Create a Maybe with a `safe` Utility Function

Andy Van Slaars
InstructorAndy Van Slaars

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Published 4 years ago
Updated 3 years ago

In this lesson, we’ll create a safe function that gives us a flexible way to create Maybes based on a value and a predicate function that we supply. We’ll verify its behavior with values that satisfy the predicate, and values that do not.

Instructor: [00:00] Here, we're importing two utility functions -- inc and toUpper. We've also pulled in the Maybe type from Crocks, and we're using a function called SafeNum to take in a value and return a Maybe.

[00:10] We'll get a Just wrapping our value if it's a number, and Nothing otherwise. We're using map on our Maybe to call increment. This will run safely, because any value that isn't a number will result in a Nothing, and our function won't be invoked.

[00:23] We also have a set of input in results using our toUpper utility. Where we're pulling in the value test, we're converting it to upper case. Then down at the bottom, we're logging out both the result n for a number example and result s for a string example.

[00:37] You'll notice that in our number example, our result is a Just of six. If I were to change this to a non-number value, say an empty array, we're going to get back Nothing. That's safely running over inc, we're not getting n or any kind of weird unexpected value.

[00:54] Down here, we're taking our string, we're getting a string out. Which is great right up until we pass this, a number. Then, we're going to see that toUpperCase is not a function, we'd like to be able to make this safe.

[01:08] What we're going to do is we'll come up here, and we can declare a new function. We'll call this one SafeString, and this is going to take in a value just like our other one did. Then, we're going to check our typeof on val, and we'll see if that equals a string.

[01:31] If it does, we're going to return that wrapped and a Just. We'll call maybe.just with our value, otherwise, we'll return maybe.nothing. Now, I'm going to come down here. What I'm going to do is I'm going to take whatever value is being passed in for input, and I'm going to wrap that as a Maybe.

[01:50] I'm going to call SafeString on my value here, then I need to update this to call map. We'll call, passing in our toUpper function. We'll see down in our result we're going to get Just with our capitalized string.

[02:11] If I were to change this to, say, a five, we'll get a Nothing. We have two different functions that will take two different types and wrap them up in Maybes. That's great, except every time we need to create a new Maybe based on a different condition, we're going to have to create one of these functions.

[02:30] Let's see if we can make this a little more generic. I'm going to start by defining a function called isNumber. IsNumber is going to take in a value, and it's going to return the result of checking its type. I'm just going to copy this, paste that in there.

[02:47] I'm going to do the same thing, isString. We'll take in a value, check its type, and return result in Boolean. Now that we have these defined, let's create a generic utility that will take in one of these predicate functions and return our Maybe.

[03:07] I'll define another constant, I'm going to call this one Safe. Safe is going to be a function that takes in a predicate function and a value. All we're going to do here is we're going to call the predicate on the value, and based on the result of that, we'll return either a Just or a Nothing.

[03:26] If it results to true, we have the type of value we want. We'll wrap that up in a Just. Otherwise, we'll call the Nothing constructor and return a Nothing. With that created, let's get rid of these SafeNum and SafeString functions, because we won't need those anymore.

[03:45] I'm going to come down here, and I'm going to call Safe to get my number. I'm going to pass it isNumber as its predicate function. I'll do the same thing for the string, calling Safe isString, then passing in my string value.

[04:04] We'll see that if I change my number to a non-number, I'll get the Nothing in the result. If I change my string to something that's not a string, say a number, we'll get a Nothing in the result for that one. Everything is working the way we expect it to.

[04:21] This approach to creating a Maybe is standard enough that the Crocks library actually provides this Safe and these predicate functions for us. I can come up here and import Safe. That will just be from require, and we'll require that from crocks/maybe/safe. Then, we can get rid of our Safe function here, and everything will continue to work.

[04:49] I can also pull in two predicates. We'll pull in isNumber, and that will be from require. That will come from crocks/predicates isNumber. We're also going to have the string version of that, we'll have isString. That's going to come from isString, I can take care of these.

[05:16] We'll see we still got our results, so we can clean this code up. As an added bonus, this version of Safe is curried. Which means we can pass it a single argument and get back a function that will wait for that second argument.

[05:30] I could go back to using a function called SafeNum, but I can create that using Safe, passing it isNumber. Now I have this reusable function, I find myself calling Safe with the same predicate multiple times, I can just do this and reuse that wherever I need it.

[05:48] I also have a function that can be used easily in a composition, where it's just waiting for that single value as a result of the previously invoked function.