Pattern Matching with TypeScript by Manuel Alabor

TypeScript does not have any pattern matching functionality built in. This article shows several ways how you can replicate the core of a simple pattern matcher using a few simple structures and functions within TypeScript.

Resulting code will have improved maintainability and better runtime type safety when done right.

What is Pattern Matching?

Pattern matching is a fundamental and powerful building block to many functional programming languages like Haskell or Scala.

We will not be able to replicate pattern matching on the level of sophistication as those languages provide. Though we can reuse the fundamental approach of pattern matching as inspiration to build something helpful in TypeScript.

Essentially a pattern can be defined as a thing describing one or more cases. Each case itself represents a specific behavior which has to be applied once the case matches.

You might think: “Hey! That sounds like a switch statement to me!”. And you are right indeed:

Match with Switch Statement

function matchNumber ( n : number ): string { switch ( n ) { case 1 : return 'one' ; case 2 : return 'two' ; case 3 : return 'three' ; default : return ` ${ n } ` ; } } function randomNumber (): number { return Math . floor ( Math . random () * ( 10 - 1 ) + 1 ); // Random number 1...10 } const result = matchNumber ( randomNumber ()); // result === One, Two, Three or 4...10

We can use a switch statement to map number s to its desired string representation.

Doing so is straightforward, but we can make out flaws for matchNumber :

The behavior for each case is baked into the matchNumber function. You have to reimplement the complete switch block if you want to map to something else than a string , for example, a boolean . Functional requirements can be misinterpreted and behavior for a case gets lost. What about 4 ? What if a developer forgets about default ? The possibility of bugs multiplies easily when the switch is reimplemented several times as described under point 1.

These flaws can be translated into a set of characteristics for a solution:

Separate matching a specific case from its behavior Make reuse of matcher simple to prevent bugs through duplicated code Implement matcher once for different types

Separation of Concerns

Let’s define an interface containing functions for each case we want to be able to match. This allows separating behavior from actual matcher logic later.

interface NumberPattern { One : () => string ; Two : () => string ; Three : () => string ; Other : ( n : number ) => string ; }

Having NumberPattern , we can rebuild matchNumber :

function matchNumber ( p : NumberPattern ): ( n : number ) => string { return ( n : number ): string => { switch ( n ) { case 1 : return p . One (); case 2 : return p . Two (); case 3 : return p . Three (); default : return p . Other ( n ); } }; }

The new implementation consumes a NumberPattern . It returns a function which uses our switch block from before with an important difference: It does no longer map a number to a string on its own, it delegates that job to the pattern initially given to matchNumber .

Applying NumberPattern and the new matchNumber to the task from the previous section results in the following code:

const match = matchNumber ({ One : () => 'One' , Two : () => 'Two' , Three : () => 'Three' , Other : ( n ) => ` ${ n } ` }); const result = match ( randomNumber ()); // result === One, Two, Three or 4...10

We clearly separated case behaviors from the matcher. That first point can be ticked off. Does it further duplicating code and improve maintainability of the matcher?

const matchGerman = matchNumber ({ One : () => 'Eins' , Two : () => 'Zwei' , Three : () => 'Drei' , Other : ( n ) => ` ${ n } ` }); const result = matchGerman ( randomNumber ()); // result === Eins, Zwei, Drei or 4...10

Another tick! Because we have split concerns by introducing NumberPattern , changing behavior without reimplementing the underlying matcher logic is straightforward.

Truly Reusable

Map a number to something different than a string still needs reimplementation of matchNumber . Can we solve this without doing so for each target type over and over again? Sure! Generics provide an elegant solution:

interface NumberPattern < T > { One : () => T ; Two : () => T ; Three : () => T ; Other : ( n : number ) => T ; } function matchNumber < T > ( p : NumberPattern < T > ): ( n : number ) => T { return ( n : number ): T => { // ... }; }

Introducing the generic type parameter T makes NumberPattern and matchNumber truly reusable: It can map a number to any other type now. For example a boolean :

const isLargerThanThree = matchNumber ({ One : () => false , Two : () => false , Three : () => false , Other : n => n > 3 }); const is100Larger = isLargerThanThree ( 100 ); // is100Larger === true const is1Larger = isLargerThanThree ( 1 ); // is1Larger === false

This fulfills the last point in our requirement list to implement the matcher once for different types. The final example will probably never make it to production code but it demonstrates the basic mechanic how a pattern and a corresponding matcher can be implemented in TypeScript.

Match Union Types

Union types are a convenient way to model more sophisticated types. Knowing what specific type you are handling can be tedious though:

type Argument = string | boolean ; const a = 'Hello World' ; if ( typeof a === 'string' ) { // do string stuff } else if ( typeof a === 'boolean' ) { // do boolean stuff }

Let’s assume I am lazy and desperately need that if block somewhere else too. I simply copy-and-paste the block and introduce successfully the first part of maintenance hell:

A new requirement wants me to allow number s as an argument in the application, so I modify the type definition of Argument accordingly and update one of the if blocks (because I already forgot about its twin):

type Argument = string | boolean | number ; // ... } else if ( typeof a === 'number' ) { // do number stuff }

The duplicated code with different type handling for Argument bears huge potential for runtime errors and undiscovered bugs.

With the pattern matcher from the section before we already know a handy tool to defuse this situation. The ArgumentPattern describes all possible cases when handling an Argument and the matchArgument matcher encapsulates the cumbersome code and makes it reusable:

interface ArgumentPattern < T > { String : ( s : string ) => T ; Boolean : ( b : boolean ) => T ; Number : ( n : number ) => T ; } function matchArgument < T > ( p : ArgumentPattern < T > ): ( a : Argument ) => T { return ( a : Argument ): T => { if ( typeof a === 'string' ) { return p . String ( a ); } else if ( typeof a === 'boolean' ) { return p . Boolean ( a ); } else if ( typeof a === 'number' ) { return p . Number ( a ); } throw new Error ( `matchArgument: Could not match type ${ typeof a } ` ); }; } const aString = 'Hello World' ; const result = matchArgument ({ String : ( s ) => console . log ( `A string: ${ s } ` ), Boolean : ( b ) => console . log ( `A boolean: ${ b } ` ), Number : ( n ) => console . log ( `A number: ${ n } ` ) })( aString ); // result === "A string: Hello World"

The big advantage of this solution plays once I have to modify the Argument type again: I Simply adapt ArgumentPattern accordingly and TypeScript will light up all code occurrences where action needs to be taken. A consistent evaluation of a union type becomes much easier this way.

Real Life Problem Domain

Following final example takes techniques introduced earlier and applies them to a more real live alike problem domain. An imaginative cash register application provides different ways how a customer can pay his bill. This requirement is modeled using the Payment type and two specializations CreditCardPayment and CashPayment . A PaymentPattern interface is implemented along with those types:

interface PaymentPattern < T > { CreditCard : ( card : CreditCardPayment ) => T ; Cash : ( cash : CashPayment ) => T ; } interface PaymentMatcher { match < T > ( p : PaymentPattern < T > ): T ; } abstract class Payment implements PaymentMatcher { constructor ( public readonly amount : number ) {} abstract match < T > ( p : PaymentPattern < T > ): T ; } class CreditCardPayment extends Payment { constructor ( amount : number , public readonly fee : number ) { super ( amount ); } match < T > ( p : PaymentPattern < T > ): T { return p . CreditCard ( this ); } } class CashPayment extends Payment { constructor ( amount : number , public readonly discount : number ) { super ( amount ); } match < T > ( p : PaymentPattern < T > ): T { return p . Cash ( this ); } }

You may notice the absence of a distinct matchPayment function when comparing to former examples. This slightly different approach applies a variation of the visitor pattern by using a PaymentMatcher interface and a pinch of polymorphism magic instead.

Doing so prevents a set of cumbersome and potentially harmful instanceof compares by baking PaymentMatcher into the abstract Payment base type. Each specialized payment implements PaymentMatcher.match then on its own.

The matcher function is called on the concrete type now. calculatePaymentAmount showcases this by applying different calculation strategies depending on what kind of payment is processed:

function calculatePaymentAmount ( payment : Payment ) { return payment . match ({ CreditCard : ( card ) => card . amount + ( card . amount * card . fee ), Cash : ( cash ) => cash . amount - cash . discount }); } const creditCardPayment = new CreditCardPayment ( 100 , 0.02 ); const creditCardAmount = calculatePaymentAmount ( creditCardPayment ); // creditCardAmount === 102 const cashPayment = new CashPayment ( 100 , 2 ); const cashAmount = calculatePaymentAmount ( cashPayment ); // cashPayment === 98

An obvious extension might be the introduction of an additional payment type or the change of an existing calculation strategy. Each of those are nicely secured by compile time checks which help to minimize the potential for new bugs.

Conclusion

The presented solution to bring pattern matching to TypeScript based applications is a powerful way to keep growing code bases better maintainable. It is a tool to keep code duplication low and keep conditions separated from actual behavior. More readable code is an additional side effect.

I do work with these paradigms for quite some time up to today. After writing this article one thing is even more clear to me than before: To replicate pattern matching for a language like TypeScript means to introduce a lot of boilerplate code. And indeed this boilerplate can be overkill for small, simple applications.

My personal experience proofs for myself that exactly this boilerplate can help to mitigate risks and potential bugs as a code base grows, developers hop on and off and requirements increase in their complexity.

Similar to other ways of structuring code, pattern matching is no silver bullet either. Have it in your toolbox and apply it with care when the situation seems reasonable.

Repository, Discussion & Credits

All code examples used in this article are available along with a Jest test suite in a complementary Github repository: https://github.com/swissmanu/pattern-matching-with-typescript

There is a thread on Hacker News with a discussion triggered by this article available: Pattern Matching with TypeScript @ HN

Further, I would like to thank @dbrack and @mweibel for proofreading and reviewing this article during its making. Thank you, guys! ❤️

Revision Notes

05.07.2017:

Initial publish 🎉

10.07.2017: