Re: [Haskell-cafe] basic field questions

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 writes:

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jamin1001 wrote:

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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:

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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.

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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 Jon.Fairbairn@cl.cam.ac.uk

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