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Today's Topics:
1. Scoping within arrow notation (using HXT)? (Michael Alan Dorman)
2. Re: Scoping within arrow notation (using HXT)?
(Ertugrul S?ylemez)
----------------------------------------------------------------------
Message: 1
Date: Fri, 08 Jun 2012 11:48:23 -0400
From: Michael Alan Dorman <[email protected]>
Subject: [Haskell-beginners] Scoping within arrow notation (using
HXT)?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain
Hey, Haskellers,
I'm trying to use state threaded through an arrow in some HXT code to
avoid passing explicit parameters through several layers of functions,
but I think I'm not understanding quite what the arrow notation is
doing, because when I try to use a value I'm extracting from the state,
I'm getting a scope error.
I had many ways I was prepared for the code to be wrong, but that one
has me baffled. Any suggestions?
Mike.
{-# LANGUAGE Arrows, NoMonomorphismRestriction #-}
module HXTTest () where
import Text.XML.HXT.Core
data Info = Info {
value :: String
} deriving (Show)
info = Info { value = "foo" }
html = "<html><head></head><body><div class='foo'>llama</div></body>";
-- print (runSLA (getState >>> arr value) info html)
-- Div class is static, no reference to state
findFoo =
proc content -> do
(deep (isElem >>>
hasName "div" >>>
hasAttrValue "class" (== "foo"))) -< content
-- print (runSLA (hread >>> findFoo) info html)
-- Extract class from state, but don't use it
findFoo' =
proc content -> do
divName <- (getState >>> arr value) -< content
content >- (deep (isElem >>>
hasName "div" >>>
hasAttrValue "class" (== "foo")))
-- print (runSLA (hread >>> findFoo') info html)
-- Extract class from state, try to use it: "Not in scope: `divName'"
-- findFoo'' =
-- proc content -> do
-- divName <- (getState >>> arr value) -< content
-- content >- (deep (isElem >>>
-- hasName "div" >>>
-- hasAttrValue "class" (== divName)))
------------------------------
Message: 2
Date: Fri, 8 Jun 2012 18:20:13 +0200
From: Ertugrul S?ylemez <[email protected]>
Subject: Re: [Haskell-beginners] Scoping within arrow notation (using
HXT)?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
Hello there,
the structure of an arrow computation cannot depend on inputs. All
arrow variables (to the left of '<-' or '->') are inputs to following
computations. For instance:
proc x1 -> do
x2 <- c1 -< x1
x3 <- c2 -< x2
returnA -< f x2 x3
The variables x1, x2 and x3 are arrow variables and are out of scope
to the left of '-<', because if they were in scope, the structure of the
computation could depend on arrow variables, and you would in fact have
a monad instead of an arrow.
Note also that 'proc x -> c -< x' is the same as 'c', and 'do' notation
is an extension to 'proc' notation.
You may be interested in my (unfinished) arrow tutorial:
<http://ertes.de/new/tutorials/arrows.html>
Greets,
Ertugrul
Michael Alan Dorman <[email protected]> wrote:
> I'm trying to use state threaded through an arrow in some HXT code to
> avoid passing explicit parameters through several layers of functions,
> but I think I'm not understanding quite what the arrow notation is
> doing, because when I try to use a value I'm extracting from the
> state, I'm getting a scope error.
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
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