"No hash" error in offside-rule left recursive grammar - lepl

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Message from discussion "No hash" error in offside-rule left recursive grammar
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Date: Sun, 18 Apr 2010 19:27:30 +0300
Message-ID: <l2w53d4a6461004180927pca244aecof88d94c7272bd334@mail.gmail.com>
Subject: Re: [LEPL] Re: "No hash" error in offside-rule left recursive grammar
From: Daniel Armak <danar...@gmail.com>
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You're right - after thinking about it some more, left recursion isn't
necessary. I can define a rule for several calls in a row and then replace
it with left-associative Call tokens in the AST. IOW, the grammar wouldn't
specify the associativity at all.

Here's the updated code, and it doesn't trigger the bug, either:

CLine = ContinuedBLineFactory(Token(r'\\'))

expr0 = Token("[A-Za-z_][A-Za-z0-9_]*")

expr1 = Delayed()

def calls_to_call(tokens):
    if len(tokens) == 1:
        return tokens[0]
    elif len(tokens) == 2:
        return Call(tokens)
    else:
        return Call((Call((tokens[0], tokens[1])),
calls_to_call(tokens[2:])))

calls = expr0[2:] > calls_to_call
expr1 += calls | expr0

program = (CLine(expr1) & Eos())
program.config.default_line_aware(block_policy=rightmost)

parsed = program.parse("a b c d")
print(parsed[0])

Thanks again - I've tried using several python parsing libraries and lepl is
by far the most powerful!

Daniel Ermak

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<div dir=3D"ltr">You&#39;re right - after thinking about it some more, left=
 recursion isn&#39;t necessary. I can define a rule for several calls in a =
row and then replace it with left-associative Call tokens in the AST. IOW, =
the grammar wouldn&#39;t specify the associativity at all.=A0<div>
<br><div>Here&#39;s the updated code, and it doesn&#39;t trigger the bug, e=
ither:</div><div><br></div><div><div>CLine =3D ContinuedBLineFactory(Token(=
r&#39;\\&#39;))</div><div><br></div><div>expr0 =3D Token(&quot;[A-Za-z_][A-=
Za-z0-9_]*&quot;)</div>
<div><br></div><div>expr1 =3D Delayed()</div><div><br></div><div>def calls_=
to_call(tokens):</div><div>=A0=A0 =A0if len(tokens) =3D=3D 1:</div><div>=A0=
=A0 =A0 =A0 =A0return tokens[0]</div><div>=A0=A0 =A0elif len(tokens) =3D=3D=
 2:</div><div>=A0=A0 =A0 =A0 =A0return Call(tokens)</div>
<div>=A0=A0 =A0else:</div><div>=A0=A0 =A0 =A0 =A0return Call((Call((tokens[=
0], tokens[1])), calls_to_call(tokens[2:])))</div><div><br></div><div>calls=
 =3D expr0[2:] &gt; calls_to_call</div><div>expr1 +=3D calls | expr0=A0</di=
v><div><br></div><div>
program =3D (CLine(expr1) &amp; Eos())</div><div>program.config.default_lin=
e_aware(block_policy=3Drightmost)</div><div><br></div><div>parsed =3D progr=
am.parse(&quot;a b c d&quot;)</div><div>print(parsed[0])</div><div><br></di=
v>
</div><div>Thanks again - I&#39;ve tried using several python parsing libra=
ries and lepl is by far the most powerful!</div></div><div><br></div><div>D=
aniel Ermak</div></div>

<p></p>

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