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Asked: 2026-06-09T04:58:38+00:00

I’m having trouble modeling a data structure in Haskell. Suppose I’m running an an

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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?

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1 Answer

  1. Editorial Team

    Here’s a short file I used for testing:

    type Weight = Double
type Volume = Double

data Rat = Rat Cage Experiment Weight deriving (Eq, Ord, Show, Read)
data Cage = Cage Volume               deriving (Eq, Ord, Show, Read)
data Experiment = Experiment String   deriving (Eq, Ord, Show, Read)

volume     = 30
name       = "foo"
weight     = 15
cage       = Cage volume
experiment = Experiment name
rat        = Rat cage experiment weight

    Then I started ghci and imported System.Vacuum.Cairo, available from the delightful vacuum-cairo package.

    *Main System.Vacuum.Cairo> view (rat, Rat (Cage 30) (Experiment "foo") 15)

    not-shared

    *Main System.Vacuum.Cairo> view (rat, Rat (Cage 30) experiment 15)

    shared-experiment

    (I’m not really sure why there’s doubled-up arrows in this one, but you can ignore/collapse them.)

    *Main System.Vacuum.Cairo> view (rat, Rat cage experiment weight)

    shared-args

    *Main System.Vacuum.Cairo> view (rat, rat)

    shared-all

    *Main System.Vacuum.Cairo> view (rat, Rat cage experiment (weight+1))

    shared-modified

    The rule of thumb, as should be illustrated above, is that new objects are created exactly when you call a constructor; otherwise, if you just name an already-created object, no new object is created. This is a safe thing to do in Haskell because it is an immutable language.

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