What does `def` in scala evaluate to?

Question

I wrote this is Scala Repl

def sum(a: Int, b: Int) = a + b

This is evaluated as sum: (a: Int, b: Int)Int in Repl. def in Scala is lazily evaluated. So, what is the type that Repl displays? Also, how is this eagerly evaluated when sum(1,2) is called or how is (a: Int, b: Int)Int evaluated to Int ?

I noticed this when I was playing with val in Scala. If I write val sum = (a: Int, b: Int) = a + b this is eagerly evaluated into (Int, Int) => Int = <function2> which is fine as the apply function call is made. But I don't understand what happens in case of def .

Answer 1

The sum: (a: Int, b: Int)Int displayed by the REPL is a bit of compiler internals leaking to the userland.

sum is a method, so there is no actual value of type (a: Int, b: Int)Int , but the compiler internally associates sum with something called method type - it's a type that holds method signature - its parameter names and types + result type. This type exists only in the compiler and cannot be directly written in Scala code.

Answer 2

A def is not a variable, it becomes a method and so does not have a type. To be able to look at its signature, you would have to convert it into a partial function or something similar to that. A val, however becomes a Function2 object. Look at some decompiled code if you want. This post is pretty helpful https://alvinalexander.com/scala/fp-book-diffs-val-def-scala-functions .