Advice defining a data structure in Haskell

Question

I'm having trouble modeling a data structure in Haskell. Suppose I'm running an an animal research facility and I want to keep track of my rats. I want to track the assignment of the rats to cages and to experiments. I also want to keep track of the weight of my rats, the volume of my cages, and keep notes on my experiments.

In SQL, I might do:

create table cages (id integer primary key, volume double);
create table experiments (id integer primary key, notes text)
create table rats (
    weight double,
    cage_id integer references cages (id),
    experiment_id integer references experiments (id)
);

(I realize that this allows me to assign two rats from different experiments to the same cage. That is intended. I don't actually run an animal research facility.)

Two operations that must be possible: (1) given a rat, find the volume of its cage and (2) given a rat, get the notes for the experiment it belongs to.

In SQL, those would be

select cages.volume from rats
  inner join cages on cages.id = rats.cage_id
  where rats.id = ...; -- (1)
select experiments.notes from rats
  inner join experiments on experiments.id = rats.experiment_id
  where rats.id = ...; -- (2)

How might I model this data structure in Haskell?

One way to do it is

type Weight = Double
type Volume = Double

data Rat = Rat Cage Experiment Weight
data Cage = Cage Volume
data Experiment = Experiment String

data ResearchFacility = ResearchFacility [Rat]

ratCageVolume :: Rat -> Volume
ratCageVolume (Rat (Cage volume) _ _) = volume

ratExperimentNotes :: Rat -> String
ratExperimentNotes (Rat _ (Experiment notes) _) = notes

But wouldn't this structure introduce a bunch of copies of the Cages and Experiments? Or should I just not worry about it and hope the optimizer takes care of that?

Advice defining a data structure in Haskell

4 Answers