Question

My question is about Odersky’s lecture 3.5 (not the assignment!):

The exercise at 19:15 asks: “What is the type of: if (true) 1 else false“

In his explanation, Odersky argues that the type is AnyVal because it is the most specific supertype of Int and Boolean (the types of the two conditional branches).

I am asking myself why the type checker is not smart enough to see that only the first branch is actually relevant, and the type could thus be inferred to Int?

Answer 1

It’s a similar question to why the following does not compile in Java:

throw new Foo();
return 42;  //unreachable statement

but this compiles:

if(true)
  throw new Foo();
return 42;

Basically the compiler has to stop somewhere. Especially the Scala compiler, which is already quite slow. Once it recognizes true as always passing condition, one might ask: what about: 1 == 1, 2 * 21 == 42 or even x == 7 where x is val.

It’s hard to draw a line so the compiler simply assumes every if can have both branches. It’s the responsibility of IDE or code validation tools to discover such impossible or probably incorrect expressions.