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Today's Topics:
1. Re: Custom type classes (Daniel Hinojosa)
2. Re: compilation error for jhc (Fabien R)
3. Re: Parsing 'A's and then ('A's or 'B's) (Francesco Ariis)
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Message: 1
Date: Mon, 25 Jan 2016 01:08:10 -0700
From: Daniel Hinojosa <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Custom type classes
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<caoxuh-5fqj-ibtysrox82ffj-fk42wq2b+m2j_3kjhegcfg...@mail.gmail.com>
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Ah. I had a suspicion it had to do with extensions. Got to study up on it.
Thanks
On Jan 25, 2016 12:42 AM, "Imants Cekusins" <[email protected]> wrote:
> Hello Daniel,
>
> it works with these tweaks:
>
> -- begin
>
> {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-}
> module TupInst where
>
>
> data Tuple3 a b c = Tuple3 a b c deriving (Show)
>
> data Tuple2 a b = Tuple2 a b deriving (Show)
>
> class Indexable idx a where
> first :: idx -> a
>
>
> instance Indexable (Tuple2 a b) a where
> first (Tuple2 a0 b0) = a0
>
>
> instance Indexable (Tuple3 a b c) a where
> first (Tuple3 a0 b0 c0) = a0
>
> -- end
>
> call it in ghci like this:
>
> first $ Tuple3 (1::Int) 'a' False::Int
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Message: 2
Date: Mon, 25 Jan 2016 09:26:56 +0100
From: Fabien R <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] compilation error for jhc
Message-ID: <[email protected]>
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On 24/01/2016 14:59, David McBride wrote:
> It looks like the binary package got its own instance for the version
> datatype. You could probably make it build by just removing that instance
> from Binary.hs.
Thanks David,
It worked fine.
--
Fabien
------------------------------
Message: 3
Date: Mon, 25 Jan 2016 10:15:13 +0100
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Parsing 'A's and then ('A's or 'B's)
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
On Sun, Jan 24, 2016 at 10:56:31PM +0100, Simon Jakobi wrote:
> > if I understood your specification correctly, there would be multiple
> > ways to parse the string "AAA":
> >
> > - 3 'x' elements ("A", "A", "A")
> > - 2 'x' elements ("AA", "A")
> > - 2 'x' elements again (first one shorter) ("A", "AA")
> > - 1 'x' element ("AAA")
> >
>
> There would be even more ways because 'y', too, can represent one or more
> 'A's.
>
> [...]
>
> Is this due to attoparsec not being able to "backtrack" (not sure if
> this is the right term)? Is backtracking something that parsers generally
> are incapable of?
Ah, indeed you are right. attoparsec, parsec and friends handle failure
with `try`. From Attoparsec documentation:
Attempt a parse, but do not consume any input if the parse fails.
One way to deal with cases like yours is for every parser to compute a
"list of successes". Crude example:
import Text.Parsec
import Text.Parsec.String
foo :: Parser [String]
foo = anyChar >>= \h ->
(foo <|> e) >>= \t ->
return ([""] ++ map (h:) t)
where
e = return [""]
-- ?> parseTest foo "bar"
-- ["","b","ba","bar"]
Then you can chain those with `try`/`choice` and compute your result(s)
(I guess using the list monad to handle the mechanism could do).
Ambiguous grammars are an age old problem, and some searching [1] leads
me to believe there are already viable solution in Haskell.
[1]
http://stackoverflow.com/questions/13279087/parser-library-that-can-handle-ambiguity
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