Contexts differ in length

Robert van Herk rherk at
Tue May 24 08:30:57 EDT 2005

Hi all,

A while ago I sent an email to the glasgow haskell users maillinglist to 
explain how the "Contexts differ in length" feature (or bug :-)) 
restricted me in writing a haskell application. I was hoping for a 
reply, however I didn't receive one (yet).

Therefore, I will explain the problem again:

I am writing (for my master's thesis project) a webdevelopment framework 
in Haskell, with features somewhat comparable to Apple's WebObjects. 
Amongst others, session state and database interaction is transparent, etc.

In my framework, functions that generate HTML are called WFComponents. 
These functions are monadic since they can generate IO (because they may 
do database interaction etc). Also, components can generate links to 
other components. However, since component a may generate a link to 
component b (so that when the user clicks that link component b will be 
evaluated) and component b may link to component a, there will occur 
errors when I try to do this, since the contexts of component a and b 
may not be the same. A minimal example of this will be something like:

{-# OPTIONS -fglasgow-exts #-}

module Main where
import Data.IORef

class MyReader r v | r -> v where
  myRead :: r -> IO v
data R v = R (IORef v)
instance MyReader (R v) v where
  myRead (R v) =
    do v <- readIORef v
       return v

a :: IO ()
a =
  do r <- createReader
     b r
b :: MyReader r Int => r -> IO ()
b r =
  do i <- myRead r
     if i > 10
       then a
       else putStrLn (show i)
createReader :: IO (R Int)
createReader =
  do ref <- newIORef 0
     return (R ref)

A real example will be a bit more complicated, but this is basically 
what I need to do and currently am not able to. Of course, when needed, 
I can show you the real example. Somewhere in the history of this 
mailling list I read that people have had this program before, but only 
in toy programs. However, I am experincing this problem currently in 
something that is not a toy program. Therefore, my question is if it 
would be possible to lift this constraint on the language, and also, if 
the developers of GHC are currently planning to do this...



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