Source code for joy.utils.stack

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#    Copyright © 2014, 2015, 2017 Simon Forman
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'''
When talking about Joy we use the terms "stack", "quote", "sequence",
"list", and others to mean the same thing: a simple linear datatype that
permits certain operations such as iterating and pushing and popping
values from (at least) one end.

There is no "Stack" Python class, instead we use the  `cons list`_, a 
venerable two-tuple recursive sequence datastructure, where the
empty tuple ``()`` is the empty stack and ``(head, rest)`` gives the
recursive form of a stack with one or more items on it::

    stack := () | (item, stack)

Putting some numbers onto a stack::

    ()
    (1, ())
    (2, (1, ()))
    (3, (2, (1, ())))
    ...

Python has very nice "tuple packing and unpacking" in its syntax which
means we can directly "unpack" the expected arguments to a Joy function.

For example::

    def dup((head, tail)):
        return head, (head, tail)

We replace the argument "stack" by the expected structure of the stack,
in this case "(head, tail)", and Python takes care of unpacking the
incoming tuple and assigning values to the names.  (Note that Python
syntax doesn't require parentheses around tuples used in expressions
where they would be redundant.)

Unfortunately, the Sphinx documentation generator, which is used to generate this
web page, doesn't handle tuples in the function parameters.  And in Python 3, this
syntax was removed entirely.  Instead you would have to write::

    def dup(stack):
        head, tail = stack
        return head, (head, tail)


We have two very simple functions, one to build up a stack from a Python
iterable and another to iterate through a stack and yield its items
one-by-one in order.  There are also two functions to generate string representations
of stacks.  They only differ in that one prints the terms in stack from left-to-right while the other prints from right-to-left.  In both functions *internal stacks* are
printed left-to-right.  These functions are written to support :doc:`../pretty`.

.. _cons list: https://en.wikipedia.org/wiki/Cons#Lists

'''
from .errors import NotAListError


[docs]def list_to_stack(el, stack=()): '''Convert a Python list (or other sequence) to a Joy stack:: [1, 2, 3] -> (1, (2, (3, ()))) :param list el: A Python list or other sequence (iterators and generators won't work because ``reverse()`` is called on ``el``.) :param stack stack: A stack, optional, defaults to the empty stack. :rtype: stack ''' for item in reversed(el): stack = item, stack return stack
[docs]def iter_stack(stack): '''Iterate through the items on the stack. :param stack stack: A stack. :rtype: iterator ''' while stack: item, stack = stack yield item
[docs]def stack_to_string(stack): ''' Return a "pretty print" string for a stack. The items are written right-to-left:: (top, (second, ...)) -> '... second top' :param stack stack: A stack. :rtype: str ''' f = lambda stack: reversed(list(iter_stack(stack))) return _to_string(stack, f)
[docs]def expression_to_string(expression): ''' Return a "pretty print" string for a expression. The items are written left-to-right:: (top, (second, ...)) -> 'top second ...' :param stack expression: A stack. :rtype: str ''' return _to_string(expression, iter_stack)
_JOY_BOOL_LITS = 'false', 'true' def _joy_repr(thing): if isinstance(thing, bool): return _JOY_BOOL_LITS[thing] return repr(thing) def _to_string(stack, f): if not isinstance(stack, tuple): return _joy_repr(stack) if not stack: return '' # shortcut return ' '.join(map(_s, f(stack))) _s = lambda s: ( '[%s]' % expression_to_string(s) if isinstance(s, tuple) else _joy_repr(s) )
[docs]def concat(quote, expression): '''Concatinate quote onto expression. In joy [1 2] [3 4] would become [1 2 3 4]. :param stack quote: A stack. :param stack expression: A stack. :raises RuntimeError: if quote is larger than sys.getrecursionlimit(). :rtype: stack ''' # This is the fastest implementation, but will trigger # RuntimeError: maximum recursion depth exceeded # on quotes longer than sys.getrecursionlimit(). ## return (quote[0], concat(quote[1], expression)) if quote else expression # Original implementation. ## return list_to_stack(list(iter_stack(quote)), expression) # In-lining is slightly faster (and won't break the # recursion limit on long quotes.) temp = [] while quote: if not isinstance(quote, tuple): raise NotAListError(repr(quote)) item, quote = quote temp.append(item) for item in reversed(temp): expression = item, expression return expression
[docs]def dnd(stack, from_index, to_index): ''' Given a stack and two indices return a rearranged stack. First remove the item at from_index and then insert it at to_index, the second index is relative to the stack after removal of the item at from_index. This function reuses all of the items and as much of the stack as it can. It's meant to be used by remote clients to support drag-n-drop rearranging of the stack from e.g. the StackListbox. ''' assert 0 <= from_index assert 0 <= to_index if from_index == to_index: return stack head, n = [], from_index while True: item, stack = stack n -= 1 if n < 0: break head.append(item) assert len(head) == from_index # now we have two cases: diff = from_index - to_index if diff < 0: # from < to # so the destination index is still in the stack while diff: h, stack = stack head.append(h) diff += 1 else: # from > to # so the destination is in the head list while diff: stack = head.pop(), stack diff -= 1 stack = item, stack while head: stack = head.pop(), stack return stack
[docs]def pick(stack, n): ''' Return the nth item on the stack. :param stack stack: A stack. :param int n: An index into the stack. :raises ValueError: if ``n`` is less than zero. :raises IndexError: if ``n`` is equal to or greater than the length of ``stack``. :rtype: whatever ''' if n < 0: raise ValueError while True: try: item, stack = stack except ValueError: raise IndexError n -= 1 if n < 0: break return item