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33 lines
948 B
Markdown
33 lines
948 B
Markdown
# Using Optional Labeled Function Arguments
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If you are constructing a function that takes some arguments, but one of
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those arguments has a reasonable default value, then you can use an optional
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labeled argument. Labeled arguments are those arguments prefixed with a `~`.
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If you give the argument a default value, then it becomes optional.
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```reason
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let thing = (~a=1, b: int, c: int) => {
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a + b + c;
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};
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```
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In this case `~a` is a labeled argument. It is also optional and will
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default to `1` if not specified. The other two arguments, `b` and `c`, are
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positional arguments and thus required in order for the function to
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evaluate.
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Here are two ways of using this function either by specifying `~a` or
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excluding it so that it defaults to `1`.
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```reason
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thing(~a=2, 1, 1)
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|> string_of_int
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|> print_endline /* 4 */
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thing(1, 1)
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|> string_of_int
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|> print_endline /* 3 */
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```
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See more details [here](https://reasonml.github.io/docs/en/function).
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