Exhaustive pattern matching in Typescript - Step-by-Step Execution

Concept Flow - Exhaustive pattern matching

Start with a value

↓

Match against patterns

↓

Check each pattern

↓

Handle

↓

All cases covered?

Yes↓

Return matched result

↓

End

The program checks a value against all possible patterns. If all patterns are covered, it returns the matched result. If any pattern is missing, it raises an error.

Execution Sample

Typescript

type Shape = { kind: 'circle', radius: number } | { kind: 'square', size: number };

function area(shape: Shape): number {
  switch (shape.kind) {
    case 'circle': return Math.PI * shape.radius ** 2;
    case 'square': return shape.size ** 2;
  }
}

Calculates area for a shape by matching its kind, covering all possible shape types.

Execution Table

Step	Input Value	Pattern Checked	Match Result	Action Taken
1	{ kind: 'circle', radius: 3 }	case 'circle'	Match	Calculate area = π * 3^2 = 28.2743
2	{ kind: 'circle', radius: 3 }	case 'square'	No Match	Skip
3	All patterns checked	Exhaustiveness	All covered	Return result 28.2743
4	{ kind: 'triangle', base: 4, height: 5 }	case 'circle'	No Match	Skip
5	{ kind: 'triangle', base: 4, height: 5 }	case 'square'	No Match	Skip
6	All patterns checked	Exhaustiveness	Missing pattern	Error: Non-exhaustive patterns

💡 Execution stops when a matching pattern is found or an error occurs if patterns are not exhaustive.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
shape	undefined	{ kind: 'circle', radius: 3 }	{ kind: 'circle', radius: 3 }	{ kind: 'circle', radius: 3 }	{ kind: 'triangle', base: 4, height: 5 }	{ kind: 'triangle', base: 4, height: 5 }	{ kind: 'triangle', base: 4, height: 5 }
matched	false	true	false	false	false	false	false
result	undefined	28.2743	28.2743	28.2743	undefined	undefined	error

Key Moments - 2 Insights

Why does the program throw an error if a pattern is missing?

Because exhaustive pattern matching requires all possible cases to be handled. The execution_table rows 4-6 show that when 'triangle' is not matched by any case, the program detects missing patterns and raises an error.

What happens when the first pattern matches?

As shown in execution_table rows 1-3, once a matching pattern is found (case 'circle'), the program calculates the result and stops checking further patterns.

Visual Quiz - 3 Questions

Look at the execution_table, what is the 'matched' variable after step 2?

A false

B undefined

C true

D error

💡 Hint

Check variable_tracker row 'matched' column 'After Step 2' to see if a pattern matched.

At which step does the program detect missing patterns and raise an error?

A Step 4

B Step 6

C Step 3

D Step 2

💡 Hint

Look at execution_table rows 4-6 where the input is a 'triangle' and no pattern matches.

If we add a case for 'triangle', how would the execution_table change for input { kind: 'triangle', base: 4, height: 5 }?

A It would still raise an error at step 6.

B It would skip all cases and return undefined.

C It would match at step 4 and return a result without error.

D It would match 'circle' case incorrectly.

💡 Hint

Adding a matching case prevents the missing pattern error shown in step 6.

Concept Snapshot

Exhaustive pattern matching checks a value against all possible cases.
All cases must be handled to avoid errors.
Use switch or match expressions with all variants.
If any case is missing, TypeScript signals an error.
This ensures safe and predictable code behavior.

Full Transcript

Exhaustive pattern matching means checking a value against every possible pattern it can have. In TypeScript, this is often done with a switch statement on a discriminated union type. The program tests each case one by one. If a case matches, it runs that code and stops checking further. If no cases match, and some patterns are missing, the program raises an error to warn the developer. This helps catch bugs early by ensuring all possible inputs are handled. The execution table shows how the program checks each pattern step-by-step, and the variable tracker shows how variables change during this process. Key moments clarify why missing patterns cause errors and how matching stops further checks. The visual quiz tests understanding of these steps. The quick snapshot summarizes the main points for easy recall.