    The middle case isn't actually implemented.  It's implied by the
+    base case and the "multiple conditions" case.
+    '''
+    conditions, stack = get_n_items(1, stack)
+    isnt_stack(conditions)
+    if not conditions:
+        raise StackUnderflowError('cond without default clause')
+
+    condition_clause, conditions = conditions
+    isnt_stack(condition_clause)
+
+    if not conditions: # This is the default clause, run it.
+        expr = push_quote(condition_clause, expr)
+    else:
+        if_, then = get_n_items(1, condition_clause)
+        isnt_stack(if_)
+        else_ = (conditions, (S_cond, ()))
+        stack = (else_, (then, (if_, stack)))
+        expr = push_quote((S_ifte, ()), expr)
+
+    return stack, expr, dictionary
+
+
 if __name__ == '__main__':
     import sys
 
@@ -2041,5 +2091,10 @@ if __name__ == '__main__':
 ##        stack = J(dictionary=dictionary)
 ##    except SystemExit:
 ##        pass
-    stack, _ = run("5 10 [>][++][*]ifte", (), dictionary)
+    jcode = "5 10 [>][++][*]ifte"
+    jcode = '1 2 [[+]] cond'
+    jcode = '1 2 [[[>] -] [[<] +] [*]] cond'
+    jcode = '2 1 [[[>] -] [[<] +] [*]] cond'
+    jcode = '3 3 [[[>] -] [[<] +] [*]] cond'
+    stack, _ = run(jcode, (), dictionary)
     print(stack_to_string(stack), '•')