6_PatternMatching.md

Pattern matching (applies as well to destructuring)

Patterns are a programming mechanism that helps with processing data. They serve two purposes:

• Check what structure data has.
• Extract parts of data.

How it works

We match patterns against data: Syntax of patterns match the syntax of processing data

Pseudo code below x matches 3 :x = 3

(x, y) matches (1, 3) :x = 1, y = 3

(1, y) matches (1, 4) :y = 4

End of pseudo code

So pattern matching is about matching data structure with actual data filling the missing pieces by creating bindings

If we don't want to create bindings, then instead of variable name we provide special name _

Alternatives

Pseudo code below

(x, 0) | (0, x) matches (1, 0) :x = 1

end of pseudo code

bind and match

(8, x) as y gives you ability to both pattern match and bind the pattern result

let { coordinates: (x, y) as coordinates } = marsDestination;

Pattern Matching with let === destructuring

Pattern Matching with Switch

match multiple patterns against data

Pseudo code switch «value» { | «pattern1» => «result1» | «pattern2» => «result2» ··· } end of pseudo code

let result = switch 1 {
 | 1 => "One"
 | 2 => "Two"
 | x => "Unknow result" 
};

using alternatives and as

let result = switch (1, 0)  {
 | (x, 0) | (0, x) => "Some X:" ++ string_of_int(x)
 | (3, 14) as test => "Almost pi"
 | (_, _) => "Unknown"
};

let somePoint: point = Point(7., 6.);

let circ = Circle(somePoint, 5.0);

 /* 1 arctan = π/4 rad */
 
 let pi = 4.0 *. atan(1.0);

let computeArea = (s: shape) =>
  switch s {
  | Rectangle(Point(x1, y1), Point(x2, y2)) =>
    let width = abs_float(x2 -. x1);
    let height = abs_float(y2 -. y1);
    width *. height;
  | Circle(_, radius) => pi *. (radius ** 2.0)
  };