clarify wording on pure-functions overview to allow immutable hidden state, and add an example #3134
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there are still some typos, sorry, marking it as draft until they are fixed |
SethTisue
reviewed
Dec 21, 2024
| @@ -57,7 +57,7 @@ Conversely, the following functions are _impure_ because they violate the defini | |||
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| Impure functions often do one or more of these things: | |||
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| - Read from hidden state, i.e., they access variables and data not explicitly passed into the function as input parameters | |||
| - Read from hidden mutable state, i.e., they access non-constant data that was not explicitly passed into the function as input parameters | |||
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"non-constant" is not really a synonym for "mutable". I would suggest using the word "mutable" again even if it feels a bit repetitive; it's more important to be precise.
SethTisue
reviewed
Dec 21, 2024
| @@ -98,6 +98,21 @@ def double(i: Int): Int = i * 2 | |||
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| {% endtabs %} | |||
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| The next example is bit more tricky. Here, `i` is not passed as a parameter, but instead referenced directly from the outside. | |||
| This works in Scala because functions act as closures - they can capture the state around them. As long as that state is *immutable*, such a closure is still considered pure. | |||
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Not sure this is the best wording since we usually use "state" to talk about mutable state. Not all of the time. I would suggest "capture values from enclosing scopes".
SethTisue
reviewed
Dec 21, 2024
| @@ -129,7 +144,7 @@ If you understand that code, you’ll see that it meets the pure function defini | |||
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| The first key point of this section is the definition of a pure function: | |||
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| > A _pure function_ is a function that depends only on its declared inputs and its implementation to produce its output. | |||
| > A _pure function_ is a function that depends only on its declared inputs, captured constants, and its implementation to produce its output. | |||
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suggest "closed-over values" rather than "captured constants"
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Updates the "pure functions" overview at https://docs.scala-lang.org/scala3/book/fp-pure-functions.html