Example

A simple example of how to use Lepl:

from lepl import *

# For a simpler result these could be replaced with 'list', giving
# an AST as a set of nested lists
# (ie replace '> Term' etc with '> list' below).

class Term(List): pass
class Factor(List): pass
class Expression(List): pass

def build():

    # Here we define the grammar

    # A delayed value is defined later (see penultimate line in block)
    expr   = Delayed()
    number = Digit()[1:,...]                        >> int

    # Allow spaces between items
    with DroppedSpace():
        term    = number | '(' & expr & ')'         > Term
        muldiv  = Any('*/')
        factor  = term & (muldiv & term)[:]         > Factor
        addsub  = Any('+-')
        expr   += factor & (addsub & factor)[:]     > Expression
        line    = Trace(expr) & Eos()

    return line.get_parse()

if __name__ == '__main__':
    parser = build()
    # parser returns a list of tokens, but line
    # returns a single value, so take the first entry
    print(parser('1 + 2 * (3 + 4 - 5)')[0])

Running this gives the result:

Expression
 +- Factor
 |   `- Term
 |       `- 1
 +- '+'
 `- Factor
     +- Term
     |   `- 2
     +- '*'
     `- Term
         +- '('
         +- Expression
         |   +- Factor
         |   |   `- Term
         |   |       `- 3
         |   +- '+'
         |   +- Factor
         |   |   `- Term
         |   |       `- 4
         |   +- '-'
         |   `- Factor
         |       `- Term
         |           `- 5
         `- ')'