Question

我正在试图弄清楚如何做一个左关联表达式，其中递归（不包含在任何东西）表达式是可能的。例如，我想这样做：

expr + OP + expr

将1 x 2 x 3等2个操作解析为(expr OP expr) OP expr个结果。

如果我试图阻止expr解析无限递归，我可以做类似的事情：

expr -> Group(simple_expr + OP + expr)
      | simple_expr

然后我会得到expr OP (expr OR expr)结果。

如何强制左侧绑定？

修改：我知道operatorPrecedence，但当运算符为"IS" + Optional("NOT")或类似时，它似乎不匹配。

Answer 1

这是一个示例解析操作，它将获取令牌的平面列表并将它们嵌套，就好像左递归解析一样：

from pyparsing import *

# parse action -maker
def makeLRlike(numterms):
    if numterms is None:
        # None operator can only by binary op
        initlen = 2
        incr = 1
    else:
        initlen = {0:1,1:2,2:3,3:5}[numterms]
        incr = {0:1,1:1,2:2,3:4}[numterms]

    # define parse action for this number of terms,
    # to convert flat list of tokens into nested list
    def pa(s,l,t):
        t = t[0]
        if len(t) > initlen:
            ret = ParseResults(t[:initlen])
            i = initlen
            while i < len(t):
                ret = ParseResults([ret] + t[i:i+incr])
                i += incr
            return ParseResults([ret])
    return pa


# setup a simple grammar for 4-function arithmetic
varname = oneOf(list(alphas))
integer = Word(nums)
operand = integer | varname

# ordinary opPrec definition
arith1 = operatorPrecedence(operand,
    [
    (None, 2, opAssoc.LEFT),
    (oneOf("* /"), 2, opAssoc.LEFT),
    (oneOf("+ -"), 2, opAssoc.LEFT),
    ])

# opPrec definition with parseAction makeLRlike
arith2 = operatorPrecedence(operand,
    [
    (None, 2, opAssoc.LEFT, makeLRlike(None)),
    (oneOf("* /"), 2, opAssoc.LEFT, makeLRlike(2)),
    (oneOf("+ -"), 2, opAssoc.LEFT, makeLRlike(2)),
    ])

# parse a few test strings, using both parsers
for arith in (arith1, arith2):
    print arith.parseString("A+B+C+D+E")[0]
    print arith.parseString("A+B+C*D+E")[0]
    print arith.parseString("12AX+34BY+C*5DZ+E")[0]

打印：

（正常）

['A', '+', 'B', '+', 'C', '+', 'D', '+', 'E']
['A', '+', 'B', '+', ['C', '*', 'D'], '+', 'E']
[['12', 'A', 'X'], '+', ['34', 'B', 'Y'], '+', ['C', '*', ['5', 'D', 'Z']], '+', 'E']

（LR-等）

[[[['A', '+', 'B'], '+', 'C'], '+', 'D'], '+', 'E']
[[['A', '+', 'B'], '+', ['C', '*', 'D']], '+', 'E']
[[[[['12', 'A'], 'X'], '+', [['34', 'B'], 'Y']], '+', ['C', '*', [['5', 'D'], 'Z']]], '+', 'E']

Answer 2

Pyparsing产生左解析树。添加语义操作以在解析expr后立即编辑解析树。

具有pyparsing的递归表达式

2 个答案: