[Haskell-cafe] Record update fusion (or how should I call it?)

[Peter Verswyvelen]

Yes of course Haskell records do this, but these updates are not first class.

But this kind of fusion seems rather general.

I haven't checked if the compiler already does it.

On Sun, Sep 6, 2009 at 9:19 PM, Alberto G. Corona<agocorona at gmail.com> wrote:
> Use Haskell records:
>
> defaultWindowDescription= WindowDescription {title="", size=(0,0) ......}
>
> Many modifications can be fused in a single statement. for example:
>
> newWindowDescription= defaultWindowDescription{itle= "Haskell" ; size=
> (640,480),background= Blue}
>
>
>
> 2009/9/6 Derek Elkins <derek.a.elkins at gmail.com>:
>> The first thing I would do i is verify that the compiler is not
>> already doing this.
>>
>> On Sun, Sep 6, 2009 at 7:50 AM, Peter Verswyvelen<bugfact at gmail.com> wrote:
>>> I've seen a couple of package being announced that provide first class
>>> labels, and other packages already existed for this (Grapefruit
>>> Record, HList, Accessor, ...)
>>>
>>> Regarding this, I have a question about the performance of multiple
>>> composed field updates. Maybe an example.
>>>
>>> Suppose I have a large record - say WindowDescription - which contains
>>> a lot of fields.
>>>
>>> Suppose I have a couple of default window description values, e.g.
>>> defaultWindowDesc, dialogBoxDesc, etc
>>>
>>> Using accessors it is easy to take such a default value, and "modify"
>>> a couple of fields, like:
>>>
>>> let myWindowDesc = set title "Haskell" . set size (640,480) . set
>>> background Blue . set fontFamily Arial $ defaultWindowDesc
>>>
>>> However, I guess this would make a lot of intermediate
>>> WindowDescription copies no (whether the fields are strict or not)? So
>>> ideally for performance, all these "updates" should be fused, maybe
>>> running inside an ST monad?
>>>
>>> I'm not sure if any of this is valid, but I would like to understand
>>> more about this, so any links and hints are welcome :-)
>>>
>>> Peter Verswyvelen
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>>> Haskell-Cafe at haskell.org
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>>>
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