"fromJust" vs. "Just =" in Haskell

Question

Haskell newbie here. I'm trying to understand this example from Haskell's diagrams library. Specifically, there's a line like this:

Just t = <thing> where <thing> is of type Maybe (Tree a)

I don't understand what this is doing. I understand that we need to get the value out from the Maybe. I replaced that line of code with

t = fromJust <thing>

and it works the same. Is there any difference between the two lines, and can anyone explain what the first line is doing?

Answer 1

fromJust is pretty much equivalent to:

fromJust :: Maybe a -> a
fromJust (Just t) = t

Note that it’s the same pattern match! If you’re sure that your Maybe going to be a Just and not a Nothing, you can use fromJust to get its value without pattern matching, but matching is cleaner in most cases, so you don’t need it here.