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Side effects in English usually represent something you didn't intend to happen, the definition is a bit more strict in Functional Programmingville.
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In an effort to keep our functions pure,
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we need to make sure that they
are free of side effects.
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In English, a side effect is something
that happens when you didn't intend it to.
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For instance,
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a side effect of eating a taco might be
that you got a salsa stain on your shirt.
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Now, you didn't intend it to happen but
it did and boy, was that taco good.
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So typically, we think of a side effect as
something that we didn't intend to happen.
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Now, unfortunately,
the saying is a little overloaded.
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Here in functional programming land, a
side effect refers to anything outside of
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the function scope that gets modified,
even if you intended to do so.
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It's still called a side effect.
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If it helps, I remember this discrepancy
by calling it a out side effect.
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For instance, if we write a function that
updates a list that is outside of its
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scope even if I intended to do so,
it is still considered a side effect.
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This outside of its scope holds true for
all IO, or Input Output.
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Now, reading from a file is off limits,
because that file could change,
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and therefore,
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would make the function behave differently
depending on the state of the file.
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Where adding to a file is
against the rules as well.
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Pure functions are meaning you should
be able to call them multiple times.
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And it won't affect anything.
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This is why output is bad news.
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Also, remember that one of the goals
of pure functions is that they can
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be memorized.
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So the code doesn't need to run anymore.
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After it knows the result so
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if you were relying on your function to
output to a file and it was memorize,
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that output wouldn't happen because
the function actually isn't running.
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It's just using that stored value.
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So the same goes for System dot out.
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Writing to that is a side effect.
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That is why we were doing that
inside of the forEach method.
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forEach, takes a consumer and
a consumer remember that takes a value and
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returns nothing.
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Consumers are kind of built for
side-effects,
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I mean what good is a pure function
that returns nothing right?
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It has no other way to do anything.
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So when you see a consumer used it is
always gonna be not pure function.
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There is one more handy function that I'd
like to show you that is intended for
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you to use to produce side effects and
it can be used inside streams.
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It's called peek.
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Here, let's go take a peek at it in use.
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#badjoke so I was at a meet up and
somebody was telling me about their
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office and where they worked and they know
that their company started with an n and
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I can't remember what it is,
so let's do this.
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Let's just filter these companies, and
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remember these are just the company names
up here, these distinct company names.
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And I want to filter those,
so let's do that.
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So let's say we have a company stream
here and I want to filter, and
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I know that it's the company and know that
it started with an N, so let's do that.
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Let's start with N here.
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And then I'll use this side effect,
right, the consumer here for each.
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And again, it's a side effect because
it is not returning anything so
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the only thing it can do in side
effect of printing out right, Okay.
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So let's run that.
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That's all?
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I think maybe we should
peek just to make sure.
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Now, I'm gonna do this.
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I'm gonna add this to the stream I'm
gonna come in here and I'm gonna peak
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because I'm pretty sure that, maybe it
was something that sounded like an n.
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We'll do company and I'll just print out,
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and I'm gonna make this
big arrow thing here.
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So that in the output we know something
fat arrow that we're debugging here,
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and we'll add the company there.
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Notice that this took a consumer, so
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we know that this is side effect friendly,
okay?
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So here we go, so
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now when I run this, you'll see that
I've printed out everything else.
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The n's are still in there but
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we also printed out everything
that doesn't start with an n.
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And this allows for handy debugging,
but don't forget to remove them.
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So with that, I think we can cross
some items off of our parking lot.
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Let's switch over to there.
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I think we've got our pure taken care of.
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Let's strike that through, and
I think we just finished our side effects.
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Don't forget, outside effects.
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So these methods on streams like peek,
they're super handy, aren't they?
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They accept functions as arguments and
they really let you be more declarative,
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as well as not coding
yourself into a corner.
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Now when this happens,
it is known as a higher order function.
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And we can actually build these ourselves.
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Let's do that after this quick break.
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