Rewrote by-name parameters section of tour #757
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| _By-name parameters_ are only evaluated when used. They are in contrast to _by-value parameters_. To make a parameter called by value, simply prepend `=>` to its type. |
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"To make a parameter called by name", you mean
| _By-name parameters_ are only evaluated when used. They are in contrast to _by-value parameters_. To make a parameter called by value, simply prepend `=>` to its type. | ||
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| ```tut | ||
| def goToSleep(tired: Boolean, seconds: => Int) = if (tired) Thread.sleep(seconds) |
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too long for a one-liner, especially given the site layout
| def lengthOfNap = 3600 * 8 | ||
| goToSleep(tired = false, seconds = lengthOfNap) | ||
| ``` | ||
| The type of `seconds` is `=> Int` which means `seconds` is a by-name parameter. Therefore, when we call `goToSleep` it with `tired = false`, `lengthOfNap` will not be evaluated. |
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This is not that compelling of an example. I would suggest leading with an example where avoiding unwanted evaluation is actually crucial. Computing a number of seconds is highly unlikely to be an expensive computation.
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What did you see as the downsides with the examples in the original version...?
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Looking back at the original example, it does look better. I'll put it back.
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| i -= 1 | ||
| } // prints 2 1 | ||
| ``` | ||
| The method `whileLoop` uses multiple parameter lists to take a condition and a body of the loop. If the `condition` is true, the `body` is executed and then a recursive call to whileLoop is made. If the `condition` is false, the body is never evaluated because we prepended `=>` to the return type. |
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=> wasn't appended to the return type; it was appended to body's type.
| ``` | ||
| The method `whileLoop` uses multiple parameter lists to take a condition and a body of the loop. If the `condition` is true, the `body` is executed and then a recursive call to whileLoop is made. If the `condition` is false, the body is never evaluated because we prepended `=>` to the return type. | ||
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| Now when we pass `i > 0` as our `condition` and `println(i); i-= 1` as the `body`, it behaves like the standard while loop in many languages. Here is the output: |
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I'll remove that sentence since I have the output in a comment.
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