Haskell记录字段依赖性

Question

data MyRecord = MyRecord {
  numberOfSides :: Int,
  shape :: Shape
}

Shape can be Triangle, Quadrilateral etc. depending on numberOfSides .

My question is: Those two fields are related. Is there a "trigger" mechanism to update the other when one field is updated? and if the two fields are in conflict then report a type error?

Another related scenario:

data MyRecord = MyRecord {
  players      :: [Player],
  numOfPlayers :: Int
}

I want to somehow automatically calculate numberOfPlayers based on the size of players .

PS: Is this somehow related to GADT and dependant types ?

Answer 1

In Haskell, unlike in OO languages, you can't modify a field in place. So the usual approach, which is what I think you're thinking of, of having an explicit "setter" doesn't make sense in Haskell. The easiest thing, and this is well in line with the DRY principle (which is very relevant in functional languages as well as OO languages), is to only store one of the fields and have the other be an accessor, as mentioned in a comment.

-- deriving (...) clauses and other boilerplate omitted for brevity

data Shape = Triangle | Quad | Pentagon | Other

data MyRecord = MyRecord Int

numSides :: Shape -> Int
numSides Triangle = 3
numSides Quad = 4
numSides Pentagon = 5
numSides Other = 0

shape :: Int -> Shape
shape 3 = Triangle
shape 4 = Quad
shape 5 = Pentagon
shape _ = Other

makeMyRecord :: Shape -> MyRecord
makeMyRecord = MyRecord . numSides

numberOfSides :: MyRecord -> Int
numberOfSides (MyRecord x) = x

shapeOf :: MyRecord -> Shape
shapeOf = shape . numberOfSides

Now, Shape can be Enum and Bounded , if you haven't done that already. From there, shape and numSides can be shortened in terms of the automatically generated Enum instance. If you want to be able to construct MyRecord from Int , you can either export the data constructor or define another makeMyRecord' to do it. And now, since we're using the DRY principle, you can just let users update the field freely and the other "field" will magically be updated.

Answer 2

My question is: Those two fields are related. Is there a "trigger" mechanism to update the other when one field is updated? and if the two fields are in conflict then report a type error?

PS: Is this somehow related to GADT and dependant types?

This is related to dependent types, but not GADTs. Since this is Haskell, I would suggest simply storing the number of sides in the record and then writing a function numTypes :: Int -> Shape .