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A type of records

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Record types are available or conveniently expressible in many programming languages.

Record types in Haskell

Language:Haskell provides syntactic sugar for algebraic data types such that constructor components can be be labeled so that they can be accessed in a record-like manner. Consider, for example, the following algebraic data type for points:

data Point = Point Float Float

The data constructor can be defined using record notation instead:

data Point = Point { getX :: Float, getY :: Float }

Here is an example of constructing a record:

myPoint :: Point
myPoint = Point { getX = 42, getY = 88 }

Here is an example of accessing record components:

-- Compute the distance between two points
distance :: Point -> Point -> Float
distance p1 p2 = sqrt (deltax^2+deltay^2)
  where
    deltax = abs (getX p1 - getX p2) 
    deltay = abs (getY p1 - getY p2) 

The constructors and component selectors are of these types:

> :t Point
Point :: Float -> Float -> Point
> :t getX
getX :: Point -> Float
> :t getY
getY :: Point -> Float

We can also update records in the sense that we can construct new records from existing records by updating specific components. For instance:

myPoint' :: Point
myPoint' = myPoint { getY = 77 }

For what it matters, the position-based approach to construction, as with normal algebraic data types, can also be used. Thus, component selectors can be omitted at will. This is demonstrated with constructing the same point as above:

myPoint :: Point
myPoint = Point 42 88

Component selectors are also omitted during pattern matching:

-- Represent point as pair
toPair :: Point -> (Float, Float)
toPair (Point x y) = (x, y)

The record notation can also be used in algebraic data types with multiple constructors, e.g.:

data Shape = Circle { getRadius :: Float }
           | Rectangle { getWidth :: Float, getHeight :: Float }

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