Resolving TODO tags, Part 2 #1874
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| > If you’re interested in how methods can be used like functions, see the [Eta Expansion][eta] discussion. | ||
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| ## Examples of impure functions | ||
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| Conversely, the following functions are *impure* because they violate the definition. | ||
| Conversely, the following functions are _impure_ because they violate the definition. | ||
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| The `foreach` method on collections classes is impure because it’s only used for its side effects, such as printing to STDOUT. |
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In fact, this is not true. It will probably be called with impure functions but List(1,2,3).foreach { () } is perfectly pure.
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Is there any way to reword this, or is it best to just delete the foreach sentence and the three bullet points after it?
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If you want to keep this discussion, here’s a stab at rewording it. FWIW, the main reason I like this discussion is that it helps people keep an eye out for methods that return Unit. I know it was something I never considered many years ago. Here’s the rewording:
While the foreach method on collections classes is technically pure, it’s typically used for side effects, such as printing to STDOUT.
A great hint that
foreachmight be used for side effects is that it returns the typeUnit.
Because it doesn’t return anything, logically the only reason you ever call it is to achieve some side effect.
Similarly, any method that returnsUnitis a candidate to be an impure function.
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Sorry for not following up earlier. I think your argument is perfectly sensible. Maybe it is easiest to actually say exactly that: "Keep an eye out for methods that return Unit. Since they do not return anything useful, this return type indicates that they are only called for their side effects"?
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| The `foreach` method on collections classes is impure because it’s only used for its side effects, such as printing to STDOUT. | ||
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| > A great hint that `foreach` is impure is that it’s method signature declares that it returns the type `Unit`. | ||
| > Because it doesn’t return anything, logically the only reason you ever call it is to achieve some side effect. | ||
| > Similarly, *any* method that returns `Unit` is going to be an impure function. |
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Like foreach above, not completely true. Unit only indicates that it might be executed for its side-effects. You can have (useless) pure methods that return Unit.
Co-authored-by: Jonathan <[email protected]>
Co-authored-by: Jonathan <[email protected]>
Co-authored-by: Jonathan <[email protected]>
| @@ -398,7 +397,12 @@ For more details, see the remainder of these modeling lessons. | |||
| ## Abstract classes | |||
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| {% comment %} | |||
| TODO: I have some notes on when to use abstract classes, and can update this section. | |||
| LATER: If anyone wants to update this section, our comments about abstract classes and traits are on Slack. The biggest points seem to be: | |||
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| LATER: If anyone wants to update this section, our comments about abstract classes and traits are on Slack. The biggest points seem to be: | |
| LATER: If anyone wants to update this section, here are a few points that could be mentioned: |
This is a second batch of trying to resolve my TODO tags in the documents.