[Haskell-cafe] F# active patterns versus GHC's view

[andy morris]

If you don't mind using GHC extensions (which in a view pattern thread
probably isn't much of a stretch to assume :) ), there's always record
punning (-XNamedFieldPuns):

    data Foo = { [snip] }
    f (Foo { a, g }) = ...

2009/1/15 John Van Enk <vanenkj at gmail.com>:
> I've often thought having constructor "views" would be handy.
> data Foo = Foo A B C D E F G H I
> view Bar = (Foo A _ C _ _ _ G _ I) => Bar A C G I
> This does bring up problems with case alternatives though.
> I think the correct answer for these kinds of views is with the record
> pattern matching syntax, though, I wish there was a more terse way to notate
> it.
> data Foo = {
>   a :: A,
>   b :: B,
>   c :: C,
>   d :: D,
>   e :: E,
>   f :: F,
>   g :: G
> }
> f (Foo {a = var_a, g = var_g}) = ...
> /jve
>
>
> 2009/1/15 Peter Verswyvelen <bugfact at gmail.com>
>>
>> When I first read about active patterns in F#, I found it really cool
>> idea, since it allows creating fake data constructors that can be used for
>> pattern matching, giving many views to a single piece of data, and allowing
>> backwards compatibility when you completely change or hide a data structure.
>> So for example one could define a Polar pattern and a Rect pattern that
>> give different views of a Complex number, e.g (pseudo code follows)
>> pattern Polar c = (mag c, phase c)
>> pattern Rect c = (real c, imag c)
>> This seems handy:
>> polarMul (Polar m1 p1) (Polar m2 p2) = mkComplexFromPolar (m1*m2) (p1+p2)
>> However, I think it is flawed, since the following
>> case c of
>>      Polar _ _ -> "it's polar!"
>>      Rect _ _ -> "it's rect!"
>> seems like valid code but does not make any sense.
>> So I think the GHC extension of view patterns is better than the active
>> patterns in F#?
>> A good coding style is to provide constructor functions and hide data
>> constructors. But then one looses the ability to perform pattern matching,
>> which is so cool in Haskell. Would I have to conclude that it would be good
>> coding style to use view patterns as much as possible in Haskell,
>> creating auxiliary data constructors to expose the "public members" of the
>> hidden data constructors?
>>
>>
>>
>>
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>
>
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