Introduction to Programming in ATS:
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Example: Implementing Counters

We implement a counter like an object in object-oriented programming (OOP). The type counter for counters is defined as follows: 

typedef counter = '{
  get= () -<cloref1> int
, inc= () -<cloref1> void
, reset= () -<cloref1> void
} // end of [counter]

The three fields of counter are closure functions that correspond to methods associated with an object: getting the count of the counter, increasing the count of the counter by 1 and resetting the count of the counter to 0. The following defined function newCounter is for creating a counter object (represented as a boxed record of closure functions): 

fun newCounter
  (): counter = let
  val count = ref<int> (0)
in '{
  get= lam () => !count
, inc= lam () => !count := !count + 1
, reset= lam () => !count := 0
} end // end of [newCounter]

The state of each created counter object is stored in a reference, which can only be accessed by the three closure functions in the record that represents the object. This is often referred to as state encapsulation in OOP.

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