Re:What Is Good About Haskell?
by HelloNurse on Thursday October 03, 2019 @11:33AM
(#21148171)
Children of a non-leaf tree node must be optional because otherwise the node would be forced to have both children.
It's therefore obvious that Box means optional; otherwise there would be a bare Tree<A> to represent a mandatory reference.
I don't know Rust (though I'm familiar with Java & C) and there's nothing obvious about Box. In fact, I'm just learning from your comment that it means optional.
Re:What Is Good About Haskell?
by steveklabnik on Thursday October 03, 2019 @04:10PM
(#21151576)
(Box does not mean optional, the parent is wrong. Option is the type for optional. Box is basically a mallloc, placing a value in the newly allocated memory, and then a call to free automatically when it goes out of scope. The box itself is a pointer to the heap. It’s not allowed to be null, in some sense, the opposite of optional.)
You need to know that “enum” means that commas in the next section are different than usual, but only one layer deep—check nesting carefully. You need to know about type parameters in either version.
I had a similar experience learning my first ML: I couldn’t even tell how many words each word would gobble up, because I didn’t know the reserved words get. Syntax highlighting helped, and it’s not a problem after a week or two. It’s no worse than figuring out what’s a binary vs unwary operator in C and its descendants.
Also: it’s not obvious to me that Box means optional/nullable. I’d expect it to mean a required non-null pointer to a heap element.