type Weight = Integer
type Person = (Name, Address) -- supposing types Name and Address to be declared elsewhere
- then you can use a value of type `Integer` wherever a `Weight` is expected, and vice versa. `newtype` and `data` on the other hand, create genuinely new types. `newtype` is basically just an efficient version of `data` that you can use in special circumstances. `newtype` must always take one type argument and have one value constructor. For example:
+ then you can use a value of type `Integer` wherever a `Weight` is expected, and vice versa. <!-- `type` is allowed to be parameterized -->
+
+ `newtype` and `data` on the other hand, create genuinely new types. `newtype` is basically just an efficient version of `data` that you can use in special circumstances. `newtype` must always take one type argument and have one value constructor. For example:
newtype PersonalData a = PD a
You could also say:
- data PersonalData a = PD a
+ data PersonalData2 a = PD2 a
- And `data` also allows multiple type arguments, and multiple variants and value constructors.
+ And `data` also allows multiple type arguments, and multiple variants and value constructors. <!-- Subtle difference: whereas `PersonalData a` is isomorphic to `a`, `PersonalData2 a` has an additional value, namely `PD2 _|_`. In a strict language, this is an additional type an expression can have, but it would not be a value. -->
OCaml just uses the one keyword `type` for all of these purposes:
* When a type only has a single variant, as with PersonalData, Haskell programmers will often use the same name for both the type and the value constructor, like this:
- data PersonalData a = PersonalData a
+ data PersonalData3 a = PersonalData3 a
The interpreter can always tell from the context when you're using the type name and when you're using the value constructor.