Implementing simple business model in Haskell

Question

Supose we have a very simple model: `Station` has at least one `Train` `Train` has at least two `Station`s The model has to allow to check what stations any given train visits and to check what trains visit a particular station. How to model it in Haskell? I am a Haskell newbie, so please correct me: once an object is created, you cannot modify it - you can only make a new object based on that one (~immutability). Am I right? If so, I'll have to create a lot of temporary variables with semi-initialized objects (during deserialization or even in unit tests). Basically what I need is an example of modeling domain classes in Haskell - after reading "Learn you a haskell.." I still have no idea how to use this language.

Accepted Answer

Here's a naive approach:

```
data Station = Station Train [Train]
data Train = Train Station Station [Station]
```

You should never have to create an uninitialized object. For example, here's a couple sample values:

```
grandCentral = Station regional [national]
bestWestern  = Station regional [national]
regional = Train grandCentral bestWestern []
national = Train grandCentral bestWestern []
```

However, there's a lot of drawbacks to this approach; observing the in-heap cycles is impossible from within the pure subset of Haskell, so updating or using these pieces of data is tricky. The usual workaround is to model your pointers explicitly.

```
type TrainMap   = Map TrainId Train
type StationMap = Map StationId Station
type TrainId    = Int -- use newtype for more compiler checks at
type StationId  = Int -- the cost of more programmer annoyance
data Train   = Train StationId StationId [StationId]
data Station = Station TrainId [TrainId]
```

Implementing simple business model in Haskell

Solution

Problem