001_monad_parser.py

## Alexey Kachayev, 2014
## Link to slides:
## http://goo.gl/n4ylC4

## Basic:
## type Parser = String -> Tree

## Composition
## type Parser = String -> (Tree, String)

## Generalise
## type Parser a = String -> (a, String)

## Errors
## type Parser a = String -> Maybe (a, String)

## Better...
## type Parser a = String -> [(a, String)]

002_monad_parser.py

# match any letter
def any(inp):
    return [(inp[0], inp[1:])] if inp else []

any("python2.7")
# => [('p', 'ython2.7')]

any("")
# = []

# match only given letter
def match(p):
    def _inner(inp):        
        return [(inp[0], inp[1:])] if (inp and inp[0] == p) else []
    return _inner

p1 = match("p")
p1("python2.7")
# => [('p', 'ython2.7')]
p1("scala2.11")
# => []

# skip given letter
def skip(p):
    def _inner(inp):        
        return [("", inp[1:])] if (inp and inp[0] == p) else []
    return _inner

p2 = skip("(")
# => [('', 'python2.7)')]

# always produce given elem
def unit(p):
    return lambda inp: [(p, inp)]

p3 = unit("P")
p3("python2.7")
# => [('P', 'python2.7')]

003_monad_parser.py

from functools import partial
from operator import eq

# sat :: (a -> Bool) -> Parser a
def sat(pred):
    return lambda inp: filter(lambda (v,_): pred(v), any(inp))

# match :: a -> Parser a
def match(p):
    return sat(partial(eq, p))

p5 = match("P")
p5("Python2.7")
# => [('P', 'ython2.7')]

# skip :: Char -> Parser Char
def skip(p):
    return lambda inp: map(lambda (_, tail): ("", tail), match(p)(inp))

p6 = skip("P")
p6("Python2.7")
# => [('', 'ython2.7')]
p6("python2.7")
# => []

from operator import methodcaller

digit = sat(methodcaller("isdigit"))

digit("2.7python")
# => [('2', '.7python')]

digit("python2.7")
# => []

004_monad_parser.py

from fn.iters import filter, first, rest

class parser(object):

    def __init__(self, f):
        self._f = f

    def parse(self, inp):
        return self._f(inp)

    def parseAll(self, inp):
        f = first(filter(lambda (v, tail): tail == "", self.parse(inp)))
        return f[0] if f else None

    def __call__(self, inp):
        return self.parse(inp)

    def __or__(self, r):
        return parser(lambda inp: self.parse(inp) + r.parse(inp))

    def __and__(self, r):
        def _inner(inp):
            return [((v1, v2), tail2) for (v1,tail1) in self.parse(inp)
                                      for (v2, tail2) in r.parse(tail1)]
        return parser(_inner)
        
        
any = parser(lambda inp: [(inp[0], inp[1:])] if inp else [])

def unit(p):
    return parser(lambda inp: [(p, inp)])

def sat(pred):
    return parser(lambda inp: filter(lambda (v,_): pred(v), any(inp)))

def match(p):
    return sat(partial(eq, p))

def skip(p):
    return parser(lambda inp: map(lambda (_, tail): ("", tail), match(p).parse(inp)))

char = match

digit = sat(methodcaller("isdigit"))

p11 = digit & char(".") & digit
p11.parse("2.7")
# => [((('2', '.'), '7'), '')]

def many(prs):
    return (prs & parser(lambda inp: many(prs).parse(inp))) | unit(())

def string(s):
    return reduce(and_, map(match, s))

string("python").parse("python2.7")
# => [(((((('p', 'y'), 't'), 'h'), 'o'), 'n'), '2.7')]

005_monad_parser.py

from fn import Stream

combine = lambda v1,v2: (v1, v2)

class parser(object):

    def __init__(self, f):
        self._f = f

    def parse(self, inp):
        if inp == "": return []
        return self._f(inp)

    def parseAll(self, inp):
        f = first(filter(lambda (v, tail): tail == "", self.parse(inp)))
        return f[0] if f else None

    @classmethod
    def delay(cls, f, *args):
        return cls(lambda inp: f(*args).parse(inp))

    def andThen(self, r):
        def _parse(inp):
            for (v1, tail1) in self.parse(inp):
                for (v2, tail2) in r.parse(tail1):
                    yield (combine(v1, v2), tail2)
        return parser(_parse)

    def orElse(self, r):
        return parser(lambda inp: Stream() << self.parse(inp) << r.parse(inp))

    def __call__(self, inp):
        return self.parse(inp)

    def __or__(self, r):
        return self.orElse(r)

    def __and__(self, r):
        return self.andThen(r)

from operator import add

combine = add

# simple example
# expr -> "python" \d "." \d
version = digit & char(".") & digit
py_ver = string("python").andThen(version)

py_ver.parseAll("python2.7")
# => 'python2.7'
py_ver.parseAll("python3.2")
# => 'python3.2'
py_ver.parseAll("scala2.11")
# => None

# more
# parse [1,2,3,4,5]
# expr -> "[" \d ("," \d)* "]"

ints = digit.andThen(many(skip(",").andThen(digit)))
wrapped = skip("[") & ints & skip("]")

wrapped.parseAll("[1,2,3,4,5]")
# => '12345'

wrapped.parseAll("[1,2,3,4,")
# => None

006_monad_parser.hs

p :: Parser String
p = do char '['
       d <- digit
       ds <- many (do {char ','; digit})
       char ']'
       return (d:ds)

char '[' >>= \_ ->
digit >>= \d ->
digits >>= \ds ->
char ']' >>= _ ->
f d ds

007_monad_parser.py

class parser(object):
    
    def __init__(self, f):
        self._f = f

    def parse(self, inp):
        return self._f(inp)

    def parseAll(self, inp):
        f = first(filter(lambda (v, tail): tail == "", self.parse(inp)))
        return f[0] if f else None

    @classmethod
    def delay(cls, f, *args):
        return cls(lambda inp: f(*args).parse(inp))

    # unit :: a -> Parser a
    @classmethod
    def unit(cls, a):
        return cls(lambda inp: (yield (a, inp)))

    # bind :: Parser a-> (a -> Parser b) -> Parser b
    def bind(self, f):
        def _parse(inp):
            for (v, tail) in self.parse(inp):
                for (v2, tail2) in f(v).parse(tail):
                    yield (v2, tail2)
        return parser(_parse)

    # lift :: (a -> b) -> (a -> Parser b)
    @classmethod
    def lift(cls, f):
        return (lambda v: cls.unit(f(v)))

    # lifted :: Parser a -> (a -> b) -> Parser b
    def lifted(self, f):
        return self.bind(parser.lift(f))

    def __rshift__(self, f):
        return self.bind(f)

    def andThen(self, r):
        return self.bind(lambda v1:
                  r.bind(lambda v2:
                  parser.unit(combine(v1, v2))))

    def orElse(self, r):
        return parser(lambda inp: Stream() << self.parse(inp) << r.parse(inp))

    def __call__(self, inp):
        return self.parse(inp)

    def __or__(self, r):
        return self.orElse(r)

    def __and__(self, r):
        return self.andThen(r)
        
# use it

any = parser(lambda inp: [(inp[0], inp[1:])] if inp else [])
failure = parser(lambda _: [])

def sat(pred):
    return any >> (lambda v: parser.unit(v) if pred(v) else failure)

def match(p):
    return sat(partial(eq, p))

def skip(p):
    return match(p).lifted(lambda _: "")

008_monad_parser.py

digits = many(char(",") >> (lambda _:
              digit >> (lambda d:
              parser.unit(d))))

p8 = char("[") >> (lambda _:
     digit >> (lambda d:
     digits >> (lambda ds:
     char("]") >> (lambda _:
     parser.unit(d + ds)))))

p8.parseAll("[1,2,3,4,5]")
# => "12345"

009_monad_parser.py

ret = parser.unit

def do(f):
    def _runner(*args, **kwargs):
        gen = f(*args, **kwargs)
        def _parse(inp):
            s = gen.send(None)
            linp = inp
            while 1:
                r = list(s.parse(linp))
                if len(r):
                    (a, tail) = first(r)
                    try: s = gen.send(a)
                    except StopIteration, e:
                        return [(a, tail)]
                    else: linp = tail
                else: return []
        return parser(_parse)
    return _runner

@do
def p():
    yield char("[")
    d = yield digit
    ds = yield many(skip(",").andThen(digit))
    yield char("]")
    yield ret(d + ds)

p().parseAll("[1,2,3,4,5]")
# => "12345"

010_monad_parser.py

def choice(ps): return reduce(or_, map(match, ps))
space = char(" ").orElse(string("\n"))
spaces = many(space)
letter = sat(methodcaller("isalpha"))
operator = choice("-+*/")
token = plus(digit | letter | operator)

@do
def lisp():
    yield char("(")
    t = yield token.orElse(lisp())
    ts = yield body().orElse(ret([]))
    yield char(")")
    yield ret([t] + ts)

@do
def body():
    yield plus(space)
    t = yield token.orElse(lisp())
    ts = yield body().orElse(ret([]))
    yield ret([t] + ts)

lisp().parseAll("(define)")
# => ['define']
lisp().parseAll("(define diameter (lambda (r) (* 2 r)))")
# => ['define' 'diameter' ['lambda' ['r'] ['*' '2' 'r']]]

020_monad_parser.py

@do
def p():
    yield char("[")
    d = yield digit
    ds = yield many(skip(",").andThen(digit))
    yield char("]")
    yield ret(d + ds)

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p2 = skip("(")
# => [('', 'python2.7)')]

p2("(python2.7)")