📜Recap

Pattern matching

• F♯ core building block

• Combines "data structure matching" and "deconstruction"

• Used almost everywhere:

  • match and function expressions

  • try/with block

  • let binding, including function parameter

• Can be abstracted into a fold function associated with a union type

Patterns

Pattern	Example
Constant • Identifier • Wilcard	1, Color.Red • Some 1 • _
Collection : Cons • List • Array	head :: tail • [1; 2]
Product type : Record • Tuple	{ A = a } • a, b
Type Test	:? Subtype
Logical : OR, AND	1 | 2, P1 & P2
Variables • Alias	head :: _ • (0, 0) as origin

+ The when guards in match expressions

Active Patterns

• Extending pattern matching

• Based on function + metadata → first-class citizens

• 4 types: total simple/multiple, partial (simple), parametric

• At first a little tricky to understand, but we get used to it quickly

• Use for:

  • Add semantics without relying on union types

  • Simplify / factorize guards

  • Wrapping BCL methods

  • Extract a data set from an object

  • ...

Addendum

📜 Match expressions, F# for fun and profit, 2012

📜 Domain modelling and pattern matching : Roman numeral, F# for fun and profit, 2012

📜 Recursive types and folds (6 articles), F# for fun and profit, 2015

📹 A Deep Dive into Active Patterns - Repo github