[Haskell-cafe] How to specialize a type

Ruben Astudillo ruben.astud at gmail.com
Sat Oct 17 06:08:44 UTC 2015

Hi. You stack-overflow had good answers, I don't think I will shed new
light on this. But still I will give it a try.

On 16/10/15 04:32, martin wrote:
> Hello all,
> Suppose I have a type "Process" which stands either for a Train or a
> moving Belt. In both cases there is a place of departure and a place of
> arrival. However the other parameters which describe a Process differ.
> A Train has departure and arrival times, but a Belt has a speed.
> I tried the following:
> type PlaceDep = Int
> type PlaceArr = Int
> data Process = Train      PlaceDep PlaceArr TP
>              | MovingBelt PlaceDep PlaceArr BP deriving (Eq, Show)
> data TP = TP Int deriving (Eq, Show)
> data BP = BP Int deriving (Eq, Show)

You could use a newtype here. This is the intended use case of them as
you want a semantically different type but an underlying same
representation (and possible some instances of such representation).
Something like this

     newtype TP = TP Int deriving (Eq, Show)
     newtype BP = BP Int deriving (Eq, Show)

> prc1 = Train      10 11 (TP 1)
> prc2 = MovingBelt 12 13 (BP 2)
> What I don't like about this is that the fact that all Processes have
> PlaceDep and PlaceArr appears somewhat "coincidental".
> This in contrast, captures the common parts more clearly:
> type PlaceDep = Int
> type PlaceArr = Int
> data ProcessParams = DepartureTime Int | Speed Int
>                     deriving (Eq, Show)
> data Process = Process PlaceDep PlaceArr ProcessParams
>                     deriving (Eq, Show)
> prc1 = Process   10 11 (DepartureTime 1)
> prc2 = Process   12 13 (Speed 2)

This just seems a trade-off if you want to know with what are you dealing
just matching the outer constructor or more of the problem won't care
about it. Not really a dichotomy worth caring about much anyways

> Is this the classic way of specializing a type or are there better options

I would just stick to sum types (the first approach) but I don't know the
whole context of the problem. BTW "specializing a type" has a concrete
meaning in haskell. It grabs a polymorphic function and gives it a more
concrete type. An usual example is

     id :: a -> a
     id x = x

     id2 :: Char -> Char
     id2 c = id c

which hopefully is clear on what I am going about

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