cond
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Simon Forman
parent
77bd7790f5
commit
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@ -75,99 +75,6 @@ S_times = Symbol('times')
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# return (q, (p, stack)), expression, dictionary
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@inscribe
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@FunctionWrapper
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def branch(stack, expression, dictionary):
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'''
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Use a Boolean value to select one of two quoted programs to run.
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::
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branch == roll< choice i
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::
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False [F] [T] branch
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--------------------------
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F
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True [F] [T] branch
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-------------------------
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T
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'''
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(then, (else_, (flag, stack))) = stack
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return stack, concat(then if flag else else_, expression), dictionary
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@inscribe
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@FunctionWrapper
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def ifte(stack, expression, dictionary):
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'''
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If-Then-Else Combinator
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::
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... [if] [then] [else] ifte
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---------------------------------------------------
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... [[else] [then]] [...] [if] infra select i
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... [if] [then] [else] ifte
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-------------------------------------------------------
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... [else] [then] [...] [if] infra first choice i
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Has the effect of grabbing a copy of the stack on which to run the
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if-part using infra.
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'''
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(else_, (then, (if_, stack))) = stack
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expression = (S_infra, (S_first, (S_choice, (S_i, expression))))
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stack = (if_, (stack, (then, (else_, stack))))
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return stack, expression, dictionary
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@inscribe
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@FunctionWrapper
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def cond(stack, expression, dictionary):
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'''
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This combinator works like a case statement. It expects a single quote
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on the stack that must contain zero or more condition quotes and a
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default quote. Each condition clause should contain a quoted predicate
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followed by the function expression to run if that predicate returns
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true. If no predicates return true the default function runs.
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It works by rewriting into a chain of nested `ifte` expressions, e.g.::
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[[[B0] T0] [[B1] T1] [D]] cond
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-----------------------------------------
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[B0] [T0] [[B1] [T1] [D] ifte] ifte
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'''
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conditions, stack = stack
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if conditions:
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expression = _cond(conditions, expression)
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try:
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# Attempt to preload the args to first ifte.
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(P, (T, (E, expression))) = expression
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except ValueError:
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# If, for any reason, the argument to cond should happen to contain
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# only the default clause then this optimization will fail.
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pass
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else:
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stack = (E, (T, (P, stack)))
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return stack, expression, dictionary
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def _cond(conditions, expression):
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(clause, rest) = conditions
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if not rest: # clause is [D]
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return clause
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P, T = clause
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return (P, (T, (_cond(rest, ()), (S_ifte, expression))))
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@inscribe
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@FunctionWrapper
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def dip(stack, expression, dictionary):
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@ -2031,6 +2031,56 @@ def ifte(stack, expr, dictionary):
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return stack, expr, dictionary
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S_cond = Symbol('cond')
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@inscribe
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def cond(stack, expr, dictionary):
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'''
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This combinator works like a case statement. It expects a single quote
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on the stack that must contain zero or more condition quotes and a
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default quote. Each condition clause should contain a quoted predicate
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followed by the function expression to run if that predicate returns
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true. If no predicates return true the default function runs.
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It works by rewriting into a chain of nested `ifte` expressions, e.g.::
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[[D]] cond
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---------------- (Kind of pointless)
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D
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[[[IF] THEN] [D]] cond
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---------------------------- (with single condition, same as ifte)
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[IF] [THEN] [D] ifte
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[[[IF] THEN] ...] cond
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----------------------------------- (multiple conditions)
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[IF] [THEN] [[...] cond] ifte
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The middle case isn't actually implemented. It's implied by the
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base case and the "multiple conditions" case.
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'''
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conditions, stack = get_n_items(1, stack)
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isnt_stack(conditions)
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if not conditions:
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raise StackUnderflowError('cond without default clause')
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condition_clause, conditions = conditions
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isnt_stack(condition_clause)
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if not conditions: # This is the default clause, run it.
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expr = push_quote(condition_clause, expr)
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else:
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if_, then = get_n_items(1, condition_clause)
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isnt_stack(if_)
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else_ = (conditions, (S_cond, ()))
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stack = (else_, (then, (if_, stack)))
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expr = push_quote((S_ifte, ()), expr)
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return stack, expr, dictionary
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if __name__ == '__main__':
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import sys
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@ -2041,5 +2091,10 @@ if __name__ == '__main__':
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## stack = J(dictionary=dictionary)
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## except SystemExit:
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## pass
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stack, _ = run("5 10 [>][++][*]ifte", (), dictionary)
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jcode = "5 10 [>][++][*]ifte"
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jcode = '1 2 [[+]] cond'
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jcode = '1 2 [[[>] -] [[<] +] [*]] cond'
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jcode = '2 1 [[[>] -] [[<] +] [*]] cond'
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jcode = '3 3 [[[>] -] [[<] +] [*]] cond'
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stack, _ = run(jcode, (), dictionary)
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print(stack_to_string(stack), '•')
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