流中的类型推断

Question

Why can't the compiler infer the type of the wildcard in the filter ?

scala> Stream.from(1) takeWhile(_ < 1000) filter ( ( _ % 3  == 0) || ( _ % 5 == 0)) sum

<console>:18: error: missing parameter type for expanded function ((x$2) => x$2.$percent(3).$eq$eq(0))
              Stream.from(1) takeWhile(_ < 1000) filter ( ( _ % 3  == 0) || ( _ % 5 == 0)) sum

This works fine:

scala> Stream.from(1).takeWhile( x => x < 1000).filter(x => (x % 3 == 0) || (x % 5 == 0)).sum

Answer 1

From the SLS 6.23.1 :

If an expression e binds underscore sections u1,…,un , in this order, it is equivalent to the anonymous function (u′1, ... u′n) => e′ where each u′i results from ui by replacing the underscore with a fresh identifier and e′ results from e by replacing each underscore section ui by u′i .

This means that multiple underscores using the placeholder syntax imply an anonymous function with multiple parameters.

ie

(_ > 0) expands to (x => x > 0)

(_ > 0 || _ < -10) expands to ((x, y) => x > 0 || y < -10)

(_ > 0 || _ < -10 || _ % 2 == 0) expands to ((x, y, z) => x > 0 || y < -10 || z % 2 == 0)

And so on.. The error is a little confusing because it looks like it's trying to read it as an arity-1 function within each set of parenthesis based on the error message. I'll speculate that it then doesn't know what to do with the expression x => a || y => b x => a || y => b to infer the types to somehow come up with Int => Boolean .

Answer 2

Each subsequent instance of _ refers to the next argument. If you want to reuse an argument of a lambda, you must name it like you did in the 2nd version you posted.

The first version desugared would be more like

filter ((x,y) => ( x % 3 == 0) || ( y % 5 == 0))