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Today's Topics:
1. Re: combining a trifecta parser with FreshMT from unbound
(Andreas Reuleaux)
----------------------------------------------------------------------
Message: 1
Date: Thu, 31 Jul 2014 00:42:49 +0100
From: Andreas Reuleaux <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] combining a trifecta parser with
FreshMT from unbound
Message-ID: <[email protected]>
Content-Type: text/plain
OK, I have made some progress, and have uploaded two programs to
lpaste.net
* one that works:
trifecta + unbound, works:
http://lpaste.net/108457
* and one that doesn't
trifecta + unbound, fails
http://lpaste.net/108458
the working version is simpler, in that it defines PiParser just as a
type synonym
type PiParser = FreshMT InnerParser
maybe I should be happy already that I got that working, e.g.
> parseTest (runInnerParser $ runFreshMT $ whiteSpace *> many identifier) " wow
> {- bla -} this rocks"
> ["wow","this","rocks"]
to install: cabal install trifecta + unbound in a sandbox e.g.
Now in the second version, I have in addition wrapped the FreshMT in a newtype
newtype FreshT m a = F {
runF :: FreshMT m a
} deriving (Monad, Functor, Applicative, MonadPlus, MonadTrans)
type PiParser = FreshT InnerParser
While this might not be necessary in this simple case here,
I can imagine situations, where doing so might come in handy.
Consequently have to add a few more instances for the parser:
instance CharParsing PiParser where
etc.
and of course I need an additional step of unwrapping with runF
runInnerParser $ runFreshMT $ runF someparser
(In both versions TokenParser is defined on the PiParser)
I would expect that I can parse something like the above test string
similarily, but unfortunately this is not the case:
> parseTest (runInnerParser $ runFreshMT $ runF (whiteSpace *> many
> identifier)) " oh too {- bla -} bad"
[]
I can at most parse one letter:
> parseTest (runInnerParser $ runFreshMT $ runF (whiteSpace *> identifier)) "oh
> too {- bla -} bad"
"o"
I don't really see, how wrapping the FreshMT in a newtype harms the parser?
Thanks in advance
-Andreas
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