[Haskell-cafe] Re: Trouble with record syntax and classes

[Steve Schafer]

On Mon, 26 Feb 2007 23:41:14 -0600 (CST), you wrote:

>Here's my second attempt at the code:
>...

You've left out a bunch of constructors, and there are various other
errors here and there. I think this will do what you want:

> data ISine = 
>   Sine Integer Integer Integer [Char] |
>   MetaSine Integer Integer Integer [ISine]
>
> letter (Sine _ _ _ l) = l
> sub_sines (MetaSine _ _ _ xs) = xs
> period (Sine p _ _ _) = p
> period (MetaSine p _ _ _) = p
> offset (Sine _ o _ _) = o
> offset (MetaSine _ o _ _) = o
> threshold (Sine _ _ t _) = t
> threshold (MetaSine _ _ t _) = t
>
> on :: Integer -> ISine -> Bool
> on time iSine = (mod (time-(offset iSine)) (period iSine)) < (threshold iSine)
>
> act :: Integer -> ISine -> [[Char]]
> act time (MetaSine p o t s) =
>   if on time (MetaSine p o t s)
>     then foldr1 (++) (map (act time) (sub_sines (MetaSine p o t s)))
>     else []
>
> act time (Sine p o t l) =
>   if on time (Sine p o t l)
>     then [letter (Sine p o t l)]
>     else []

But note that you have to write the equations for period, offset and
threshold twice. If you want to avoid that, you can move the
Sine/MetaSine discrimination into the "tail" of the data structure,
something like this:

> data ISineTail = 
>   SineTail [Char] |
>   MetaTail [ISine]
>
> data ISine = ISine Integer Integer Integer ISineTail
>
> letter (ISine _ _ _ (SineTail l)) = l
> sub_sines (ISine _ _ _ (MetaTail xs)) = xs
> period (ISine p _ _ _) = p
> offset (ISine _ o _ _) = o
> threshold (ISine _ _ t _) = t
>
> on :: Integer -> ISine -> Bool
> on time iSine = (mod (time-(offset iSine)) (period iSine)) < (threshold iSine)
>
> act :: Integer -> ISine -> [[Char]]
> act time (ISine p o t (MetaTail s)) =
>   if on time (ISine p o t (MetaTail s))
>     then foldr1 (++) (map (act time) (sub_sines (ISine p o t (MetaTail s))))
>     else []
>
> act time (ISine p o t (SineTail l)) =
>   if on time (ISine p o t (SineTail l))
>     then [letter (ISine p o t (SineTail l))]
>     else []

Steve Schafer
Fenestra Technologies Corp.
http://www.fenestra.com/