---------- Forwarded message ----------
From: David Barbour <dmbarbour@gmail.com>
Date: 2011/9/6
Subject: Re: [Haskell-cafe] Is there any way to parametrize a value update using record syntax?
To: Poprádi Árpád <popradi_arpad@freemail.hu>

2011/9/6 Poprádi Árpád <popradi_arpad@freemail.hu>

But it's ugly. Always the same, only the record selector has another
name. Is it possible to generalize it?

You can generalize using template haskell. I believe Oleg's HList already provides such mechanisms, so you don't need to do this yourself. If you're doing this a lot, try the HList package. (If not, just do the

Also, I would say you've too tightly coupled your BigData to the MonadicEnv. I suggest you reduce it instead to:

setX :: X -> BigData -> BigData

setX x' bd = bd { dataX = x' }

updX :: (X -> X) -> BigData -> BigData

updX fx bd = bd { dataX = fx (dataX bd) }

Then in your state monad you can use:

modify (setX x')

gets dataX

And use of 'updX' is much more composable.

That aside, from personal experience, I'm usually okay just using:

modify (\ s -> s { dataX = x' })

in the few places I need it.