Re: [Haskell-cafe] Is there any way to parametrize a value update using record syntax?
2011/9/6 Poprádi Árpád
> updateData1 :: X -> MonadicEnv()
> updateData1 d = do; env <- get; put env {data1 = d}
>
> updateData1 d = modify (\r -> r {data1 = d})
But there is, sadly, no eta-reduced version of record update to make the "\r
-> r ..." boilerplate go away; recognition of the syntax requires an
expression before the braces. (Consider the ambiguity of the eta-reduced
expression "modify {data1 = d}".) Also, and much more annoyingly, "data1"
must be constant.
If you can decipher the documentation, there are several alternative record
packages on Hackage based on functional lenses. Also there are packages
which use Template Haskell to automate the above.
--
brandon s allbery allber...@gmail.com
wandering unix systems administrator (available) (412) 475-9364 vm/sms
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Re: [Haskell-cafe] Is there any way to parametrize a value update using record syntax?
On 07.09.2011 00:56, Brandon Allbery wrote:
2011/9/6 Poprádi Árpád mailto:popradi_ar...@freemail.hu>>
updateData1 :: X -> MonadicEnv()
updateData1 d = do; env <- get; put env {data1 = d}
> updateData1 d = modify (\r -> r {data1 = d})
But there is, sadly, no eta-reduced version of record update to make the
"\r -> r ..." boilerplate go away; recognition of the syntax requires an
expression before the braces. (Consider the ambiguity of the
eta-reduced expression "modify {data1 = d}".) Also, and much more
annoyingly, "data1" must be constant.
In principle ambiguity could be resolved with a slash:
modyfy \{data = d}
But that's from land of hypothetical language extension and dog headed
people.
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Re: [Haskell-cafe] Is there any way to parametrize a value update using record syntax?
2011/9/6 Poprádi Árpád :
> i have a record with a lot of items used in a state monad.
>
> data BigData = BigData {
> data1 :: X
> , data2 :: X
> -- and so on
> }
>
> updateData1 :: X -> MonadicEnv()
> updateData1 d = do; env <- get; put env {data1 = d}
>
> updateData2 :: X -> MonadicEnv()
> updateData2 d = do; env <- get; put env {data2 = d}
>
> But it's ugly. Always the same, only the record selector has another
> name.
> Is it possible to generalize it?
You can use the fclabels package [1] for this. It makes record labels
first class, and also provides functions to update parts of a record
in the state monad [2]. You would be able to write something like:
updateData1 = puts data1 d
It has a function for modifcation as well, which is even uglier with
regular record syntax.
Erik
[1] http://hackage.haskell.org/package/fclabels
[2]
http://hackage.haskell.org/packages/archive/fclabels/1.0.4/doc/html/Data-Label-PureM.html#v:puts
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Re: [Haskell-cafe] Is there any way to parametrize a value update using record syntax?
2011/9/6 Erik Hesselink > You can use the fclabels package [1] for this. It makes record labels > first class, and also provides functions to update parts of a record > in the state monad [2]. That's pretty nifty. Thanks for mentioning it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is there any way to parametrize a value update using record syntax?
Hi Erik,
thanks a lot!
fclabels is an amazing package!
My code become much clearer.
Greetings,
Árpád
On Wed, 2011-09-07 at 00:04 +0200, Erik Hesselink wrote:
> 2011/9/6 Poprádi Árpád :
> > i have a record with a lot of items used in a state monad.
> >
> > data BigData = BigData {
> > data1 :: X
> > , data2 :: X
> >-- and so on
> >}
> >
> > updateData1 :: X -> MonadicEnv()
> > updateData1 d = do; env <- get; put env {data1 = d}
> >
> > updateData2 :: X -> MonadicEnv()
> > updateData2 d = do; env <- get; put env {data2 = d}
> >
> > But it's ugly. Always the same, only the record selector has another
> > name.
> > Is it possible to generalize it?
>
> You can use the fclabels package [1] for this. It makes record labels
> first class, and also provides functions to update parts of a record
> in the state monad [2]. You would be able to write something like:
>
> updateData1 = puts data1 d
>
> It has a function for modifcation as well, which is even uglier with
> regular record syntax.
>
> Erik
>
> [1] http://hackage.haskell.org/package/fclabels
> [2]
> http://hackage.haskell.org/packages/archive/fclabels/1.0.4/doc/html/Data-Label-PureM.html#v:puts
>
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