Multiline function literal as arguments in Scala

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There are just a couple of simple syntax rules. The appendix of the spec is worth perusing.

A function literal or anonymous function (6.23) will look like x => Expr or x => Block depending on whether the context is an Expr or a ResultExpr, respectively.

A function application (6.6) will look like f(Expr, Expr) or f BlockExpr, i.e., f{ Block }. That is, a BlockExpr is just a sequence of block statements inside {...}.

When you call f(g), then g is an Expr, so as a function literal, x => Expr. The Expr can be a BlockExpr, x => { ... }.

When you call f{ Block }, then f { x => ... } has the function literal in ResultExpr of a Block (which is just a sequence of statements, no braces required).

Here, it’s obvious that the anon func is at the bottom of a block:

scala> def m(x: Int=>Int) = x(5)
m: (x: Int => Int)Int

scala> m {
     | val y = 7
     | x => // no brace
     | x+y+1
     | }
res0: Int = 13

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