Pattern Matching Intro

Pattern Matching / Switch

Consider the factorial function from the last post:

While an if/else or ternary expression works just fine for two branches, there is another way to express this in Reason using a switch statement:

As you can see, the final case of the switch can be _, which is Reason OCaml's syntax for a throwaway variable.

Continuing also from the last post, consider the function we wrote to determine if a character is a vowel:

Let's convert this to using the switch statement:

Here I've also used the String.make function, which can make a string from a char.

Consider the Fibonacci series, 1, 1, 2, 3, 5, 8, 13..., where each number is the sum of the prior two numbers. To write a function that gives the _n_th Fibonacci number, one can use of pattern matching. This shows that one doesn't need to discard the default case and can reuse it as a variable in a subsequent function call:

Variants / Synonyms

The complement of pattern matching is variants. Variants are synonyms or equivalence classes that are constructed with the single bar character: (|)

Both pattern matching and variants are much more important to Reason than we've let on here. Before we touch on that we need to introduce more data structures. Hopefully this post gives a taste of what it's like to use switch to do make clear code paths.

Pattern Matching with Arrays

This example is powered by a Single Argument Match Function.

Explorations

Use pattern matching to write a function that ...

1. returns the sum of integers 1 through n
2. computes x to the power of n

Links

1. Dr. Axel Rauschmayer has written an extensive post on this topic, Pattern matching in ReasonML: destructuring, switch, if expressions

Image Credit: Wall by Nigel Appleton on Flickr

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