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    Unwrap Values from a Maybe

    Andy Van SlaarsAndy Van Slaars

    Applying functions to values within the context of a Maybe means we can perform our transformations with more confidence. At some point, we’ll need to extract our value to use it in places where our code expects a raw value. In this lesson, we’ll see how to get the value back out of a Maybe.

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    Transcript

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    Instructor: 00:00 We have a function defined that takes a number, then using the utility functions we've pulled in, increments it, and doubles the result of that and returns the value.

    00:08 We're passing in a two, and we're getting the expected result of six. But if we were to pass in something that's not a number, such as two as a string, we're going to start to get unexpected results.

    00:17 In this case, we get a 42, because our value ends up being coerced to a string, concatenated, then coerced back to a number in a multiplication step. If we change the value to something that's not coercible into a number, we're going to get not a number as our result.

    00:32 Let's use a Maybe inside this function to avoid these unexpected results. We'll come up here, and we've already imported Safe and isNumber. I'm going to start by defining a constant.

    00:42 We'll call it SafeNum, and we're going to call Safe with isNumber. Then, we're going to pass in that number value that's been passed into this function. To apply our increment function to the value, we're going to call SafeNum.map, passing in that increment function.

    00:58 Then we want to apply double so we can map again, passing in double. We want this to be our return value, we'll get rid of these and we'll return SafeNum with our two calls to map.

    01:11 We'll see that our result is right, except that our number is still wrapped up in a Just. Rather than returning our Maybe, we need to unwrap this value and return that. So that all of our consuming code can continue to work with numbers like it was.

    01:25 To do that after map, we're going to add a call to option. The option method is going to take a default value, and this default value is going to be returned in the case of a Nothing. Otherwise, it's just going to unwrap the value from our Just and return that.

    01:39 Since we're dealing with numbers, a nice Safe to fall would be zero. We'll pass that zero, we'll see that our result has updated to six. It's been unwrapped from the Just. Then we can pass in an invalid value and verify that we get back our default value of zero.

    01:54 That's working. Let's clean up these comments, and we really don't need to assign this to a variable. What we could do is take SafeNum out of the equation here and put these maps on Safe.

    02:14 Then if we really wanted to remove some more noise from our code, we could get rid of the curly braces. Now, we don't need the return statement either. We could take this refactoring one step further.

    02:28 We have two maps here, what's going to happen is our Maybe is going to go into the first map. It's going to see that there's a Just with a value, it will unwrap it, call inc, rewrap it, then go to the next map. It's going to go through that same process again.

    02:40 What we can do is we can save the work of unwrapping that value twice by turning these two functions into a single function composition. Then passing that in the map. To do that, I'm going to come up here, and I'm going to import compose.

    02:56 I can pull that right out of the Crocks library through crocks/helpers/compose. Then what I can do is I can compose here, and we can say that we call double after inc. Get rid of that second map, and we'll see that we still have our expected result.

    03:18 But now, we're only unwrapping and rewrapping that value one time.

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