Why is there a value constructor in addition to the type constructor in Haskell?

Question

I'm a newcomer to Haskell and am currently going through Real World Haskell. The book says the type constructor is used only in the type signature while the value constructor is used in actual code. It also gives an example of a declaration to show that the names for both are independent of each other. Why are two constructors needed in the first place, if only one of them is used in actual code? Since we would not use the type constructor in actual code, what purpose does the type constructor serve?

Answer 1

Maybe the names are a little bit misleading. A type constructor represents the name of the type you're declaring. They're called like that because they build types, not values: indeed, being (possibly) parameterized on type variables they define a family of types. They act something like C++'s templates and Java's generics. In data MyType a b = Constr a b, MyType is a type constructor that takes two types a and b to build a new type (MyType a b).

A value constructor is the only part you would call a "constructor" in other (object oriented) languages, because you need it in order to build values for that type. So, in the previous example, if you take the value constructor Constr :: a -> b -> MyType a b, you may build a value Constr "abc" 'd' :: MyType [Char] Char.

Answer 2

It's a bit like saying "why do we need classes and objects if objects are the only thing that actually runs?"

The two sorts of constructors do different jobs. Type constructors go in type signatures. Value constructors go in runnable code.

In the simplest case, a type "constructor" is just a type name. In the simplest case, a type has only one value constructor. So you end up with things like

data Point = Point Int Int

You might say to yourself "now why the heck to I need to write Point twice?"

But now consider a less trivial example:

data Tree x = Leaf x | Branch (Tree x) (Tree x)

Here Tree is a type constructor. You give it a type argument, and it "constructs" a type. So Tree Int is one type, Tree String is another type, and so on. (Like templates in C++, or generics in Java or Eiffel.)

On the other hand, Leaf is a value constructor. Given a value, it makes a 1-node tree out of it. So Leaf 5 is a Tree Int value, Leaf "banana" is a Tree String value, and so on.

Similarly for Branch. It takes two tree values and constructs a tree node with those trees as children. For example, Branch (Leaf 2) (Leaf 7) is a Tree Int value.