问题
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?
回答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.
回答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.
回答3:
One convenient way to get the intuition about Types and Values is that the former are compile-time values while the latter are run-time values. In other words, Type constructors are constructors of values in the set of Haskell Types, for the sole aim of typing your program at compile time. That also means you can not construct a type at run-time, and you can not construct a value at compile-time.
So, because you can not explicitly branch at run-time on the basis of a type value (though you can implicitly with typeclasses), Type constructors are totally useless as run-time objects and in many cases are totally absent in the final binary. Conversely, because values constructors allows to construct values in the set of their type at run-time, they are totally useless as compile-time object.
Because of this simple property, Type constructors and Value constructors can share names unambiguously.
来源：https://stackoverflow.com/questions/10242709/why-is-there-a-value-constructor-in-addition-to-the-type-constructor-in-haskell