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Today's Topics:
1. Re: Simple parser question (Martin Drautzburg)
2. Building with shared libraries on OS X (James Nelson)
----------------------------------------------------------------------
Message: 1
Date: Mon, 18 Feb 2013 21:48:36 +0100
From: Martin Drautzburg <[email protected]>
Subject: Re: [Haskell-beginners] Simple parser question
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Message-ID: <[email protected]>
Content-Type: Text/Plain; charset="iso-8859-1"
Thanks, that kind of worked. As long as I am only dealing with literal
integers at the beginning of pluses is works fine.
*Main> parse "(1+2+3)"
Plus (Lit 1) (Plus (Lit 2) (Lit 3))
But this fails already:
*Main> parse "(1+2)+3"
Error "parse error"
There is no problem with expressions in parenthes, but my "pluses" can only
start with an integer (even one in parentheses), but not with an expression,
since I had to get rid of the expression at the left side, to avoid left-
recursion.
I am also confused that saying "a plus expression is an integer followed by
many "plus somethings" is not what the language says. So this requires a lot
of "paying attention" to get right. I'd much rather say "a plus expression is
two expressions with a '+' in between".
Any other ideas?
On Thursday, 14. February 2013 15:23:03 Twan van Laarhoven wrote:
> Left-recursion is always a problem for recursive-descend parsers. The
> solution is to rewrite the parser as:
> * first always parse an expression without a Plus
> * followed by zero or more "+ exp" parts.
>
> How exactly you write this depends on the combinators that the book defines
> for writing parsers. In Parsec you would write something like:
>
> parseExp = do
> lit <- parseLit
> pluses <- many (parsePlusToken *> parseLit)
> return (combinePlusesWithLit lit pluses)
>
> combinePlusesWithLit = foldr Plus -- or foldl
>
> I hope you get the idea.
>
> Note that the parsec library has functions chainl and chainr that do
> something like this under the hood, so you would never actually write the
> above code.
>
>
> Twan
>
> On 14/02/13 13:59, Martin Drautzburg wrote:
> > Hello all,
> >
> > I just hit a sticking point when trying to parse something like
> >
> > data Exp = Lit Int -- literal integer
> >
> > | Plus Exp Exp
> >
> > where something like "1+2" should be parsed to "Plus (Lit 1) (Lit 2)".
> >
> > When I try to parse "1+2" my parser enters an infinite loop. I can
> > understand why: it thinks
> >
> > "hmm, this expression could be a plus, but then it must start with an
> > expression, lets check".
> >
> > and it tries to parse expression again and again considers Plus.
> >
> > When I change the rules, so it first checks for "Lit", it does parse the
> > "1" just fine, but then gives up, because the remainder is not an
> > expression anymore, but just a "+2".
> >
> > My parser is written in the style shown in Graham Hutton's book:
> >
> > Parser a :: String -> (a, String).
> >
> > I believe I am missing something obvious, but I can't see it.
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
--
Martin
------------------------------
Message: 2
Date: Tue, 19 Feb 2013 11:17:24 +0900
From: James Nelson <[email protected]>
Subject: [Haskell-beginners] Building with shared libraries on OS X
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Hi,
I've been trying to install Haskell in such a way that I can compile things
with the -dynamic option on an OS X machine.
It seems that with the standard Haskell Platform, the shared versions of
libraries are not included, so enabling the "shared" option in ~/.cabal/config
doesn't work.
I've tried installing haskell-platform from scratch with homebrew (with and
without --devel), and this hasn't worked either
Finally, I've tried re-installing Haskell Platform with the provided .dmg from
scratch, but I get errors like:
- Perhaps you haven't installed the "dyn" libraries for package `text-0.11.2.3'?
- Codec/Zlib.hs:47:8:
Could not find module `Codec.Compression.Zlib'
Perhaps you haven't installed the "dyn" libraries for package
`zlib-0.5.4.0'?
Is there any way to add the shared libraries to everything included in haskell
platform for OS X?
Regards,
James
------------------------------
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