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List:       haskell-cafe
Subject:    Re: [Haskell-cafe] Is there any way to parametrize a value update
From:       Alexey Khudyakov <alexey.skladnoy () gmail ! com>
Date:       2011-09-06 21:13:49
Message-ID: 4E668D0D.5000103 () gmail ! com
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On 07.09.2011 00:56, Brandon Allbery wrote:
> 2011/9/6 Poprádi Árpád <popradi_arpad@freemail.hu
> <mailto:popradi_arpad@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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