[Haskell-cafe] reading existential types

Andrea Rossato mailing_list at istitutocolli.org
Fri Jul 13 13:59:29 EDT 2007

On Mon, Jul 09, 2007 at 09:41:32PM +0100, Claus Reinke wrote:
>  hiding concrete types in existentials sometimes only defers problems
>  instead of solving them, but exposing class interfaces instead of types is a 
>  useful way to mitigate that effect. it just so happens that this particular 
>  problem, reading an existential type, slightly exceeds that pattern, as 
>  'read' needs to know the hidden type to do its job ('read' does not 
>  determine the type from the input form, but uses the type to determine what 
>  form.the input should have). 
>  a workaround is to try to read all possible types, then hide the type again 
>  once a match is found. the main disadvantage of this method is that we need 
>  a list of all the types that could possibly be hidden

As a follow up, mainly meant to thank you, I wanted to let you know
that I adopted this approach in a piece of software I'm writing.

It's a status bar for the XMonad Window Manager, the tiling WM written
in Haskell.[1] 

Actually it is a text based status bar that can be used with any
WM, but we love XMonad particularly...;-)

More information about this status bar can be found here:
with link to the source code, a screen shot and eve a link to a

I obviously credited you for the help and the code![2]
One again, thank you.

All the best,

[1] http://xmonad.org/
[2] http://gorgias.mine.nu/repos/xmobar/Runnable.hs

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