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FAQ: add question about _ in lambdas #2142

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Expand Up @@ -149,6 +149,23 @@ has a good summary of all the meanings it has.
Note that, even if the specific meaning is different,
according to the situation, it usually means _"anything"_.

### Why doesn't my function literal with `_` in it work?

Not all function literals (aka lambdas) can be expressed with the `_`
syntax.

Every occurrence of `_` introduces a new variable. So `_ + _` means
`(x, y) => x + y`, not `x => x + x`. The latter function cannot be
written using the `_` syntax.

Also, the scope of `_` is always the smallest enclosing expression.
The scope is determined purely syntactically, during parsing, without
regard to types. So for example, `foo(_ + 1)` always means `foo(x =>
x + 1)`; it never means `x => foo(x + 1)`. The latter function cannot
be written using the `_` syntax.

See also [SLS 6.23.2](https://scala-lang.org/files/archive/spec/2.13/06-expressions.html#placeholder-syntax-for-anonymous-functions).

### Can I chain or nest implicit conversions?

Not really, but you can [make it work](https://stackoverflow.com/a/5332804).
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