[Haskell-beginners] Scoping within arrow notation (using HXT)?

Fri Jun 8 17:48:23 CEST 2012

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)))