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Node(A, Box<Tree<A>>, Box<Tree<A>>), is obviously nested (a Node contains an A and two Trees) and the trees are optional (they are contained in a Box, whose pu

Node(A, Box<Tree<A>>, Box<Tree<A>>), is obviously nested (a Node contains an A and two Trees) and the trees are optional (they are contained in a Box, whose purpose is obvious without even knowing the language) | Node a (Tree a) (Tree a) might or might not be nested, given the cheerful taste for currying and juxtaposition without punctuation that prevails in Haskell syntax, and it isn't obvious what purpose the parentheses serve (just grouping ?) and whether the trees are optional.

tom_mellior 7y ago
How is the Haskell example not "nested" according to your definition? It contains an "a" and two Trees, just like the Rust example. Nothing is optional. (There might be a Maybe or similar type, possibly hidden behind a type name, but it would still not be optional.)
brians 7y ago
To be obvious that it’s nested, you need to know that < and > are used as approximations for ⟨ and ⟩. You need to know that they’re brackets, not operators. 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.
- HelloNurse 7y ago
  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.
  - the_af 7y ago
    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.
    - steveklabnik 7y ago
      (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.)
the_af 7y ago
> given the cheerful taste for currying Currying makes no sense in type definitions. It's like saying that in Java you aren't sure if "String name" will run something, "given Java's cheerful taste for running things". To me, it's obvious the parentheses in "(Tree a)" are grouping things, which is the most immediate (and correct) interpretation, but I'll agree this is more debatable.
- HelloNurse 7y ago
  They are clearly grouping "Tree a", but do they mean that it is optional?
  - the_af 7y ago
    Why would you assume it's optional? In which popular programming language do parentheses mean "this is optional"? When you read a formula such as (x + 1) / 2 do you assume part of it is optional?

tom_mellior 7y ago

How is the Haskell example not "nested" according to your definition? It contains an "a" and two Trees, just like the Rust example. Nothing is optional. (There might be a Maybe or similar type, possibly hidden behind a type name, but it would still not be optional.)

brians 7y ago

To be obvious that it’s nested, you need to know that < and > are used as approximations for ⟨ and ⟩. You need to know that they’re brackets, not operators. 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.

HelloNurse 7y ago
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.
- the_af 7y ago
  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.
  - steveklabnik 7y ago
    (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.)

the_af 7y ago

> given the cheerful taste for currying Currying makes no sense in type definitions. It's like saying that in Java you aren't sure if "String name" will run something, "given Java's cheerful taste for running things". To me, it's obvious the parentheses in "(Tree a)" are grouping things, which is the most immediate (and correct) interpretation, but I'll agree this is more debatable.

HelloNurse 7y ago
They are clearly grouping "Tree a", but do they mean that it is optional?
- the_af 7y ago
  Why would you assume it's optional? In which popular programming language do parentheses mean "this is optional"? When you read a formula such as (x + 1) / 2 do you assume part of it is optional?