Records, Variants, and Pattern Matching

Overview

Modern programming languages provide structured data through records and variants.
Records capture fixed labeled collections of values; variants encode distinct alternatives.
Together, they form the foundation for algebraic data types (ADTs) and safe pattern matching.

Note

Records = product types (fields combined).
Variants = sum types (one alternative active).
Pattern matching = selective decomposition of those structures.

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Records: Product Structures

A record groups labeled fields together into one value.
Unlike tuples, fields are identified by names rather than position.

Example (OCaml):

type point = { x: float; y: float }
 
let move p dx dy = { x = p.x +. dx; y = p.y +. dy }

Records make code more self-documenting and robust to field reordering.

Concept	Tuple	Record
Identification	by position	by label
Extensibility	rigid	flexible (with optional defaults)
Example	(1.0, 2.0)	{x=1.0; y=2.0}

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Variants: Sum Structures

A variant (also called a tagged union) represents a value that can take one of several labeled forms.

Example:

type shape =
  | Circle of float
  | Square of float
  | Rectangle of float * float

Each constructor (Circle, Square, Rectangle) tags its data.
The compiler tracks which variant is active at runtime.

Note

Variants correspond to sum types:
Shape = Circle(radius) + Square(side) + Rectangle(width,height).

Variants can be nested, parameterized, or recursive — enabling expressive hierarchies like:

type 'a tree = Leaf | Node of 'a * 'a tree * 'a tree

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Pattern Matching

Pattern matching destructures values according to their constructors or record fields.

Example:

let area s =
  match s with
  | Circle r -> Float.pi *. r *. r
  | Square a -> a *. a
  | Rectangle (w, h) -> w *. h

Each clause tests the constructor and binds its contents to names.
Pattern matching replaces chains of conditionals with a concise, declarative form.

Tip

Matching is exhaustive and typed — every case is checked for consistency by the compiler.

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Nested and Guarded Patterns

Patterns can be nested or combined with guards for finer control:

match s with
| Rectangle (w, h) when w = h -> "square"
| Rectangle _ -> "rectangle"
| Circle _ -> "circle"

Guards (when ...) allow conditional matching beyond constructor names.
Nested patterns can directly decompose structures:

match node with
| Node (x, Leaf, _) -> x
| Node (_, l, r) -> traverse l; traverse r
| Leaf -> ()

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Exhaustiveness and Redundancy

Compilers perform exhaustiveness checking to ensure that every possible variant is handled.
They also warn about redundant (unreachable) patterns.

Example:

match s with
| Circle r -> r*r
| _ -> 0  (* covers all remaining variants *)

Warning

Non-exhaustive matches cause runtime errors in many languages.
Some, like Haskell and OCaml, statically detect missing cases and issue warnings.

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Record Patterns

Pattern matching also works on record fields:

match p with
| { x; y } when x = y -> "diagonal"
| { x; y } -> "non-diagonal"

Each field can be selectively matched or ignored:

| { x; _ } -> x

Renaming is allowed during binding:

| { x = x_pos; y = y_pos } -> (x_pos, y_pos)

Note

Partial record patterns are permitted — unmentioned fields are ignored.

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Variants with Parameters

Constructors can carry arbitrary payloads:

type value =
  | Int of int
  | Float of float
  | Pair of value * value

Each case introduces new bindings during a match:

match v with
| Int n -> string_of_int n
| Pair (a, b) -> "(" ^ show a ^ ", " ^ show b ^ ")"

This recursive style forms the basis of interpreters, compilers, and type checkers.

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Design Pitfalls

Warning

• Missing constructors → non-exhaustive matches.

• Shadowed patterns → redundant code that never executes.

• Variant misuse → applying the wrong constructor or mismatched arguments.

• Overly broad _ patterns → suppress exhaustiveness checking, hiding bugs.

Tip

Treat _ as a temporary placeholder during prototyping — replace it with explicit cases for maintainability.

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Conceptual Summary

Concept	Description	Example
Record	Labeled product of values	{x=1; y=2}
Variant	Tagged alternative forms	`Circle r
Pattern Match	Structured deconstruction	match s with ...
Exhaustiveness	All cases handled	compiler check
Guards	Conditional refinement	when w = h

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See also

• Parametric Polymorphism & ADTs

• Objects, Classes, and Dispatch

• Values, Variables & Environments

• Type Systems — Goals & Guarantees