Question

I’ve seen several examples of code using the symbol “That” with Generics. i.e.

def map[B, That](f : (A) => B) : That

But, due to the lack of google-ability of that word, I can’t find any documentation on what it does or how I use it.

Is it simply a normal type placeholder, or does it do something special?

Answer 1

Any identifiers inside [...] are treated as type parameters.

So in case of def map[B, That](f : (A) => B) : That That only means a generic return type. Replace it with Z for example: def map[B, Z](f : (A) => B) : Z would have the exact same effect.