C[omp]ute

From: "andrew cooke" <andrew@...>

Date: Sat, 7 Feb 2009 09:10:54 -0300 (CLST)

I doubt I could have written code like this a few years ago.  That's quite
a reassuring feeling - that in some ways you can continue to improve.

Anyway, this is a visitor.  It's the usual visitor pattern (a
catamorphism).  And it's invoked depth-first on a tree of objects.  node()
method is called with arguments that reflect the constructor arguments
used to construct the original tree.


 class GraphStr(Visitor):

   def __init__(self):
     super(GraphStr, self).__init__()
     self.loop = lambda first, rest, name: \
                   [first + name + ' <loop>']

   def node(self, *args, **kargs):
     def fun(first, rest, name, type_=self.type_):
       spec = []
       for arg in args:
	 spec.append(('+- ', '|  ', '', arg))
       for arg in kargs:
	 spec.append(('+- ', '|  ', arg, kargs[arg]))
       if spec:
	 spec[-1] = ('`- ', '   ', spec[-1][2], spec[-1][3])
       yield first + name + (' ' if name else '') + type_.__name__
       for (a, b, c, f) in spec:
	 for line in f(a, b, c):
	   yield rest + line
     return fun

   def arg(self, value):
     return lambda first, rest, name: \
       [first + name + ' ' + repr(value)]

   def postprocess(self, f):
     return '\n'.join(f('', '', ''))


Note the following:

- 'spec' is a list of calls.  Preparing a specification for the calls
  before making them allows the *last* call to be modified simply.

- Generators are used where appropriate (spec cannot be as we need to
  use a -1 index on it to access the last element)

- The walker is depth-first (bottom-up), but the final processing is
  top-down.  So the first stage generates a set of functions that
  implement the second stage (much like implementing foldr in foldl -
  or have I got that backwards? :o)


When applied to the tree generated by this code:

  expression  = Delayed()
  number      = Digit()[1:,...]
  expression += (number | '(' / expression / ')')

It generates:

Delayed
`- matcher Or
   +- Apply
   |  +- function <function add at 0x7f30bf3da738>
   |  +- matcher DepthFirst
   |  |  +- start 1
   |  |  +- stop None
   |  |  +- rest Any
   |  |  |  `- restrict '0123456789'
   |  |  `- first Any
   |  |     `- restrict '0123456789'
   |  +- args False
   |  `- raw True
   `- And
      +- And
      |  +- Literal
      |  |  `- text '('
      |  +- Apply
      |  |  +- function <function add at 0x7f30bef4d380>
      |  |  +- matcher DepthFirst
      |  |  |  +- start 0
      |  |  |  +- stop None
      |  |  |  +- rest Any
      |  |  |  |  `- restrict ' \t'
      |  |  |  `- first Any
      |  |  |     `- restrict ' \t'
      |  |  +- args False
      |  |  `- raw True
      |  `- Delayed
      |     `- matcher <loop>
      +- Apply
      |  +- function <function add at 0x7f30bf176d98>
      |  +- matcher DepthFirst
      |  |  +- start 0
      |  |  +- stop None
      |  |  +- rest Any
      |  |  |  `- restrict ' \t'
      |  |  `- first Any
      |  |     `- restrict ' \t'
      |  +- args False
      |  `- raw True
      `- Literal
         `- text ')'

Andrew