[Haskell-cafe] Fwd: Is there any way to parametrize a value update using record syntax?

[David Barbour]

forgot to CC list.

---------- Forwarded message ----------
From: David Barbour <dmbarbour at 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 at freemail.hu>


2011/9/6 Poprádi Árpád <popradi_arpad at 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.
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