Well, it seems this approach doesn't allow you to group some fields together
like "colour" and "weight", but instead you need to relist them piecemeal
for each new data constructor. Also, you get a run-time error (rather than
compile-time) if you happen to reference a field that didn't happen to exist
in another data constructor, say like:
sq = squishiness $ Table {colour = Black, weight=1, height= 2}
main = putStr $ show sq
main: No match in record selector Main.squishiness
-Jamin
Jon Fairbairn wrote:
>
> Ketil Malde <[EMAIL PROTECTED]> writes:
>
>> jamin1001 wrote:
>> > What if I want to do something like data Chair = Chair
>> > {pos:: Int, color :: Int} data Table = Table {pos:: Int,
>> > color :: Int}
>> data Properties = Props { pos, color :: Int }
>> data Chair = Chair Props
>> data Table = Table Props
>>
>> or:
>>
>> data Chair = Chair Int Int
>> data Table = Table Int Int
>>
>> class Furniture a where
>> pos :: a -> Int
>> color :: a -> Int
>>
>> instance Furniture Chair where
>> pos (Chair x _) = x
>> color (Chair _ c) = c
>
>
> I'm not sure I follow what's really wanted here. Whats wrong
> with this:
>
>> data Colour = Black | Brown deriving (Enum, Show)
>> data Furniture = Table {colour:: Colour, weight:: Double, height::
>> Double}
>> | Chair {colour:: Colour, weight:: Double, squishiness::
>> Double}
>> deriving Show
>
> and then...
>
> ___ ___ _
> / _ \ /\ /\/ __(_)
> / /_\// /_/ / / | | GHC Interactive, version 6.4.2, for Haskell
> 98.
> / /_\\/ __ / /___| | http://www.haskell.org/ghc/
> \____/\/ /_/\____/|_| Type :? for help.
>
> Loading package base-1.0 ... linking ... done.
> Prelude> :l /tmp/foo.lhs
> Compiling Foo ( /tmp/foo.lhs, interpreted )
> Ok, modules loaded: Foo.
> *Foo> colour $ Table {colour = Black, weight=1, height= 2}
> Black
> *Foo> colour $ Chair {colour = Brown, weight = 1, squishiness=100}
> Brown
> *Foo>
>
>
> The requirement is that all the "colour" fields have the
> same type.
>
>> The record system is somewhat wartish,
>
> Sure, but it's not obvious to me that we're looking at one
> of the warts here, unless the problem is that my "Furniture"
> above isn't extensible in the appropriate sense or someone
> wants the different coulour fields to have different types
> (confusing, surely?).
>
> --
> Jón Fairbairn [EMAIL PROTECTED]
>
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>
>
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