[Haskell-cafe] dependent types

[Jason Dagit]

On Mon, Apr 12, 2010 at 4:32 AM, Ben Millwood <haskell at benmachine.co.uk>wrote:

>
> Personally I think this approach is all rather OO. The way that seems
> most natural to me is:
>
> moveVehicleAcrossBridge :: Bridge -> Vehicle -> Maybe Move
> moveVehicleAcrossBridge bridge { maxWeight = max } vehicle { weight = w }
>  | w > max = Nothing
>  | otherwise = {- ... moving stuff ... -}
>
> so you just test the properties directly as and when they are
> interesting to you.
>

The problem with this is that it doesn't address the question the OP had.
The OP's question was how to enforce things at the type level, but this,
while being a valid approach, allows heavy vehicles to attempt the bridge
crossing.  I agree it's a nice way of encoding it, but if type level
enforcement is called for then I don't see how it applies.

Jason
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